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Hybridization Experiments with an All-Female Fish of the Genus Poeciliopsis
Abstract
1. Hybridization experiments between an all-female fish, Poeciliopsis "Cx," and the males of two bisexual species, P. lucida and P. latidens, have provided genetic evidence that hereditary materials of both the mother and the father combine to form the F, progeny. The entire male genome, however, appears to be simply "on loan" for each generation. None of the male's chromosomes, apparently, are incorporated in the ova; thus, the integrity of the maternal line is maintained, even though the all-female form is forced to rely on sperm from other species to maintain itself.2. This mode of reproduction provides an isolating mechanism as complete as that of parthenogenesis or gynogenesis.
... The bottom is larger than it and concave shape, the surface of the jaws attached on each other, Occlusal Surface, the outer edge of bottom jaw covered the upper jaw in pharynx, and this harmony between the jaws may have a role in correlation between them, which reflected on processing of food particles during feeding process of fish. Although there have been few studies on teeth in some species of Poecilia genus, to which molly fish belongs to it, but these studies did not give detailed information, and dealt of teeth in mouth region only (Schultz, 1966(Schultz, : 1969Lewis et al., 1999). The current study showed that molly fish has teeth in both mouth and pharynx regions, and it is therefore necessary to conduct a detailed study on teeth of other species of this genus. ...
... This was not described in study of Lewis et al. (1999), which was conducted on the three species mentioned above, in which the analysis of teeth (shape, number and distribution pattern) for taxonomic purposes. Also this has not been mentioned in the previous studies on some species such as P. lucida and P. monach (Schultz, 1966 ;1969). ...
... Tooth base connected to bones of bearing structures in both oral and pharyngeal regions. Schultz, 1966;Darnell and Abramoff, 1968). ...
The present study aimes to determine the location of the teeth in the sailfin molly fish Poecilia latipinna and the dentition pattern, In addition to studying the structure of teeth, both morphological and anatomical. Bringing a number of Sailfin molly fish, they were dissected in the laboratory and samples were prepared using the whole mount method, and some were prepared for imaging by a scanning electron microscope (SEM). The results show the P. latipinna fish have two types of teeth. First, the oral teeth that are located on oral jaws. Second is the pharyngeal teeth that are located on pharyngeal jaws. The dentition pattern of two types of teeth are different at different shape, size and distribution of teeth on each their jaws. The oral teeth are organized in two rows; a row of outer teeth (OT) and a row of inner teeth (IT) on both upper and lower oral jaws, The outer teeth were larger than the inner teeth, also different shape. The outer tooth was similar to the Sickle. Whereas the inner tooth was cobra-like. The dentition pattern of pharyngeal teeth differed from the oral teeth; the teeth were distributed in two regions in each part of the pharyngeal jaw, Arrangement Teeth Region (ATR) and the other is Less Arrangement Teeth Region (LATR). The teeth in these regions were arranged with the primary and secondary rows, the order of these rows and their poles varied between the upper and lower pharynx jaws.
... Three distinct diploid all-female forms of Poeciliopsis arose as hybrids between sexual species in this genus. Each unisexual form contains a maternal genome derived from P. monacha, and a paternal genome from one of three species, P. lucida, P. occidentalis, or P. latidens, with which the unisexuals live and mate (Schultz, 1969(Schultz, , 1971(Schultz, , 1973. The F1 hybrid genotype of the unisexuals is sustained through a reproductive process termed hybridogenesis (Schultz, 1969) in which segregation is not random; the monacha genome passes intact to the haploid egg while the paternal genome is lost during meiosis Present addresses: Department of Zoology, Rutgers University, New Brunswick, New Jersey 08903. 1 Biological Sciences Group, University of Connecticut, Storrs, Connecticut 06268. ...
... Each unisexual form contains a maternal genome derived from P. monacha, and a paternal genome from one of three species, P. lucida, P. occidentalis, or P. latidens, with which the unisexuals live and mate (Schultz, 1969(Schultz, , 1971(Schultz, , 1973. The F1 hybrid genotype of the unisexuals is sustained through a reproductive process termed hybridogenesis (Schultz, 1969) in which segregation is not random; the monacha genome passes intact to the haploid egg while the paternal genome is lost during meiosis Present addresses: Department of Zoology, Rutgers University, New Brunswick, New Jersey 08903. 1 Biological Sciences Group, University of Connecticut, Storrs, Connecticut 06268. (Cimino, 1972;Schultz, 1966). ...
... The F1 hybrid genotype of the unisexuals is sustained through a reproductive process termed hybridogenesis (Schultz, 1969) in which segregation is not random; the monacha genome passes intact to the haploid egg while the paternal genome is lost during meiosis Present addresses: Department of Zoology, Rutgers University, New Brunswick, New Jersey 08903. 1 Biological Sciences Group, University of Connecticut, Storrs, Connecticut 06268. (Cimino, 1972;Schultz, 1966). Upon mating with a male of the sexual host species, fertilization restores the hybrid genotype to the zygotes. ...
... The morphology of the three all-female forms indicates a hybrid origin, with the two gonochoristic (bisexual) species of the complex, P. Lucida Miller and P. monacha Miller, as parental forms. Of the three unisexual forms, one is diploid (2n =48) like the gonochorists, and is intermediate in morphology between them (Schultz, 1961(Schultz, , 1969. It apparently possesses one genome of each; for this reason, it has been named P. monacha-lucida (formerly "ex"; Schultz, 1969). ...
... The morphology of the three all-female forms indicates a hybrid origin, with the two gonochoristic (bisexual) species of the complex, P. Lucida Miller and P. monacha Miller, as parental forms. Of the three unisexual forms, one is diploid (2n =48) like the gonochorists, and is intermediate in morphology between them (Schultz, 1961(Schultz, , 1969. It apparently possesses one genome of each; for this reason, it has been named P. monacha-lucida (formerly "ex"; Schultz, 1969). ...
... Of the three unisexual forms, one is diploid (2n =48) like the gonochorists, and is intermediate in morphology between them (Schultz, 1961(Schultz, , 1969. It apparently possesses one genome of each; for this reason, it has been named P. monacha-lucida (formerly "ex"; Schultz, 1969). The other two unisexuals are triploid (3n = 72). ...
... Initially a number of different Poeciliopsis unisexuals were identified. Some forms, known as P. Cx and P. Cz, were diploid with 2n = 48 chromosomes; others, for example, P. Cy; were triploid with 3n = 72 chromosomes (Miller and Schultz, 1959;Schultz, 1961Schultz, , 1966Schultz, , 1967Schultz, , 1969Schultz, , 1971. The Poeciliopsis diploid unisexuals maintain the all-female characteristic from generation to generation by a unique mechanism called hybridogenesis (Schultz, 1969), which prevents independent assortment of maternally and paternally derived chromosomes. ...
... Additional important features of hybridogenesis were revealed by studies on the unisexual P. Cx. This form, which inhabits the Rio Mocorito, resembled another hybridogen, Poeciliopsis monacha-lucida, in several morphological features, but differed slightly in attributes characteristic of the bisexual species, Poeciliopsis viriosa (Schultz, 1961(Schultz, , 1966. From biochemical and morphological studies (Schultz, 1969;Vrijenhoek, 1972), it was evident that P. monachalucida had arisen from interspecific hybridization between Poeciliopsis monacha and Poeciliopsis lucida, some 200 km to the north in the Rio Fuerte. ...
... 317). The dramatic spread of P. monacha-lucida via hybridogenetic combinations with the paternal genomes of Poeciliopsis latidens and Poeciliopsis occidentalis is now well documented (Schultz, 1961(Schultz, , 1966(Schultz, , 1969(Schultz, , 1971Moore et al., 1970;Vrijenhoek and Schultz, 1974). The adaptive advantages and evolutionary implications ofhybridogenesis (and gynogenesis) are too lengthy to be considered here. ...
Cytogenetics refers to the study of heredity through the study of chromosomes (the bearers of the genes), and the cytological mechanisms of inheritance. Procedures involving preparations of mitotic chromosomes from actively dividing somatic tissues of live specimens or from embryos have been the most widely used among fish cytologists and have the dual advantages of being rapid and inexpensive. The soft organs (kidney, spleen, and liver) have proved to be fine sources of chromosomes. The discipline of “cytogenetics,” along with its practical application, has yet to be used extensively in fish breeding or fish culture. In this chapter, emphasis is placed on cytogenetics of fishes. The various modes of sexuality found among fishes are briefly considered, with the emphasis placed on the genetic and/or cytogenetic mechanisms that influence sexuality. As a group, the fishes display an almost complete range of sexuality from hermaphroditism to unisexuality to bisexuality or gonochorism. This diversity is unparalleled among the vertebrates. A few instances of unisexuality are known in the ambystomid salamanders and in a few lizard species, but hermaphroditism is unknown elsewhere. A few potentialities for the practical application of cytogenetics to fish culture are briefly discussed in the chapter.
... Hybridogenesis in an interspecific hybrid mainly occurs in fish and frogs [3][4][5][6][7][8]. The hybrids develop normally and produce fertile gametes comprising either genome of the parental species. ...
... We conclude that these Russian populations of P. kl. esculentus possess three different cytoplasmic origins: P. lessonae, P. ridibundus and P. bedriagae (Table 1). (1) Haplotype of mitochondrial cytochrome b. (2) Haplotype or genotype of Serum albumin intron 1. (3) The letter of species genome, of which serum albumin intron 1 fragment band depth is reduced in gonadal DNA compared to that of finger DNA; (4) Fertility is the ratio of normal tail-bud embryos to the total number of layed eggs (cf. Table S2). ...
