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Variation in the sexual behaviour of Achnanthes longipes (Bacillariophyta). II. Inbred monoecious lineages
Abstract
This paper continues a series of studies of the allogamous raphid diatom Achnanthes longipes, which has a complex reproductive system combining unisexual, bisexual and monoecious behaviour. Following earlier work on the effects of inbreeding in the progeny of crosses between two unisexual clones, we studied the progeny of clones that are capable of a high degree of selfing in monoclonal culture (‘monoecious clones’). Three generations of selfed progeny were examined. In addition, we investigated the F1 generation obtained after crossing two different monoecious clones. Monoecious clones produced monoecious or, more rarely, bisexual progeny, but did not give rise to unisexual progeny. As in inbred lineages made by crossing closely related unisexual clones, inbreeding in monoecious lineages leads to a reduction in the number of gametes formed by the gametangia, from two to one. Inbred clones exhibit marked inbreeding depression and only three inbred generations were possible in monoecious lineages. In the third, final inbred generation, monoecious sexual reproduction was initiated in monoclonal cultures but gametes rarely fused and none of the auxospores and initial cells that were formed were viable ; this also occurred when the inbred clones were crossed with any of the other clones available. The significance of inbreeding and other aspects of the breeding system in A. longipes is discussed.
... Differentiation of clones into two mating types, or "male" and "female" wherever it is possible to determine, assumes sex to be determined diplogenotypically, but very little is known about the genetics of sex-determination in diatoms (mann et al. 2003;DaviDovich et al. 2006). In pennates, including members of Fragilariaceae, heterothally seems to be the common form of mating (roshchin 1994;mann et al. 1999;), but infrequent intraclonal reproduction of male clones was observed in T. tabulata and F. delicatissima (roshchin 1994). This coexistence of two types of mating behaviour within a single sympatric population was interpreted as an alternation of monoecy and dioecy ("monoecydioecy") in consecutive generations (roshchin 1994;roshchin & chEPurnov 1999). ...
... In either case, the role of heterothallic reproduction in T. fasciculata is principal and common, while intraclonal reproduction should be regarded as secondary and occasional. Other authors have postulated that intraclonal reproduction plays a secondary role not only because of lower frequency, but also because what is essentially inbreeding seems to have higher sensitivity to external factors and is restricted to a shorter period of the life cycle, as in T. tabulata (roshchin 1994) and Achnanthes longipes agarDh (chEPurnov & mann 1997(chEPurnov & mann , 1999. This notion is not supported in T. fasciculata populations because we observed our clones reproducing intraclonally when cell size was close to the upper limit of the sexual size range (Figs 15, 19). ...
... This notion is not supported in T. fasciculata populations because we observed our clones reproducing intraclonally when cell size was close to the upper limit of the sexual size range (Figs 15, 19). Initial cells produced as a result of such intraclonal reproduction were viable and gave normally growing cultures, in contrast to species showing inbred depression in intraclonal progeny (chEPurnov & mann 1999). Moreover, initial cells produced intraclonally were the largest of those observed in our study (Fig. 19), as was expected from the largest gametangia, and grew well in 4 years of culturing. ...
Sexual behaviour, pattern of auxosporulation, and breeding system of the araphid diatom Tabularia fasciculata were studied using 64 clones derived from two natural populations from the Canadian Atlantic seashore and the Black Sea. Both inter– and intraclonal reproductions were observed. Heterothallic reproduction was morphologically and behaviourally heterogamous. The heterothallic nature of sexual reproduction in this species allowed the determination of sex ratio in the population, which was 1:1 male:female. A novel type of active movement of gametes designated as male was described and pathways traced and recorded. A few male clones could reproduce intraclonally, but this was not abundant compared to interclonal reproduction. The advantages and inferred significance of each mode of reproduction are discussed. In the semi–natural population of T. fasciculata, two size classes were detected among sexualized individuals, and they gave rise to two size groups of initial cells. Sexual compatibility of all derived clones was independent of cell size class. In cultures, the apical length of initial cells was also strongly correlated to the size of parental gametangia. Cell sizes corresponding to cardinal points in the life cycle were established. Interbreeding of clones derived from the Canadian Atlantic and the Black Sea suggests a pan–Atlantic reproductive compatibility of populations of diatoms conforming to current morphological delineation of T. fasciculata.