Hybridogenesis in an interspecific hybrid frog is a coupling mechanism in the gametogenic cell line that eliminates the genome of one parental species with endoduplication of the remaining genome of the other parental species. It has been intensively investigated in the edible frog Pelophylax kl. esculentus (RL), a natural hybrid between the marsh frog P. ridibundus (RR) and the pool frog P. lessonae (LL). However, the genetic mechanisms involved remain unclear. Here, we investigated the water frogs in the western Russian territory. In three of the four populations, we genetically identified 16 RL frogs living sympatrically with the parental LL species, or with both parental species. In addition, two populations contained genome introgression with another species, P. bedriagae (BB) (a close relative of RR). In the gonads of 13 RL frogs, the L genome was eliminated, producing gametes of R (or R combined with the B genome). In sharp contrast, one RL male eliminated the L or R genome, producing both R and L sperm. We detected a variation in genome elimination within a population. Based on the genetic backgrounds of RL frogs, we hypothesize that the introgression of the B genome resulted in the change in choosing a genome to be eliminated.
... Hybridogenesis, a rare form of clonal inheritance found in a few fish, a frog, and a stick insect (Schultz 1969;Uzzell and Berger 1975;Mantovani and Scali 1992;Carmona et al. 1997) takes nuclear replacement to an extreme. In diploid hybridogens, this hemiclonal form of inheritance clones the maternal nuclear genome (M) and substitutes the paternal nuclear genome (P) in each generation ( Fig. 1A). ...
... Collection localities are shown in Figure 2. Rí o Mocorito samples were collected in 1978, 1999, and 2000 from the same two localities (LH and SB) used in previous studies (Schultz 1966;Vrijenhoek and Schultz 1974). Several thousand Poeciliopsis were examined morphologically in the field to roughly sort the hybrid biotypes from coexisting sexual species. ...
Crossing experiments revealed that a diploid hybridogenetic fish (genus Poeciliopsis) from the Río Mocorito (Sinaloa, Mexico) is trihybrid. Its haploid maternal genome is inherited clonally (i.e., hemiclonally), and it expresses a mixture of morphological traits found in the closely related species P. monacha and P. viriosa. Its haploid paternal genome is replaced in each generation by mating with males of a more distantly related sexual species, P. lucida. However, expression of mixed (monacha × viriosa) traits by this hemiclone is also consistent with retention of shared ancestral polymorphisms. If true, this hemiclonal lineage would be one of the few examples of an ancient asexual taxon. We used mitochondrial DNA and allozymes to test whether the maternal progenitor of the Mocorito hybridogen was a recent P. monacha × P. viriosa hybrid or a remnant of their most recent common ancestor. Our results clearly link the hemiclonal genome to contemporary P. monacha and therefore support the hypothesis of a recent origin. Additionally, our findings suggest that this unisexual fish may serve as a vehicle for introgression between two allopatric sexual species. Corresponding Editor: S. Karl
... These clones are diploids that replicate the genetic material normally, but expel the paternal contribution during oogenesis (Cimino 1972b). Thus, the matriline is perpetuated identically, while a new paternal contribution is acquired each generation (Schultz 1961(Schultz , 1966. ...
The present study looks at the interaction between species recognition
and mate choice within a set of sympatric species. In Mexico, a complex of
species of internally fertilizing, viviparous fishes (Genus Poeciliopsis) live
sympatrically with each other and with an array of clonal females. The clones
depend on inseminations from males of the various species to initiate
embryogenesis. The clones are considered "sperm parasites" since none of the
DNA from the males is incorporated into the offspring. Considering the costs
incurred by males in time, energy, inter-male aggression, exposure to predation
and, of course, the wasted gametic material, the question investigated was why
males from these species court and mate with clonal females? Many hypotheses
were considered, including whether males are allocating mating effort in
proportion to the probability of achieving sexual offspring, or whether males are
deriving other fitness benefits from clonal inseminations, such as discarding old
sperm, gaining valuable practice or increasing their attractiveness to sexual
females. The hypothesis found to be most consistent with the evidence was the
one that tested whether males must interact and mate with clonal females while
learning to discriminate between clonal and conspecific females. It was
demonstrated that males can learn clonal discrimination only if they are given
extensive experiences with a sexual female first and then given extensive
experience with a clonal female. The reverse order of experiences did not result
in males exhibiting clonal discrimination. Further, it was shown that males more
rapidly learned cues associated with sexual females than they learned cues
associated with clonal discrimination. Finally, a model of the proximate
mechanism involved in learning clonal discrimination was proposed.
... The morphology of female genital region and urogenital papilla is one of these unexplored traits. In the present study, we describe, discuss and taxonomically explore these traits, supporting the description of a recently discovered species and expanding the meagre published knowledge on the subject (e.g., Peden, 1972Peden, , 1973Schultz, 1966). Most of the 20 known species of Phalloceros differ in genital morphology, which in addition to helping species recognition presumably act as a prezygotic reproductive isolation mechanism (Jennings et al., accepted;Lucinda, 2008). ...
A new species of Phalloceros is described from the Tijucas, Maruim, Aririú, Cubatão do Sul, Rio da Madre, D'Una and Tubarão river basins in southern Brazil. The new species is diagnosed among congeners by the bilateral asymmetry of a hood‐like structure located immediately anterior to the urogenital papilla of females. The hood is fused with the urogenital papilla on the right side, but free from the papilla on the left side. This hood is reported for the first time in the genus and is present in six of the 21 valid species of Phalloceros, but its bilateral asymmetry is an exclusive condition of the new species. These species do not constitute a monophyletic group. The new species is likely to be phylogenetically related to P. caudimaculatus, whose females have a symmetrical hood.
... No. AF047343 and AF047344), verifying their hybrid status. The rare occurrence of males among hybridogenetic Poeciliopsis is not unprecedented (e.g., Schultz 1966Schultz , 1967 reported rare P. monacha-latidens males, which were sterile). Further work is needed to verify the reproduction mode of the MJ hybrids. ...
Poeciliopsis jackschultzisp. nov. , is described based on seven specimens (17.9–26.7 mm SL) from the Río Concepción (also known as Río Magdalena), Sonora, Mexico. The new species belongs to the Leptorhaphis species group and can be distinguished from other members of this group by features of the skeleton and colouration. The new species is sympatric with P. occidentalis , a hybridogenetic all-female biotype P. monacha-occidentalis , and hybrids between P. monacha-occidentalis females and P. jackschultzi males. The distribution of P. jackschultzi is highly restricted, and the main habitat, spring-fed marshy streams and pools, is susceptible to loss and degradation in a desert environment with increasing human water demand.
... Ãèíîãåíåç îòìå÷åí òàêaeå ó äèïëîèäíûõ è òðèïëîèäíûõ ðàñ ìåëêèõ aeèâîðîäÿùèõ òðî-ïè÷åñêèõ ðûáîê, îòíîñÿùèõñÿ ê ðîäàì Poecilia è Poeciliopsis, ñåì. Poeciliidae (Hubbs, Hubbs, 1932Miller, Schultz, 1959;Schultz, 1961Schultz, , 1966Schultz, , 1967Schultz, , 1969. Êðîìå òîãî, îí íàáëþäàåòñÿ ó òðèïëîèäíûõ è òåòðàïëîèäíûõ ùèïîâîê Cobitis, ðûá ñåìåéñòâà Cobitidae (Sofradûija, Berberovic, 1978;Âàñèëüåâ, Âàñèëüåâà, 1982;Âàñèëüåâ, 1985;Âàñèëüåâà, 1999), ó òðèïëîèäíîé ôîðìû ïëîòâû Rutilus, Cyprinidae (Collares-Pereira, 1985, 1986, öèò. ...
... The paternallucida set (L) is expelled in a premeiotic cell division, thereby precluding synapsis and recombination (Cimino, 1972). In nature, the haploid M ova produced by ML females are fertilized by lucida sperm (L') and a new ML' hybrid expressing traits encoded by both parental genomes is produced (Schultz, 1966;Vrijenhoek, 1972). Only the maternal M genome of a hybridogenetic strain is transmitted clonally; the paternal L genome is substituted in each generation. ...
The frozen-niche-variation model was proposed to account for the coexistence of genetically related clones in naturally occurring unisexual populations. This model is based on two assumptions: 1) ecologically different clones have multiple independent origins from sexual ancestors; and 2) the population of sexual ancestors contains genetic variability for ecologically relevant traits. To test these assumptions, we produced 14 new "hemiclones" (nonrecombining haploid genotypes) of fish (Poeciliopsis: Poeciliidae). Our ability to synthesize many new hemiclones demonstrates the feasibility of multiple independent origins of nonrecombining genotypes. A substantial proportion (10-50%) of the phenotypic variation among hemiclones in size at birth, juvenile growth rate, and fecundity had a genetic basis. Thus, we conclude that multiple origins can give rise to an assemblage of genetically distinct hemiclones, each with a unique combination of life-history traits. Additionally, a comparative analysis of two natural hemiclones revealed that the synthetic strains represent a broad field of variation from which natural hemiclones can be selected.
... Three Poeciliopsis strains were analyzed in this experiment: an outcrossed sexual strain (Poeciliopsis monacha) and two associated unisexual, triploid strains (Poeciliopsis 2-monacha-lucida: MML/I and MML/II; Vrijenhoek et al. 1977). The triploid Poeciliopsis strains reproduce gynogenetically, a strictly clonal mode of reproduction whereby the entire triploid genome is faithfully replicated among generations (Schultzz1967)..Although sperm fromma sexual species is required to activate embryogenesis in the triploid ova, paternal genes make no contribution to the genotype of the offspring (Schultz 1967;Cimino 1972;Vrijenhoek 1972). For further information on this reproductive complex, see Schultz (1969). ...
The validity of the assumptions of the Red Queen model for the maintenance of sexual reproduction was reassessed using life- history data collected from clonal and sexual fish in the genus Poeciliopsis. A previous study using these strains (one sexual, two clonal) indicated that sex might be maintained by coevolutionary processes outlined in the Red Queen model. However, the previous study did not test the assumption that parasitism negatively affects the host's fitness. The current study was undertaken to address this issue, as well as to reevaluate the assumptions of the Red Queen model in this complex. Data on size, fecundity, and parasitic intensity were collected from fish from seven Mexican pools. Parasitic intensity was highest for one clone, but intensity was not correlated with clonal frequency, as assumed by the Red Queen model. Variance in parasitic intensity was not reduced in the clonal strains relative to the sexual strain, and intensity was not correlated with fecundity. These data, combined with aspects of the parasite's biology, indicate that the Red Queen model is unlikely to maintain sex in this clonal-sexual complex.
... In the swordtail, the genital pore was noted as being wider and farther from the anus than in the platyfish (Clark et al., 1954). The genital opening in Poeciliopsis-species is a scaleless depression with a fleshy coneshaped papilla with the genital opening at its tip (Schultz, 1966). A large, presumably distensible, foliate genital pad was mentioned, but not pictured, for female Pseudoxiphophorus (= Heterandria). ...
A review of the literature and personal research have been integrated in a survey of morphological and behavioral traits associated with the transfer of sperm to achieve internal fertilization in the tribe Poeciliinae. The structure of the intromittent organ, depth of insertion into the female genital opening, and its structure appear to be related to mating tactics (displaying or non-displaying prior to copulation) and the efficiency of
inseminations. Male gonopodial armaments (hooks, claws, serrae) serve as holdfasts that can also injure the female inside the genital sinus during copulations. An outgrowth of the integument over the third fin rays of the gonopodium may limit the depth of insertion and/or steady the gonopodium when touching the genital pore. The female genitalia show a striking heterogeneity with prominent or reduced papillae, sinuses of different sizes, and elaborated external skin folds that may function as guiding structures and/or may be
even involved in the uptake of spermatozeugmata. The length of the gonopodium as related to reproduction is poorly defined in the literature. Males of a species, that do not display when courting females, tend to possess a relatively long and “unarmed” gonopodium, lacking hooks, claws, or serrae. They transfer sperm exclusively by gonopodial thrusting (= rape, coercive, or sneak copulation), and insert their gonopodium only slightly, if at all, into the female genital pore during copulation. Males of displaying species have relatively short and “armed” gonopodia, court the female prior to copulation, and insert the gonopodium deeper, but exhibit gonopodial thrusting as an alternative mating tactic. Inconsistencies with this “rule” suggest that a more complex scenario may exist and perhaps there is a multiple reduction, a secondary loss, or a new acquisition of a given mating tactic. Some species, however, regarded as non displaying may send out other visual signals.
... In order for us to be able to communicate about them, all taxa must bear nomina. Kyons, just like mayrons, must be referred to by Latin binomina governed by the international nomenclatural rules of the Code, and not by any other system of numbers, letters, codes, abbreviations or plurinominal designations that are not real nomina (Avise, 2008;Bogart et al., 2007;Schultz, 1961Schultz, , 1966Schultz, , 1967Schultz, , 1969. When what is at stake is simply to mention these taxa in official lists, in publications of physiology or in the medias, it is often enough to mention the two terms of the binomen, substantive and epithet. ...
Eidonomy is the field of taxonomy, which addresses the “species problem”. The latter has several dimensions. The first is the confusions frequently made between species as an evolutionary unit, a taxon, a taxonomic category and a nomenclatural rank. The second is the reductionist temptation to recognize in nature only one kind of “basic entities” of biodiversity. From a practical viewpoint and for all non-systematist users of specific nomina, any animal organism should be referred to a taxon of nomenclatural rank species, designated by a Latin binomen. These nomina are indispensable for administrative and legal documents, for management and conservation of biodiversity, as well as for all biological research. But this does not imply that all these taxa should be referred to a single taxonomic category, a “unified concept of species”. In nature, several kinds of entities correspond to different “species concepts” or specions that are irreducible one to another. These categories can be defined according to the modalities of their reproduction, i.e., of their gametopoiesis (mode of formation of gametes), of their kinetogenesis (mode of initiation of the development of the ovum) and of the gene flow between individuals. The best known of these categories is that of mayron or “mixiological species concept”, which points to an independent bisexual panmictic entity, constituting a protected gene pool, but others do exist. In order to better understand their differences, it is useful to consider the various patterns of speciation. These can be referred to three main categories: monogeny (change within a single lineage), diplogeny (birth of two specions from a single one) and mixogeny (hybridization between two specions). The splitting in two of a single initial gene pool is a rather long process, which often shows different stages. The latter can be expressed taxonomically through the use of particular eidonomic categories. Speciation can be considered completed only when it has become irreversible in case of new contact between the two stocks after a period of separation. In allopatry, it is difficult or impossible to know if this stage has been reached. This is not a reason for abandoning the mixiological criterion for identifying sympatric or parapatric specions. In allopatry, the method of inference can be used and, in the absence of sufficient information, it is fully possible to use “by default” the “phylogenetic concept of species” or simpson. The situation is further complicated by the fact that some of the basic entities of biodiversity of hybrid origin, the kyons, are not mayrons, as they have particular reproductive modes. Contrary to isolated and “normal” hybrids, such entities may persist in the long term in nature. Their gametopoiesis implies either an ameiosis (mitoses) or a metameiosis (modified meiosis), and their kinetogenesis implies zygogenesis (fertilization), gynogenesis or parthenogenesis. Kyons are of two main categories. The klonons are unisexual female entities in which genetic transmission is clonal, reproduction being often through parthenogenesis, or following other mechanisms with similar results in genetic terms. The kleptons are unisexual or bisexual entities, which depend for their reproduction, at each generation, on a mayron or another klepton. Their metameiosis produces particular gametes, which start their development either by zygogenesis (zygokleptons), by gynogenesis (gynokleptons) or by a combination of both systems (tychokleptons). All these particular cases do not constitute, as it has long been believed, “evolutionary dead ends”. On the contrary, some of these forms are advantaged in some conditions, and may also, in some cases, be at the origin of allopolyploid specions. The latter include both “normal mayrons” or eumayrons (allotetraploid bisexual entities) and heteromayrons (allotriploid bisexual entities with metameiosis). Mayron, klepton, klonon and simpson, and their subcategories, are different taxonomic categories which correspond to a single nomenclatural rank, that of species. The existence of interspecific hybridization in nature is a very common phenomenon in animals, the importance of which has long been underestimated. Reticulous relationships between specions have played a major role in the scenario of evolution. This should be acknowledged by the rejection of the “universal tree of life” model for organic evolution and its replacement by a “network of life” model.
... The triploid Poeciliopsis reproduce gynogenetically, a strictly clonal mode of reproduction whereby the entire triploid genome is faithfully replicated among generations (Schultz 1969). Although sperm from a sexual species is required to activate embryogenesis in the triploid ova, paternal genes make no contribution to the genotype of the offspring (Schultz 1967;Cimino 1972;Vrijenhoek 1972). For further information on this reproductive complex, see Schultz (1969). ...
The evolution and subsequent maintenance of sex has been debated for many years, and there are numerous aspects that remain poorly understood. When comparing sexual with asexual reproduction, there are many more apparent benefits to being asexual than sexual. The frozen niche variation (FNV) model describes how asexual clones can arise from a sexual population and how the two reproductive types can coexist. Herein we compared three sympatric populations of sexual and asexual fish (one sexual population, Poeciliopsis monacha, and two clonal populations, P. 2-monacha-lucida) to test the assumption of the FNV model that sexual populations have a broader dietary niche (as measured by gut contents analysis) than clonal populations. Individual sexual fish had similar dietary breadth when compared with clonal individuals. However, dietary breadth for sexual populations as a whole was broader than for either clonal population, indicating differences in between-individual dietary choice. Our results support the primary assumption of the FNV model and thereby provide a possible explanation for the maintenance of sexual reproduction in this clonalsexual complex.
... Hybridogenesis is a hemiclonal mode of reproduction that has been observed in several species complexes from a range of different taxa: Bacillus (Mantovani & Scali 1992), Poeciliopsis (Schultz 1966), Rana (Berger 1973a), Trophidophoxinellus (Carmona et al. 1997). Such systems seem to be highly complex and interesting for evolutionary ecology due to the various impacts hybridogenesis has on the ecology and genetics of the involved species (e.g. ...
Hybridogenesis is a hemiclonal mode of reproduction occurring in several species complexes with various effects on the ecology and genetics of the involved species. The complexity of these systems is illustrated with the example of Western Palearctic waterfrogs. Deviations from original models of hybridogenesis are discussed, proofs for recombination between the parental genomes of the hybrid are reviewed, and the sex ratio disturbance found in many populations is explained. The review shows that benefits deriving from hybridogenesis, such as increased genetic variability and colonization ability, seem to be primarily directed toward R. ridibunda, but also have an impact on the other parental species. It is concluded that, due to mating possibilities and fertilization success, regions with mixed populations of two (or more) parental species and their hybridogenetic associate might be especially of interest in investigating recombination patterns and relating these to the population dynamics of the parental species.