... Kremp and Parrow, 2006). Cultures of 148 homothallic diatom species are generally expected to manifest sexuality because the 149 gradual decrease in cell size observed during normal asexual division is commonly 150 reversed through an obligatory sexual phase (Chepurnov et al., 2004). Sexual 151 reproduction will have consequences for the genetic diversity within cultures. ...
... However, recovery was 277 rapid, and sexual lines adapted at an overall greater rate and to a greater extent in certain 278 M a n u s c r i p t 14 14 order to expedite research for which biomass accumulation is rate-limiting. For the samelight levels may be reduced to slow growth and thereby allow a greater time between 295 transfers (Mann and Chepurnov, 2004). Finally, conditions in nature tend to change 296 continuously whereas conditions in culture are generally held fairly constant. ...
... Diatoms 297 are thought to be highly adapted to growth in turbulent, rapidly changing environments 298 (Margalef, 1978). The imposition of constant conditions on diatom cultures may disrupt 299 natural life cycle processes allowing, for example, the formation of unusually small cells 300 (Mann and Chepurnov, 2004; von Dassow et al., 2006). 301 302 ...
Laboratory cultures are important tools for investigating the biology of microalgae, allowing experimentation under controlled conditions. This control is critical for comparative studies, such as those often used to investigate intra-specific variation in properties of interest. By holding the environment constant, the experimentalist can gain insight into the genetic basis of phytoplankton phenotypes and by extension, into the adaptive history of those genotypes. In most cases the adaptations of interest are those that the algae have evolved in response to their natural environment. However, here it is argued that such experiments may instead reveal evolutionary adaptations to, and/or non-adaptive changes induced by, the culture conditions under which the alga is maintained. We present a review of the processes of evolution as they pertain to microalgal culture, and illustrate this discussion with examples of in-culture evolution from both within and outside the field of phycology. With these considerations in mind, recommendations are made for experimental practice focusing on comparative physiology, for which the effects of in-culture evolution are particularly confounding. Finally we argue that, although problematic in some contexts, the evolutionary propensities of phytoplankton cultures actually present an important opportunity for experimental evolutionary research with direct environmental significance.
... Additionally, within a single species (e.g. Achnanthes longipes Agardh and Sellaphora spp.), different clones may be either homothallic or heterothallic, or may show varying capacity for inbreeding (Roshchin 1990;Chepurnov and Mann 1999; reviewed in Roshchin and Chepurnov 1999;). Self-fertilization does occur within the homothallic lineages, but tends to lead to inbreeding depression (Chepurnov and Roshchin 1995;Chepurnov and Mann 1999). ...
... Achnanthes longipes Agardh and Sellaphora spp.), different clones may be either homothallic or heterothallic, or may show varying capacity for inbreeding (Roshchin 1990;Chepurnov and Mann 1999; reviewed in Roshchin and Chepurnov 1999;). Self-fertilization does occur within the homothallic lineages, but tends to lead to inbreeding depression (Chepurnov and Roshchin 1995;Chepurnov and Mann 1999). In heterothallic lineages, each clone produces only one mating type; outcrossing is obligatory because complementary mating types are required for fertilization. ...
Sexual reproduction is an essential phase in the life history of diatoms that serves to maintain high levels of genetic diversity and to restore large cells to a population. However, complete life cycles of most diatoms are unknown, and observations of sexual reproduction in nature are rare. Culture studies and field observations of Ditylum brightwellii (West ) Grunow in Van Heurck, a centric marine diatom, were used to clarify confusion generated from previous studies of the sexual cycle, to determine the mating system, and to examine natural populations for the incidence of sexual reproduction. Clones of D. brightwellii isolated from Wadsworth Cove, Castine, Maine, were used to observe the morphology of sexual stages. These clones were then studied to determine their mating system and their potential to self-fertilize or outcross. Results from this study show that sexual reproduction in Ditylum brightwellii is homothallic; two naked, spherical eggs and an estimated 64 uniflagellate sperm are released simultaneously from each gametangium into nutrient-replete medium. Oogenesis and spermatogenesis appear to be determined by size; larger clones produced predominately eggs whereas smaller clones produced predominately sperm. Sperm were attracted to the eggs, and size regeneration progressed from a putatively fertilized egg via a true auxospore. Ditylum brightwellii had high gametogenic potential, and self-fertilization was vigorous in some clonal cultures. However, outcrossing was also important in the mating system. Vegetative cell enlargement occurred in older, nutrient-depleted cultures, and produced interme&ate-sized cells. The size structure of natural populations of D. brightwellii in Wadsworth Cove was monitored weekly from 31 August to 7 December, 2004. The size frequency distributions of these populations were mainly unimodal throughout autumn, but appeared to have gradual shifts in size. The smallest cells were found in an early autumn population, whereas, the largest cells, including cells within the size range of initial cells measured in the laboratory (59 a 5 SD pm), appeared in mid-autumn. Spennatogonangia and large, polar cells were found concurrently in Wadsworth Cove in mid-autumn, 2004, suggesting that sexual reproduction was occurring in nature at this time.