... Gynogenesis was also discovered in diploid and triploid races of small viviparous tropical fishes of the genera Poecilia (= Mollienesia ) and Poeciliopsis (Poeciliidae; Hubbs, 1932, 1946;Miller and Schultz, 1959;Schultz, 1961Schultz, , 1966Schultz, , 1969; in loaches, Cobitis (Cobitidae; Sofrad ja and Berberovic, 1978;Vasil'ev and Vasil'eva, 1982;Vasil'ev, 1985;Vasil'ev et al., 2007;Lebedeva, 2007); in triploid races of the caudate amphibian genus Ambystoma (Ambystomatidae; Macgregor and Uzzell, 1964;Cuellar, 1974); and in only some insects (triploid race of beetle Ptinus mobilis, Ptinidae, the reproduction of which depends on the presence of males of the diploid species Ptinus clavipes ; Sanderson, 1960). ...
Parthenogenesis is usually recognized as the most accepted mechanism of cloning, i.e., reproduction without genetic recombination. Transfer
from bisexual to parthenogenetic propagation causes the appearance of all-female populations, races, and species. It was ascertained
in natural populations of numerous of reptile and insect species. Clonal and hemiclonal species of fishes and amphibians propagate
by means of gynogenesis and hybridogenesis. Less known are instances of androgenesis found in some insects and mollusks. In this case offspring develops only under control of male genes supplied by spermatozoa.
Mother’s genes included into the egg nucleus have to be entirely lost. Androgenesis may be called mirroring of parthenogenesis.
... During hybridogenetic oogenesis only the haploid monacha genome is transmitted to the ova; the paternal genome is expelled from oogonia thereby preventing synapsis and recombination (Cimino, 19726). The haploid monacha ova produced by these hybrids are fertilized by sperm from the host species, reestablishing diploid hybrids that express morphological and electrophoretic traits encoded by both parental genomes (Schultz, 1966; Vrijenhoek, 1972). Hybridogenetic unisexuals are not clones in the strict sense because their paternal genomes are substituted in each generation and therefore have access to all of the allelic variation in the gene pool of the sexual host (Vrijenhoek et al., 1977). ...
SYNOPSIS. Recent genetic studies of asexually reproducing fishes in the genus Poeciliopsis (Poeciliidae) revealed abundant variation in the form of multiple sympatric clones. Recur- rent hybridizations between sexual species provides the principal source of clonal variation. The hybrids are spontaneously endowed with a clonal reproductive mechanism that per- petuates a high level of heterozygosity. Migration within and between river systems, and mutations, also contribute to clonal diversity in" these fish. Coexistence among different clones and with the sexual ancestors depends in part upon specializations characteristic of individual clones. Clonal reproduction is an efficient mechanism for freezing a portion of the niche-width variation contained in the gene pool of the more broadly adapted, sexual ancestors. Multiclonal populations achieve significantly higher densities relative to the sexual forms than do monoclonal populations. This relationship is a function of the clonal variability upon which natural selection can act and upon the capacity of a multiclonal population to better exploit a heterogeneous environment through niche diversification. In all-female organisms such as Poeciliopsis, which are dependent upon sexual species for insemination, competitive abilities probably are at a premium in the densely populated pools and arroyos of the Sonoran Desert. Competitive abilities are probably less important for truly parthenogenetic clones which rely on colonization abilities to escape from their sexual ancestors and from other clones.
Zusammenfassung Der Teichfrosch (Pelophylax esculentus, Genotypen LR, LLR oder LRR) ist ein natürlicher Hybrid zwischen dem Kleinen Wasserfrosch (P. lessonae, Genotyp LL) und dem Seefrosch (P. ridibundus, Genotyp RR). In der rein diploiden Form (LR) pflanzt sich der Hybrid via Hybridogenese fort, bei welcher ein Teil des elterlichen Genoms (entweder der L- oder der R-spezifische Teil) während der Gametogenese eliminiert und der verbleibende Teil klonal auf haploide Keimzellen übertragen wird. Rekombination zwischen dem L- und R-spezifischen Teil des Genoms ist in der Regel nicht möglich, weshalb es durch die wiederholte klonale Weitergabe innerhalb des Genoms zu einer Ansammlung schädlicher Mutationen kommt. Bei der Verpaarung zweiter Hybriden aus derselben Population sind deren Nachkommen durch die Kombination zweier klonaler Genome daher nicht überlebensfähig. Um lebensfähige Nachkommen zu zeugen, müssen sich die Hybriden mit der jeweiligen Elternart rückkreuzen, deren Teil des Hybridgenoms während der Gametogenese verloren ging. Aus dieser fortplanzungsbedingten Abhängigkeit heraus entwickelten sich verschiedene Formen eines gemischten Populationssystems aus Hybriden und Elternarten. In einigen Populationen produziert P. esculentus sowohl haploide (L oder R) als auch diploide Keimzellen (LR-Gameten, welche normalerweise nur von Individuen des Genotyps LR gebildet werden). Aus der Verschmelzung von diploiden und haploiden Keimzellen entstehen triploide Hybriden der Genotypen LLR und LRR, welche ihrerseits bei der Gametogenese jenen Genomteil ausschliessen, der nur in einzelner Ausführung vorliegt, und aus dem doppelt vorhandenen Genomteil haploide Gameten bilden (so produziert der Genotyp LLR haploide L-Gameten und der Genotyp LRR haploide R-Gameten). Dadurch entstehen über die Generationen hinweg abwechselnd diploide und triploide Hybride, welche sich unabhängig von der Anwesenheit der Elternarten erfolgreich miteinander fortpflanzen. Die reproduktive Unabhängigkeit dieser sogenannten gemischt-ploiden Systeme wird dadurch ermöglicht, dass die in den triploiden Fröschen doppelt vorhandenen Genomteile rekombiniert werden können, wodurch verhindert wird, dass sich in den Genomen zu viele schädliche Mutationen ansammeln. Sowohl die gemischten Systeme aus Hybriden und Elternarten als auch gemischt-ploide Systeme wurden in den letzten Jahrzehnten unter verschiedenen Aspekten und in unterschiedlichen geographischen Regionen untersucht. Jedoch fehlte bislang aufgrund der ungleichmässigen Verbreitung gemischt-ploider Populationssysteme in Europa ein umfassender und vergleichender populationsgenetischer Überblick im grösseren geographischen Masstab. Ferner sind die populationsgenetischen und phänotypischen Unterschiede zwischen Hybriden aus Elternarten-Populationen und gemischt-ploiden Populationen bisher noch weitgehend unbekannt, obwohl zwischen den unterschiedlichen Systemen, als auch zwischen geographischen Regionen, potentiell unterschiedliche Selektionsbedingungen herrschen. Die Zielsetzungen meiner Arbeit waren daher: a) gemischt-ploide Populationen aus unterschiedlichen Gegenden Europas genealogisch zu untersuchen und herauszufinden, ob deren ungleichmässige Verteilung auf eine unabhängige Entstehungsgeschichte zurückzuführen ist, b) zwei augenscheinlich unterschiedliche gemischt-ploide Fortpflanzungssysteme auf Unterschiede in der Keimzellenproduktion zu untersuchen, c) die bioakustischen Eigenschaften männlicher Paarungsrufe einer Anzahl gemischt-ploider Populationen unterschiedlicher geographischer Herkunft zu analysieren und mit den Eigenschaften der Paarungsrufe aus gemischten Hybrid-Elternarten-Populationen zu vergleichen, und d) die räumlichen Bewegungsmuster und das Distanzverhalten zwischen Männchen verschiedener Genotypen aus gemischt-ploiden und gemischten Hybrid- Elternart-Populationen zu untersuchen und potentielle Unterschiede auf Zusammenhänge mit phänotypischen Eigenschaften der Männchen sowie mit Eigenschaften der untersuchten Teiche zu testen. In Kapitel eins untersuchte ich durch Analysen von Mikrosatelliten-DNA und mitochondrialer DNA populationsgenetische Parameter für mehr als 2000 Gewebeproben, welche aus 72 Lokalitäten in Nord-, Mittel- und Osteuropa stammten. Die Ergebnisse dieser Studie zeigten, dass die auf der Mikrosatelliten- Analyse basierende genetische Diversität von der geographischen Lage, dem Vorhandensein der Elternarten P. lessonae und P. ridibundus sowie dem Populationstyp beeinflusst wird. Während sich die meisten gemischt-ploiden Populationen aus Mitteleuropa und dem östlichen Mitteleuropa genetisch nicht sehr unterscheiden, zeigen einige Populationen aus der Ukraine ein deutlich anderes genetisches Profil. Dieses Ergebnis wird durch den Fund ungewöhnlicher mitochondrialer DNA-Typen in Individuen jener Populationen bestätigt und legt die von den gemischt-ploiden Populationen Nord- und Mitteleuropas unabhängige Entstehung jener östlichen Populationen nahe. In der Diskussion interpretiere ich diese Ergebnisse mit Bezug auf nach- und zwischeneiszeitliche Kolonisationsszenarien in Europa. Kapitel zwei präsentiert eine Studie, welche in Zusammenarbeit mit Nicolas Pruvost durchgeführt wurde und in welcher wir Kreuzungsexperimente und Analysen von Mikrosatelliten-DNA benutzten, um fünf Populationen von unterschiedlicher Populationsstruktur zu vergleichen. Dafür untersuchten wir mit Hilfe von Indizes für Heterozygotie und genetische Differenzierung die Interaktionen zwischen verschiedenen Genotypen (LL, LLR, LR, LRR and RR). Die Ergebnisse dieser Studie erlaubten uns verschiedene Fortpflanzungssysteme zu definieren und zu unterscheiden, sowie ein evolutionäres Szenario für das Auftreten und die Aufrechterhaltung eines alternativen Systems gemischt-ploider Populationen in Mitteleuropa vorzuschlagen. In Kapitel drei befasste ich mich mit den bioakustischen Eigenschaften männlicher Paarungsrufe innerhalb und zwischen gemischt-ploider sowie gemischten Populationen aus Hybriden und den Elternarten P. lessonae und P. ridibundus. Aus der Analyse von Feldaufnahmen der Rufe leitete ich fünf Rufparameter ab, welche alle einen Dosiseffekt des jeweiligen Genoms L oder R zeigten, d.h. sie nahmen mit steigendem L:R-Verhältnis der Genotypen in der Reihenfolge LL-LLR-LR-LRR-RR entweder zu oder ab. Zwei der fünf Rufparameter unterschieden sich zudem zwischen Populationssystemen. Die Effektgrössen nahmen in der Reihenfolge Genotyp-Populationssystem –geographische Lage der Population ab. Die Rufe diploider Hybriden (LR) variierten zwischen den Populationssystemen in Abhängigkeit davon, ob die Hybriden zur erfolgreichen Fortpflanzung eine der beiden Elternarten benötigen, oder nicht. In Kapitel vier überprüfte ich innerhalb dreier Teiche (zwei mit gemischt- ploiden Populationen, einer mit einer Population aus LR-Hybriden und P. lessonae), ob sich das räumliche Mobilitätsmuster und Distanzverhalten der Männchen während der Paarungszeit zwischen Genotypen unterscheidet. Darüber hinaus testete ich die räumlichen Parameter auf Zusammenhänge mit der Körpergrösse und Kondition der Männchen, sowie mit der beobachteten Häufigkeit, in der die einzelnen Männchen im Amplexus mit Weibchen beobachtet wurden. Die Ergebnisse zeigten, dass weder Genotyp noch Kondition das räumliche Bewegungsmuster beeinflussen und deuteten darauf hin, dass vorhandene Unterschiede zwischen den Teichen wahrscheinlich auf Unterschiede in der Populationsdichte zurückzuführen sind. Die Verteilung der Genotypen der Männchen im Amplexus entsprach für gemischt-ploide Populationen der tatsächlichen Verteilung der Genotypen der Männchen im Teich. Bei den Amplexus-Männchen der gemischten Hybrid-P. lessonae-Population waren P. lessonae-Männchen leicht überproportional vertreten. In Kapitel fünf präsentiere ich eine kollaborative Studie mit Anke Stöhr über Ranavirus-Infektionen in wilden Wasserfrosch-Populationen. Die Studie kombiniert die Fallstudie eines Ranavirus-Ausbruchs unter in Gehegen gehaltenen Wasserfröschen mit der Beschreibung eines neuen Ranavirus und dessen phylogenetischer Klassifizierung. Die Kapitel eins bis vier meiner Dissertation ermöglichen ein tieferes Verständnis der Diversität, Verbreitung sowie der genetischen und phänotypischen Differenzierung von P. esculentus-Populationen. Als Schlussfolgerung daraus argumentiere ich, dass es keinen „Allerwelts“-Hybriden gibt, sondern dass Teichfrosch-Populationen in Europa so divers sind, dass sie als signifikante Evolutionseinheiten denselben Respekt und dieselbe Aufmerksamkeit verdienen wie „reine“ Arten. Summary The edible frog (Pelophylax esculentus, genotypes LR, LLR or LRR) is a natural hybrid between the pool frog (P. lessonae, genotype LL) and the marsh frog (P. ridibundus, genotype RR). Diploid hybrids (LR) reproduce by hybridogenesis, where one part of the hybrid’s parental genome (either the L or the R chromosome set) is excluded during gametogenesis and the other part is clonally transmitted into haploid gametes. Recombination between the L and R genome within the hybrid is usually not possible. Therefore, repeated clonal transmission of one part of the genome leads to the accumulation of deleterious mutations which normally renders offspring from inter-hybrid crossings within the same population unviable. In order to produce viable offspring, the hybrid is thus forced to mate with the parental species whose part of the genome was excluded. This reproductive dependence has led to several forms of mixed hybrid-parental population systems. In some populations, P. esculentus can produce both haploid (L or R) and diploid gametes, LR gametes usually coming only from LR individuals. The fusion of diploid with haploid gametes results in triploid hybrids of the genotypes LLR and LRR, which exclude the single copy genome and produce only haploid gametes of the other genome (LLR produce L, LRR produce R gametes). Thus, in a perpetuating way, diploid and triploid hybrids are generated and can successfully reproduce with each other, independent of the presence of the parental species. The reproductive independence of these so-called mixed-ploidy systems is due to the fact that triploids recombine the part of their genome which is present in a double copy and thus prevent the accumulation of deleterious alleles in the genetic pool of the population. Both the mixed hybrid-parental and the mixed-ploidy systems have been studied over the last decades in several aspects and geographic regions, but due to the patchy geographic distribution of mixed-ploidy systems in Europe, a comprehensive and comparative population genetic overview across a larger area has been lacking. Furthermore, population genetic and phenotypic differences between hybrid P. esculentus from mixed-parental and mixed-ploidy systems are vastly unknown, despite potentially different selection regimes between population types and geographic regions. The objectives of my thesis were thus: a) to compare mixed-ploidy populations from different European areas in a genealogical approach and to find out whether these patchily distributed populations are of independent origin, b) to examine the gamete production patterns between two supposedly different mixed-ploidy breeding systems, c) to study bioacoustic characteristics of male advertisement calls across a number of geographically distant mixed-ploidy populations and compare them with hybrids from mixed hybrid-parental systems, and d) to examine the spatial movement and spacing behavior of male frogs within and between mixed-ploidy and mixed hybrid-parental systems and relate potential differences to male genotype, male morphology and pond characteristics. In chapter one, I used microsatellite DNA and mitochondrial DNA analysis to obtain population genetic parameters for more than 2000 samples from 72 localities across Northern, Central and Eastern Europe. The results from this study showed that genetic diversity among populations based on microsatellites is structured by geographic latitude and longitude, the presence of parental genotypes (P. lessonae and P. ridibundus) and population type. Most mixed-ploidy populations from Central and East-Central Europe did not genetically differ substantially, but some populations from Ukraine showed a distinctively different genetic profile. This was confirmed by the novel finding of unusual types of mitochondrial DNA in specimens from there. My findings suggest an independent origin of polyploid water frogs from this area, which I discuss with reference to postglacial re-colonization scenarios in Europe. Chapter two presents a collaborative study with Nicolas Pruvost, where we used microsatellite DNA analyses and crossing experiments to compare five populations of different population structures. Indices of heterozygosity and genetic differentiation were used to depict the genetic interactions between the different genotypes (LL, LLR, LR, LRR and RR). The results from this study allowed us to define and differentiate between different breeding systems and propose an evolutionary scenario for the occurrence and maintenance of an alternative mixed- ploidy population type in Central Europe. In chapter three I studied the bioacoustic properties of male advertisement calls within and between mixed-ploidy and mixed hybrid-parental populations. From field recordings I derived five call parameters which all showed a genomic dosage effect, i.e. they either decreased or increased with the L/R ratio among genotypes in the order LL-LLR-LR-LRR-RR. Two of the five call parameters were also affected by the population system. Effect sizes decreased from genotype through population system to geographic location of the population. Calls of diploid (LR) hybrids varied between population systems, depending on whether they belonged to a system that required a sexual host for successful reproduction, or not. In chapter four I tested within three ponds whether male spatial movement and spacing behavior during the breeding season differs between genotypes. Furthermore, I related spatial parameters to male body size and condition, and to the observed frequency of amplexus by individual males. As a result, I found that neither genotype nor size nor condition affected spatial movement patterns and that differences are most likely to be explained by population density. The frequency of amplexus events among genotypes corresponded to the observed male genotype distribution in the two mixed-ploidy ponds, and slightly favored P. lessonae males in the mixed hybrid-parental population in the other pond. Chapter five represents a collaborative study with Anke Stöhr on ranavirus infection in wild populations of European water frogs. The study combines a case study of a ranavirus outbreak among captive water frogs with the description of a novel ranavirus and its phylogenetic classification. In conclusion, chapters one to four of my thesis allow a better understanding of the diversity, distribution and differentiation of P. esculentus populations in terms of genetic and phenotypic characteristics. I argue that there is no such thing as a “common” water frog hybrid, but rather that hybrid populations are so diverse that they represent evolutionary significant units which deserve the same respect and attention as other “true” anuran species.