... However, more reliable is a system suggesting that two genes are involved, because three genders (males, females and hermaphrodites) may be controlled by a single genetic locus [129,130]. In some pennate diatoms, such as for H. ostrearia, bisexual clones also occur, thus revealing the existence of both dioecy and monoecy [131]. ...
The marine pennate diatom Haslea ostrearia has long been known for its characteristic blue pigment marennine, which is responsible for the greening of invertebrate gills, a natural phenomenon of great importance for the oyster industry. For two centuries, this taxon was considered unique; however, the recent description of a new blue Haslea species revealed unsuspected biodiversity. Marennine-like pigments are natural blue dyes that display various biological activities-e.g., an-tibacterial, antioxidant and antiproliferative-with a great potential for applications in the food, feed, cosmetic and health industries. Regarding fundamental prospects, researchers use model organisms as standards to study cellular and physiological processes in other organisms, and there is a growing and crucial need for more, new and unconventional model organisms to better correspond to the diversity of the tree of life. The present work, thus, advocates for establishing H. ostrearia as a new model organism by presenting its pros and cons-i.e., the interesting aspects of this peculiar di-atom (representative of benthic-epiphytic phytoplankton, with original behavior and chemodiversity, controlled sexual reproduction, fundamental and applied-oriented importance, reference genome, and transcriptome will soon be available); it will also present the difficulties encountered before this becomes a reality as it is for other diatom models (the genetics of the species in its infancy, the transformation feasibility to be explored, the routine methods needed to cryopreserve strains of interest).
... Moreover, when selfing has been observed and the progeny followed for a few generations thereafter, the cultured strains experienced inbreeding depression, resulting in reduced rates of gamete fusion and inviable zygotes (e.g. Chepurnov & Mann, 1999). It follows that selfing is probably rare in natural populations, and the various motility mechanisms that have evolved in different life history stages (gametes vs vegetative cells) might represent alternative strategies for increasing the frequency or efficiency of sex and outcrossing. ...
Patterns of species richness are commonly linked to life history strategies. In diatoms, an exceptionally diverse lineage of photosynthetic heterokonts important for global photosynthesis and burial of atmospheric carbon, lineages with different locomotory and reproductive traits differ dramatically in species richness, but any potential association between life history strategy and diversification has not been tested in a phylogenetic framework.
We constructed a time‐calibrated, 11‐gene, 1151‐taxon phylogeny of diatoms – the most inclusive diatom species tree to date. We used this phylogeny, together with a comprehensive inventory of first–last occurrences of Cenozoic fossil diatoms, to estimate ranges of expected species richness, diversification and its variation through time and across lineages.
Diversification rates varied with life history traits. Although anisogamous lineages diversified faster than oogamous ones, this increase was restricted to a nested clade with active motility in the vegetative cells.
We propose that the evolution of motility in vegetative cells, following an earlier transition from oogamy to anisogamy, facilitated outcrossing and improved utilization of habitat complexity, ultimately leading to enhanced opportunity for adaptive divergence across a variety of novel habitats. Together, these contributed to a species radiation that gave rise to the majority of present‐day diatom diversity.
... The frequent abortion of gametes in Schizostauron is interesting and requires explanation, given the deleterious effect it might be expected to have on clone survival. Repeated inbred mating induced a rapid decrease of viability in Achnanthes longipes, accompanied by a reduction from two to one in the number of gametes produced by each gametangium and in the number of surviving auxospores in the gametangial pairs (Chepurnov & Mann 1999. Inbreeding also regularly resulted in the abortion of one of the two auxospores produced by each pair of gametangia in Nitzschia lanceolata W.Smith (Roshchin 1990). ...