The distribution of the all-female "species" P. monacha-occidentalis over the range of its "host species" P. occidentalis is presented. The all-female form achieves greatest concentration at the southern limit of its range in the Rio Mayo and is successively diminished in the four northerly drainages. Within a given drainage the proportion of P. monacha-occidentalis increases from very low levels at its western extent, along the eastern edge of the Sonoran Desert, to maximum levels midway between the eastern and western boundaries of its range. To the east of this maximum, considerable localized fluctuation in proportional levels of P. monacha-occidentalis occurs. The hypothesis which most parsimoniously explains the origin and distribution of the unisexual form, P. monacha-occidentalis, is: 1) P. monacha-occidentalis arose from P. monacha-lucida, a unisexual form of a more southern origin which has migrated north into the range of P. occidentalis. 2) It sustains itself by mating with males of the indigenous P. occidentalis. In so doing, it inherits not only some of its morphological characters but also some of its ecological requirements. All-femaleness and the maternal genome are retained, however, via hybridogenesis. 3) The level at which P. monacha-occidentalis females are found in a population is probably determined by the slight differences in niche requirements of P. monacha-occidentalis and the host species P. occidentalis. A stable equilibrium between the two forms appears to exist within a given drainage and possibly within local populations. However, the causative factors in this equilibrium cannot be resolved from these data.
Skewed sex ratios are common among several species of Poeciliopsis, a viviparous fish from northwestern Mexico. Since previous, unrelated studies from this laboratory (Angus and Schultz, 1983) suggested that deviation from a 1:1 sex ratio might be influenced by temperature, two inbred strains of P. lucida were tested for temperature-dependent sex determination by comparing sex ratios of offspring from pregnant females held at different water temperatures. Different sex ratios were produced by the two strains at the same temperature: one strain produced almost all-male offspring at 30°C and female-biased sex ratios at 24°C, while the other strain produced a 1:1 sex ratio at both temperatures. At intermediate temperatures, the labile strain produced sequentially fewer males with decreasing temperatures. The other strain produced a consistent sex ratio regardless of temperature. Poeciliopsis lucida apparently has a genetic polymorphism for temperature-influenced sex determination. An hypothesis is offered for the evolutionary origin of environmental sex determination.
Many studies in the genus Poecilia have focused on reproductive and genetic characteristics of Poecilia formosa, the Amazon molly, and its sympatric species P. latipinna and P. mexicana. The research literature of Poecilia dentition has been limited to general tooth morphology. Essentially absent are comparative analyses of dentition patterns and total numbers of teeth. The current study uses dentition analysis as a method to compare species in the genus Poecilia and to address some taxonomic issues related to these fish. The study focused on fish from the areas of southern Texas and northeastern Mexico. Through the use of scanning electron microscopy, the lower jaws of Poecilia spp were examined to determine total numbers of outer and inner teeth. In addition, the differences in distribution patterns of the inner teeth were recorded and compared. Statistical analyses were performed to determine which comparisons were significant. This study reveals several observations: 1) variations in the numbers of outer and inner teeth exist in some of these fish with respect to site of collection; 2) differences in total teeth numbers and dentition patterns were found both interspecifically and intraspecifically; and 3) in addition, dentition analysis provided evidence regarding the origin of P. formosa. This study supports the current notion that P. latipinna, the proported paternal component, and P. mexicana limantouri, the purported maternal component, are the progenitor species of P. formosa. Two unresolved taxonomic questions were addressed through dentition analysis. First, the present study supports the exclusion of the triploid associate of P. formosa as a separate species from P. formosa. Second, this study shows a significant difference in the number of inner teeth and. in dentition patterns between I! mexicana limantouri and P. mexicana mexicana. Such differences, in addition to previously known distinguishing characteristics, should prompt careful consideration of whether or not these taxa deserve specific status or retention of their current subspecific status. (C) 1999 Wiley-Liss, Inc.
The name of Th. Dobzhansky will always be associated with the concepts of heterosis and adaptive genetic polymorphisms in natural populations of sexually reproducing organisms. But the relationship between these concepts is still in part controversial and even after approximately 35 years of serious study of these phenomena in a variety of organisms, both in nature and in the laboratory, it is by no means clear how far the adaptive significance of most polymorphisms is to be ascribed to “pure heterosis” (i.e., superiority of the heterozygote in all environments) and how far to “annidation” (in the terminology of Ludwig, 1950)—i.e., an adaptive correspondence between the various genotypes present in the population and the alternative ecological niches present in the environment.
The evolutionary ecology of unisexual fishes is distinguished from that of closely related sexual species by two peculiar aspects of their genetics: First, they reproduce, in effect, asexually, and second, their genomic constitutions are invariably identical to those of F1 hybrids. Thus, a major difficulty in analyzing the evolutionary ecology of these species is to determine the extent to which the distinct phenomena of asexuality and hybridity contribute to their successes and failures. That is, where abundance indicates the success of a particular unisexual fish species, is it successful there because it reproduces asexually or because it possesses a hybrid genotype? The ecological consequences of asexuality, in turn, can be more finely divided. Asexually reproducing organisms do not pay the cost of meiosis (Williams, 1975), and, hence, an asexual species has twice the intrinsic rate of increase r of a comparable sexual species (Maynard Smith, 1978). On the other hand, asexual species lack genetic recombination and cannot generate the multiplicity of genotypes possible in a sexual species. This paucity of genotypic variation may limit the effectiveness of selection and, hence, evolutionary potential or it may reduce the aggregate effectiveness of resource utilization because there cannot be a spectrum of “specialized” genotypes in the population.
During the development of population biology as a science, genetic variation has been regarded as adaptive, maladaptive, or simply waiting to be drawn upon during times of environmental change. Attempts to evaluate the importance of variation, either in the production of heterosis or as adaptive polymorphisms in homogeneous, heterogeneous, or fluctuating environments, are generally confounded by two major problems: (1) identifying the forces of selection that are operational and (2) determining the phenotype (and associated genotype) upon which selection operates. In sexually reproducing organisms the effects of selection are continually obscured by recombination. Asexual organisms provide a simpler system for investigating the adaptive significance of genetic variation. Here, the genetic variation of a population is contained in the form of multiple genetic clones that, in the absence of recombination, magnify the effects of selection in successive generations.
Prior to 1932 the scientific community had already accepted the fact that invertebrates of all sorts reproduce parthenogenetically; none the less, it came as a surprise when Hubbs and Hubbs (1932) announced the existence of a unisexual fish, the now famous Amazon molly, Poecilia formosa. After more than 12 years of experimenting with this unique vertebrate, the Hubbses concluded that it was of hybrid origin probably involving Poeoilia sphenops (sensu lato) and P. latipinna (Hubbs, 1955). They found that either of the presumed parental species provides sperm to the unisexual form but that its function is merely to stimulate the ova into development (gynogenesis). Offspring, thus, are characteristically females and identical to their mothers. Biochemical (Abramoff et al. 1968) and cytological studies (Drewry, 1964; Schultz and Kallman, 1968), and tissue transplant experiments (Kall-man, 1962) subsequently confirmed that P. formosa is a gynogenetic hybrid; and, furthermore, that it is diploid and apparently produces diploid eggs.
The European water frog Pelophylax esculentus (genome LR) is a natural hybrid between P. lessonae (LL) and P. ridibundus (RR). It presents a peculiar quasi-sexual reproductive mode known as hybridogenesis: the hybrid excludes one of the parental species’ genomes at a pre-meiotic stage of gametogenesis, thus producing gametes containing clonal copies of the other parental genome. By mating with the parental species whose genome has been excluded it re-establishes hybridity at each generation.
Moreover, because of its hybrid nature and resulting problems of chromosome pairing at gametogenesis, P. esculentus also produces diploid gametes from time to time. These gametes often lead to the generation of triploid frogs which will allow, under certain ecological conditions, the establishment of all hybrid populations which are maintained without the genetic contribution of either parental species. Over the past decade, such populations have been well studied in the north-western part of Europe, but the presence of triploid water frogs has also been reported for various areas in Central Europe. However, for those localities details on the breeding system, i.e. the genetic contribution of the various frog types, are usually lacking. The major goals of this thesis were to (a) investigate the Central European populations more closely, (b) to compare the breeding systems there with that in all-hybrid populations from Northern Europe and (c) find out whether triploid water frogs in different areas are of mono- or polyphyletic origin.
In chapter one I used microsatellite DNA analyses and crossing experiments to compare five populations (one in Poland, two in Germany and two in Slovakia) presenting different population structures. Indices of heterozygosity and of genetic differentiation allowed to depict the genetic interactions between the different type of frogs (LL, LLR, LR, LRR and RR). I was then able to define and differentiate the breeding systems occurring in each of them and to propose an evolutionary scenario for the appearance and maintenance of the all-hybrid populations.
Chapter two presents a collaborative study with Alexandra Hoffmann.
Here we enlarged our survey to populations distributed all over Europe and used microsatellite DNA and mitochondrial DNA (mtDNA) analyses to find patterns of genetic structure among different breeding system types. We found that genetic diversity among hybrid populations is influenced by geographic location (latitude, longitude) and by the proportions of parental genotypes in the hybrid population. Furthermore, we identified genetic clusters from both microsatellites and mtDNA, which indicate that there are at least two separate polyploid hybrid clades existing today: one in Northern and East-Central Europe and one in Eastern Europe (Eastern Ukraine).
In chapter three I focused on eight Czech and Slovak populations. Using microsatellite DNA analyses, flow cytometry and crossing experiments I was able to enlighten and describe a new breeding system type of hybrid water frog populations, the “modified LE-system”. It is characterized by a triploid lineage consisting of males only.