The focus of this paper is the sexual reproduction and phylogeny of Schizostauron Grunow, which were studied using clonal cultures isolated from the Indian Ocean coast of Mozambique near Tofo and Bazaruto. Taxa of Schizostauron were characterized by light and electron microscopy and a phylogeny derived from three genes (rbcL, psbC and short subunit). Schizostauron was established in the second half of the 19th century, then forgotten, with its taxa included in Cocconeis or Achnanthes sensu lato. Schizostauron, together with Astartiella and perhaps also Kolbesia and Karayevia, seems to form a third lineage of monoraphid diatoms, which are related to biraphid diatoms belonging to the Stauroneidaceae and Parlibellus, but not to the two other lineages of monoraphids (Achnanthes and the Achnanthidiaceae-Cocconeidaceae group). In culture, size reduction was followed by release of gametes and auxosporulation in mixtures of clones. Despite some morphological differences, four clones from the Bazaruto population proved to be sexually compatible, one of the clones being sexually compatible with the three others. Before gametogenesis, cells gathered in groups of two to eight through active movement. Some groups of cells surrounded themselves by weakly visible mucilage. Reproduction was isogamous morphologically, and apparently also behaviourally. Growing auxospores were surrounded by the empty irregularly arranged frustules of parental cells. In numerous aspects the pattern of sexual reproduction of Schizostauron is similar to that of Achnanthes sensu stricto. The rbcL identity matrix (IDM) for sexually compatible clones ranged between 0.998 and 0.984. One clone derived from the Tofo population had an IDM below 0.900 and was sexually isolated from the remaining clones. A description of Schizostauron davidovichiorum sp. Nov. is provided.
... Vegetative cell enlargement through auxospore-like structures was observed in L. aporus, in agreement with the observations of (French and Hargraves 1986). A vegetative cell enlargement was first reported by von Stosch (1965) in Achnanthes longipes, Ditylum brightwellii and Biddulphia pulchella, followed by similar observations on a number of centric diatoms, for example, Skeletonema costatum (Gallagher 1983), Coscinodiscus wailesii (Nagai et al. 1995), as well as the pennate diatom Achnanthes longipes (Chepurnov and Mann 1999). The vegetative cell enlargement is believed to have a selective advantage because of the lower energy requirement as compared to sex, and because it overcomes the risk of finding a mate of the opposite mating type. ...
Centric diatoms of the genus Leptocylindrus are common in the marine plankton worldwide. Only two species, L. danicus Cleve and L. minimus Gran, so far clearly belong to this genus, whose diversity has not been fully investigated. We investigated frustule and spore morphology as well as three nuclear‐ and three plastid‐encoded markers of 85 Leptocylindrus strains from the Gulf of Naples, and one from the Atlantic US. The strains grouped into five molecularly distinct species with different levels of morphological differentiation. Two species matched the description of L. danicus and produced similar spores but differed in morphometric characters and sub‐central pore position, supporting the description of L. hargravesii Nanjappa and Zingone as a distinct species. Leptocylindrus danicus var. apora French III and Hargraves, lacking a sub‐central pore and not forming spores, was raised to the species level as L. aporus (French III and Hargraves) Nanjappa and Zingone. A fourth species with convex valves was described as L. convexus Nanjappa and Zingone. The fifth species matched the description of L. belgicus Meunier, considered as synonym of L. minimus. However, ultrastructural differences from all other Leptocylindrus supported the erection of the genus Tenuicylindrus Nanjappa and Zingone with T. belgicus (Meunier) Nanjappa and Zingone as type species. None of the sequences matched the L. minimus sequence in GenBank. The species analyzed showed different or partially overlapping seasonal distributions. Despite the addition of the new taxa, the ancient diatom lineage of the Leptocylindraceae shows a relative species poorness and considerable morphological stasis.
... The incomplete heterothallism of N. ampliatum is paralleled in other species. Nitzschia lanceolata, though predominantly heterothallic, can also reproduce in monoclonal cultures (Roshchin 1990Roshchin , 1994), and in A. longipes (see Introduction) some clones are unisexual and show only the slightest capacity for intraclonal reproduction, whereas other clones exhibit frequent intraclonal sex, although repeated inbreeding rapidly leads to a loss of vitality (Chepurnov & Mann 1999). Currently, it is difficult to detect evolutionary trends in pennate diatoms with respect to heterothally vs homothally. ...
D.G. MANN AND V.A. CHEPURNOV. 2005. Auxosporulation, mating system, and reproductive isolation in Neidium (Bacillar- iophyta). Phycologia 44: 335-350. Three allogamous Neidium demes, referrable to N. ampliatum sensu lato,coexist without intergradation in Blackford Pond, Edinburgh, and some other lakes. A slight change in valve shape has occurred in one deme during the last 20 years. Morphological deme traits persist in healthy clonal cultures, but with time or during senescence, aberrant morphologies and sizes of cell can be produced that have no parallel in nature. Simplification of valve outline occurs as cells get smaller, but the initial cells also have a simplified morphology. The rostrate apices of some N. ampliatum demes develop rapidly after auxosporulation, during the first divisions of the initial cells. The 'major' and 'minor' demes of N. ampliatum are hetero- thallic, although some inbreeding occurs. Very rarely, mixed pairs of 'major' 3 'minor' are formed, but hybrid auxospores are apparently never produced, so the demes are reproductively isolated. The 'maternal' gametangium has a nongenetic influence on initial cell size; however, the effect is slight and control of initial cell size is very well buffered to variation in gametangium size, so the concept of 'cardinal points' is valid for this species complex. The characteristics of sexual reproduction in clones and seminatural populations (including the effective, though incomplete, suppression of triplets and larger groups during copulation) prompt hypotheses about pairing mechanisms, in particular that a chemoattractant is in- volved. Preferential polyandry in Sellaphora and theoretical considerations indicate that the chemoattractant is produced by only one mating type. Size selection of mates in the 'minor' deme probably reflects progressively easier and more rapid sexualization as cells become smaller.
... those observed by Gross, 1937). Extruded protoplasts sometimes secrete new valves and enlarge vegetatively (von Stosch, 1965 ), providing an alternative size restitution mechanism to auxosporulation, though the increase in size seems always to be much less than during expansion of a sexually or asexually derived auxospore (e.g. Chepurnov & Mann, 1997). Thus, the multinucleate 'eggs' described by Steele (1967, Figs 7–10) may be reinterpreted as extruded protoplasts of stressed cells or stages in vegetative enlargement, because they (1) were found in senescent cultures, (2) formed 'auxospores' in the absence of sperm, and (3) developed into relatively small vegetative cells, approximately 40–70 mm in diameter versus 110 mm for the largest cells observed (Steele, 1967). ...
Ditylum brightwellii (T. West) Grunow in Van Heurck is now a model organism for population genetic studies of marine phytoplankton, but observations of sexual reproduction in this species are sparse and there are controversial aspects to the identification of its gametes. Culture studies and field observations of D. brightwellii from Wadsworth Cove (Castine), Maine (USA) showed that it was homothallic, producing and releasing two naked, spherical eggs from each oogonium and 64 uniflagellate sperm per spermatogonangium. Eggs and sperm were produced by larger and smaller cells, respectively, and were formed in nutrient-replete medium, without special procedures for induction. Size regeneration was achieved from a putatively fertilized egg via a true auxospore. Vegetative cell enlargement occurred in senescent cultures and resulted in intermediate-sized cells. The size structure of natural populations of D. brightwellii was monitored weekly in autumn 2004 and was unimodal throughout, with slight shifts in size accompanying low levels of gametogenesis and auxosporulation. Gametogenesis and size reduction ceased with the onset of winter. Previous reports concerning gametogenesis and auxosporulation in Ditylum are reinterpreted on the basis of our studies.
... Meister) Toyoda and Achnanthes subssesilis Kütz. (see Karsten 1899;Idei & Chihara 1991;Mizuno 1994;Chepurnov & Roschin 1995;Chepurnov & Mann 1999;Sabbe et al. 2004). These reports primarily describe the sexual reproductive behavior of the marine and brackish water species, but omit observations on the fine structure of perizonium. ...