Chapter four takes a more ecological approach where I was looking for fitness differences in larval life history traits between the three hybrid types (diploid LR and two type of triploids LLR and LRR) and their parental species (LL and RR) when raised under two temperature regimes. Diploid and triploid hybrids performed better than their parental species under colder conditions. This finding helps to explain why all hybrid populations dominate in the colder northern part of the species distribution.
Chapter five also presents a collaborative study with colleagues from Poland. A detailed microsatellite DNA analysis of 18 loci revealed the origin of an unexpected pentaploid froglet offspring obtained by artificial crosses. Using the dosage effect of seven microsatellite loci I was able to demonstrate that the pentaploid froglet was the result of the fusion of a haploid L sperm with a tetraploid egg containing two times the entire genome of the hybrid mother (LLRR egg). This study illustrates a practical application of the DNA microsatellite dosage effect which allows unraveling the ploidy level and the number of copies of the two specific genomes, P. lessonae and P. ridibundus, in the hybrids.
In conclusion, this study (a) allows a better understanding of the diversity and complexity of water frog breeding systems containing triploid individuals, (b) demonstrates the multiple origins of triploids from different populations and (c) proposes an evolutionary scenario for the origin and maintenance of all hybrid populations. I argue that such populations represent significant evolutionary units which deserve attention of biologists but also the care of decision-makers in conservation policies.
Population and evolutionary genetics have been quickly developing ?elds of biological research over the past decades. This book compiles our current understanding of genetic processes in natural populations. In addition, the book provides the author's original ideas and concepts based on the data obtained by himself and his close coworkers. The author introduces his pioneering concept of population genetic stability,and much of thebook is concerned with the factors and conditions of such stability. Why does genetic stability matter so much? Altukhov argues that the sustainable use of natural resources, including genetic resources of popu- tions, critically depends on the maintenance of their stability. The preser- tion of well-adapted genetic characteristics from one generation to the next is essential for this stability. Traditionally, population genetics has been - cusedonevolution andthe role of evolutionary factorsinshapinggenetic structures of populations. While the idea of a population as a dynamic unit of evolution has been widely accepted, the signi?cance of genetic stability and its implications for the long-term survival of populations and species have not been fully appreciated.
Clonal variability and the factors affecting variability were examined in populations of the hybridogenetic unisexual fish Poeciliopsis monacha-lucida from northwest Mexico. Tissue grafting, a sensitive indicator of genetic differences between individuals, was used to analyze the clonal structure of populations some distance apart and in different habitats. Fish scales were chosen as donor tissue since their use permits multiple tissue exchanges between individuals. Studies confirming the suitability of scales for grafting experiments with unisexual fishes are presented. Tissue grafting proved to be a more sensitive technqiue for detecting clonal variability in unisexual vertebrate populations than electrophoresis. A previous electrophoretic survey of 22 protein loci identified eight different phenotypes among the 30 unisexual strains used in this study. In contrast, tissue grafting revealed 18 distinct histocompatibility genotypes among the same 30 strains. The evidence indicates that multiple hybrid origins have been the most significant source of clonal variability in these unisexuals. Some of the clones identified are widespread and have existed for at least 12 years. The importance of both selective and nonselective processes in determining the clonal composition of populations are discussed.
The European water frog Pelophylax esculentus is a natural hybrid between P. lessonae (genotype LL) and P. ridibundus (RR). It reproduces through hybridogenesis, eliminating one parental genome from its germline and producing gametes containing the genome of the other parental species. According to previous studies, this elimination and transmission pattern is very diverse. In mixed populations, where only diploid hybrids (LR) live in sympatry and mate with one or both parental species, the excluded genome varies among regions, and the remaining genome is transmitted clonally to haploid gametes. In all-hybrid populations consisting of diploid (LR) and triploid (LLR and/or LRR) frogs, diploid individuals also produce gametes clonally (1n in males, 2n in females), whereas triploids eliminate the genome they have in single copy and produce haploid gametes containing the recombined other genome. However, here, too, regional differences seem to exist, and some triploids have been reported to produce diploid gametes. In order to systematically study such regional and genotype differences in gamete production, their potential origin, and their consequences for the breeding system, we sampled frogs from five populations in three European countries, performed crossing experiments, and investigated the genetic variation through microsatellite analysis. For four populations, one in Poland, two in Germany, and one in Slovakia, our results confirmed the elimination and transmission pattern described above. In one Slovakian population, however, we found a totally different pattern. Here, triploid males (LLR) produce sperm with a clonally transmitted diploid LL genome, rather than a haploid recombined L genome, and LR females clonally produce haploid R eggs, rather than diploid LR eggs. These differences among the populations in gamete production go along with differences in genomotype composition, breeding system (i.e., the way triploids are produced), and genetic variation. These differences are strong evidence for a polyphyletic origin of triploids. Moreover, our findings shed light on the evolutionary potential inherent to the P. esculentus complex, where rare events due to untypical gametogenetic processes can lead to the raise, the perpetuation, and the dispersion of new evolutionary significant lineages which may also deserve special conservation measures.
Ranker and Arft (1994) brought needed attention to current ambiguity in the use of the term "hybrid" and the potential confusion this ambiguity can have on the protection of allopolyploid plants under the U.S. Endangered Species Act (ESA). This legislation denies protection to hybrids under the (not explicitly stated) rationale that "hybrids" as generally understood do not constitute independent evolutionary lineages and, hence, are undeserving of protection. Ranker and Arft correctly point out, however, that the term "hybrid," denoting the F 1 progeny of interspecific matings, should not be confused with polyploid taxa that are originally of hybrid origin. The important distinction is that the former are evolutionary dead-ends that are most often sterile or have greatly reduced fertility relative to their parental species (Dobzhansky 1941; Mayr 1970), while the latter are independent lineages capable of persistence over evolutionary time scales. Ranker and Arft argue that the preclusion of protection for hybrids by the ESA pertains to the former group, while the latter, by virtue of their persistence and evolutionary independence from their parental species, qualify as species and, therefore, deserve protection under the ESA. I believe this argument to be accurate, but I would like to point out the existence among certain unisexual vertebrates of a third class of phenomena intermediate in nature to the distinction between primary, homoploid hybrids and aUopolyploid species noted by Ranker and Arft. I then consider how the questionable taxonomic standing of these unisexuals affects how they fare in current protection programs and argue that a process-oriented conception of biodiversity requires inclusion of such unisexual forms in protection efforts. Unisexual (all-female) populations of vertebrates were first described by Hubbs and Hubbs (1932) for the fish genus Poecilia. Subsequently, unisexual populations have been discovered in several families of freshwater fishes, salamanders, and lizards. Currently, unisexual lineages are known from at least 22 genera (see Vrijenhoek et al. [1989] for an almost complete listing), and a few of
The literature was reviewed to determine the number of fertile, interspecific fish hybrids produced naturally and artificially. A total of 130 interspecific, natural crosses resulted in fertile F1 hybrids, and 150 artificially produced interspecific crosses produced fertile F1 hybrids. The validity of species concepts with reproductive isolation in their definition is questioned. The presence of fertile hybrids in 45 intergeneric crosses makes their current classification questionable.
In the loach Misgurnus anguillicaudatus (Pisces: Cobitidae), triploid females derived from the hybridization of normal diploid females and natural tetraploid males generate both triploid and haploid eggs simultaneously. Diploid progenies were produced by fertilizing the haploid eggs of triploids with haploid spermatozoa of normal diploids. Ldh-1 and Mdh-2 allozyme loci were electrophoretically diagnostic, because diploid and tetraploid loach exhibited the diverged genotype aa and bbbb at each locus, respectively. Diploids from triploid (abb) × diploid (aa) crosses frequently exhibited a heterozygous ab genotype at each locus but sometimes showed genotypic frequencies that fit a random pairing model (aa:ab = 1:2) of two of three chromosomes in each set of homologues. These results show that the entire haploid genome of one parent is not completely excluded in the oogenesis of triploid loach, and the haploid eggs are not formed in a typical hybridogenetic manner. However, the two homologous chromosomes derived from the tetraploid parent are likely to pair with each other preferentially during the meiosis of triploid loach. J. Exp. Zool. 280:368–374, 1998. © 1998 Wiley-Liss, Inc.
Many studies in the genus Poecilia have focused on reproductive and genetic characteristics of Poecilia formosa, the Amazon molly, and its sympatric species P. latipinna and P. mexicana. The research literature of Poecilia dentition has been limited to general tooth morphology. Essentially absent are comparative analyses of dentition patterns and total numbers of teeth. The current study uses dentition analysis as a method to compare species in the genus Poecilia and to address some taxonomic issues related to these fish. The study focused on fish from the areas of southern Texas and northeastern Mexico. Through the use of scanning electron microscopy, the lower jaws of Poecilia spp were examined to determine total numbers of outer and inner teeth. In addition, the differences in distribution patterns of the inner teeth were recorded and compared. Statistical analyses were performed to determine which comparisons were significant. This study reveals several observations: 1) variations in the numbers of outer and inner teeth exist in some of these fish with respect to site of collection; 2) differences in total teeth numbers and dentition patterns were found both interspecifically and intraspecifically; and 3) in addition, dentition analysis provided evidence regarding the origin of P. formosa. This study supports the current notion that P. latipinna, the proported paternal component, and P. mexicana limantouri, the purported maternal component, are the progenitor species of P. formosa. Two unresolved taxonomic questions were addressed through dentition analysis. First, the present study supports the exclusion of the triploid associate of P. formosa as a separate species from P. formosa. Second, this study shows a significant difference in the number of inner teeth and in dentition patterns between P. mexicana limantouri and P. mexicana mexicana. Such differences, in addition to previously known distinguishing characteristics, should prompt careful consideration of whether or not these taxa deserve specific status or retention of their current subspecific status. J. Morphol. 239:271–282, 1999. © 1999 Wiley-Liss, Inc.
The distribution of species is limited by their ability to adapt to local environments. For adaptation by selection, genetic variability is crucial. As founder effects reduce genetic variability, extension of species' range borders is usually slow due to the reduced probability of successful colonization. However, the range limit might be extended by incorporating locally adapted genes. In western Palaearctic waterfrogs, interspecies hybrids show hemiclonal gametogenesis, are fertile and reproductively mimic one parental species. Genetic analysis, using allozyme loci, shows that they mediate gene exchange between the two parental species. Selection analysis provides evidence for local adaptation of single locus genotypes. This suggests that hybridogenesis presents a process which increases the number of neoform parental genotypes, exposing these to selection, and thereby revealing locally adapted genotypes which are essential for species range expansion.
The diploid hybridogenetic frog Rana esculenta, an interspecific hybrid of R. ridibunda and R. lessonae, usually transmits its ridibunda genome to progeny: the lessonae genome is lost before gametogenesis is completed. Samples of ovary reveal only ridibunda alleles when examined electrophoretically. Cytological and genetic evidence suggests that the lessonae genome is excluded premeiotically. Since recombination between the parental genomes occurs at a low frequency, such recombination should be detectable by examination of the alleles expressed in individual enlarged oocytes.
Examination of electrophoretic markers for four loci from 860 individual oocytes (100 each from eight females, ten each from six females) revealed no evidence of either heterozygosity or recombination, although the eight females for which 100 oocytes were examined were somatically heterozygous 29 times for these loci, so that there were 2,900 possibilities for detecting heterozygosity or recombination in the oocytes.
These data abundantly confirm the inactivity of the lessonae genome during vitellogenesis in Rana esculenta. They are consistent with a premeiotic exclusion of the lessonae genome, but do not require it. The lessonae genome may be present although inactive, which may permit occasional crossing over between the lessonae and ridibunda genomes. The exclusion of the lessonae genome possibly is a consequence of its inactivity. Exclusion of the non-ridibunda genome seems to be under control of loci located in the ridibunda genome. A regulatory endoduplication of the ridibunda genome may be necessary before an orderly meiosis can occur.
That a positive correlation exists between egg size and egg energetic content is an assumption of empirical and theoretical studies of life-history evolution. Although assumed, this relationship lacks substantiative support, particularly at the intraspecific level. To this end, we evaluated the validity of this assumption within and among clonal and bisexual females of the desert fish Poeciliopsis. As anticipated, we found generally high correlations between the two variables at all levels studied. Thus, egg volume remains a reliable and often precise indicator of maternal investment in this species.
This chapter focuses on sex differentiation as a process rather than examining sex phenotypes as they appear in adults. Fishes provide excellent material to approach the problems of sex differentiation and of the evolution of sex among animals. This chapter discusses the sexuality in fishes. First, it is unsound to consider that sexuality in all members of the class pisces is labile. There are many sex types, such as synchronous, protandrous, and protogynous hermaphrodites, as well as gonochorists. Second, sex is a phenotypic expression. In gonochorists, a male is a sperm producer and a female an egg producer. Although the combinations of sex genes are determined at the time of fertilization, sex is actually determined by sex gene-controlled sex inducers at a certain critical period of development—that is, sex genes act only indirectly upon sex differentiation.
Two all-female species of viviparous fish depend on males of bisexual species for sperm in order to reproduce. The unisexual P. monacha-latidens mates with males of P. latidens (the sexual host) while the unisexual P. monacha-lucida mates with males of P. lucida (its sexual host). Ecological divergence of each unisexual from its sexual host is promoted by any competitive interactions while ecological convergence of the unisexual toward its sexual host is promoted by the unisexual's sperm requirements. This study measured the habitat use in an artificial stream of five Poeciliopsis species (P. monacha-latidens, P. monacha-lucida, P. latidens, P. lucida and P. monacha) when alone and in several combinations. The habitat use of both unisexuals was examined for evidence of 1) competition with their sexual hosts, 2) use of monacha-like behavior, and 3) competition with each other. The two unisexuals interacted differently with their sexual hosts. P. monachalatidens competed with P. latidens. These two species differed more in their habitat use when they were sympatric than when they were allopatric. They frequented different habitats when they were sympatric, P. latidens preferring fast-water areas and P. monacha-latidens preferring slowwater areas. The distribution of P. monacha-latidens was more similar to P. monacha when the unisexual was with its sexual host than when it was alone, indicating that this unisexual may use monacha-like behavior to escape from competition with its sexual host. In contrast, P. monachalucida did not compete with P. lucida. These two species were more similar in their habitat use when they were sympatric than when they were allopatric. The habitat use of P. monacha-lucida when alone and when with P. lucida showed no consistent pattern when compared with the habitat use of P. monacha. Apparently, P. monacha-lucida did not use monacha-like behavior when with its sexual host. The two unisexuals did not compete. Thus, forces favoring ecological divergence are more important for P. monachalatidens and forces favoring ecological convergence are more important for P. monacha-lucida.
Studies of all-female Poeciliopsis populations show that differences in trophic morphology, food habits, use of space, and life history characters all might contribute to niche diversification among clones in a multiclonal assemblage. According to the frozen niche variation model, the differences among asexual clones are fixed from the range of genetic variance in niche related characters present in the sexual ancestors. Individual clones must compete with more plastic ancestors, but all-female forms potentially have a 'twofold' reproductive advantage; thus a diverse multiclonal assemblage should displace the sexual forms from contested portions of the niche. That unisexual Poeciliopsis do not completely replace the sexual forms is due in part to their sperm dependence on males of the related sexual species, ie the success of these nonrecombinant clonal forms derives from the recombinational genetic variation in their sexual ancestors.-from Author
Fishes of the genus Poeciliopsis comprise both sexual and unisexual forms that coexist in northwest Mexico. This study compares genetic variability and heterozygosity in P. occidentalis, a sexual species, and P. monacha-occidentalis, a diploid unisexual that lives and mates with P. occidentalis. The unisexuals are hybrids derived from the sexual species P. monacha and P. occidentalis. They reproduce through a mechanism termed hybridogenesis in which the P. monacha genome is transmitted clonally from mother to daughter while the paternal (occidentalis) genome is discarded each generation and replaced by mating with males of P. occidentalis. Electrophoretic variation was assessed at 25 protein loci for 202 P. occidentalis and 213 P. monacha-occidentalis from five sites: three in the Rio Mayo where P. monacha and P. occidentalis are sympatric and hybrid synthesis of unisexuals presumably still occurs, and two in northern rivers where P. monacha does not occur. P. occidentalis had low levels of polymorphism (P̄ averaged 10.7%) and heterozygosity (H̄ averaged 1.8%). Genetic similarities between sites ranged from .999 in the Rio Mayo to .800 between the northernmost river and a Rio Mayo site. P. monacha-occidentalis was more polymorphic than P. occidentalis as a result of combining variation from two species (P̄ averaged 19.4%); it was also highly heterozygous (H̄ averaged 42.5%). Unisexuals inherited occidentalis genes in the same frequencies as they occurred in local occidentalis populations. Apparently the monacha genomes of unisexuals are compatible with any occidentalis genes they encounter. The clonally inherited monacha genomes of unisexuals consisted of three different genotypes in the Rio Mayo and only one each in the other two rivers. Recurrent hybrid syntheses of unisexuals appears to be an important influence on the number of hemiclones at specific sites. The single monacha genotype in the northernmost P. monacha-occidentalis population contained unique alleles at two loci. This unisexual provides the first evidence that clonally reproducing vertebrates diverge genetically via mutation subsequent to their hybrid origins.
Despite the elegant body of theoretical literature on sexuality, recently reviewed by Williams (1975) and Maynard Smith (1978), very little empirical data exist to assess this “contest of ideas.” I will not attempt in this chapter to address the question, “Why is there sex?” Instead, I will examine a common assumption in most of these theoretical studies—that asexual populations lack genetic variation. It is commonly argued that asexual reproduction is an evolutionary dead end, that the absence of recombinational variability results in genetic inflexibility and sure extinction in a changing environment. The clonal genomes of asexual organisms are looked upon as rigid structures that can only change through mutation, and most mutations are deleterious (Muller, 1964). Despite the occasional success of some asexual populations, the long-term prospects appear grim (White, 1978). Yet, recent genetic studies have reported abundant clonal diversity in a number of asexually reproducing organisms [see reviews by Parker (1979a), Vepsalainen and Jarvinen (1979), and Vrijenhoek (1979a)]. What then is the source of this clonal diversity and how might it contribute to the ecological and evolutionary success of asexual populations?
Hybridization between the viviparous fishes Poeciliopsis monacha and P. lucida of northwestern Mexico has resulted in the formation of diploid and triploid all-female ‘species’, P. monacha-lucida and P. monacha-2 lucida. These females reproduce by mating back to P. lucida, and are essentially clonally reproducing sexual parasites superimposed on that species. In a series of behavioural experiments, one diploid clone proved to be significantly more aggressive than one triploid and four other diploid clones. No differences in aggression were exhibited among the other five clones. The aggressiveness of this one clone may explain why only two clones live in the small tributary where it is found but up to 10 diploid and triploid clones occur where it is absent.
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