The present study clarifies the fine structure of the vegetative frustules, initial valves and perizonium of Achnanthes crenulata Grunow. The valves of the vegetative cell are distinctly linear-lanceolate with an undulate margin. The valve face is quite flat and in girdle view is smoothly curved as in species of Gephyria (Bacillariophyceae). However, the valve face of the initial cells is slightly rounded and does not have an undulate margin. Furthermore, the rapheless sternum is centrally positioned along the apical axis of the araphid initial valve. As this taxon develops from auxospore to initial valve, it forms only longitudinal perizonial bands; no transverse bands arise. The perizonium consists of three silicified bands: one large, central longitudinal plate and two bands that underlie this plate; these two bands are either open or closed. This taxon has several conspicuous structures compared to other marine species of Achnanthes, but the structure of the perizonium supports the position of A. crenulata within Achnanthes sensu stricto.
... Our changed perceptions have come from experimental studies of interclonal crosses, which suggest that although inbreeding does occur and is vigorous in some species (Chepurnov and Mann, unpublished observations), pennate diatoms are basically outbreeders , inbreeding often being prevented by dioecy (heterothallism) (e.g. Roshchin 1994, Davidovich and Bates 1998, Roshchin and Chepurnov 1999, Mann et al. 1999). Even where inbreeding is permitted by the mating system, outbreeding may be the rule not the exception, so that enforced inbreeding or selfing causes inbreeding depression (Chepurnov and Mann 1997, 1999, 2000). ...
Cell division, the mating system, and auxosporulation were studied in the marine epipelic diatom Seminavis cf. robusta Danielidis & D. G. Mann. The interphase protoplast contains two girdle-appressed chloroplasts, each with an elongate bar-like pyrenoid, and also a central nucleus, located in a bridge between two vacuoles. Before cell division, the chloroplasts divide transversely and translocate onto the valves. The nucleus relocates to the ventral side for mitosis. After cytokinesis and valve formation, the chloroplasts move back to the girdle, showing a constant clockwise movement relative to the epitheca of the daughter cell. Seminavis cf. robusta is dioecious, and sexual reproduction is possible once cells are less than 50 μm. In crosses of compatible clones, gametangia pair laterally, without the formation of a copulation envelope, and produce two gametes apiece. The intensity of sexualization increases as cells reduce further in size below the 50-μm threshold. At plasmogamy, the gametangia dehisce fully and the gametes, which were morphologically and behaviorally isogamous, fuse in the space between the gametangial thecae. The auxospore forms a transverse and longitudinal perizonium. After expansion is complete, there is an unequal contraction of the protoplast within the perizonium, creating the asymmetrical shape of the vegetative cell. Apart from this last feature, almost all characteristics exhibited by the live cell and auxospores of Seminavis agree with what is found in Navicula sensu stricto, supporting the classification of both in the Naviculaceae. Haploid parthenogenesis and polyploid auxospores were found, lending support to the view that change in ploidy may be a significant mechanism in diatom evolution.
... Alternatively, the inability to manipulate these isolates through their complete life cycle may reflect the fact that both isolates have been maintained in culture for well over 10 years. Inbreeding, which can also occur after extended time in culture, can result in reduced viability and/or fertility as has been reported in the centric diatoms Stephanopyxis turris (Arnott in Greville) Ralfs (von Stosch 1965), Chaetoceros didymum Ehrenberg (von Stosch et al. 1973) and Thalassiosira punctigera (Castracane) Hasle (Chepurnov et al. submitted), and the pennate Achnanthes longipes Agardh (Chepurnov and Mann 1999). It is not clear why potential inbreeding artifacts would have differentially impacted formation of oogonia and auxospores, rather than spermatogenesis. ...
Patterns of changes in cell size, growth rate, and the inducibility of spermatogenesis were followed in eight sub-clones of two isolates of the centric diatom Thalassiosira weissflogii (Grunow) Fryxell & Hasle grown at saturating light. One isolate originated from Long Island Sound, New York, USA and the other originated from Jakarta Harbor, Indonesia. As expected from previous studies, oscillations between intervals of cell size reduction and cell size enlargement were observed for each sub-clone. For both isolates, sperm were easily detected, but cells resembling eggs and auxospores were rarely observed and fertilization was not confirmed, suggesting that the observed cell size increases may have resulted from a combination of asexual cell enlargement and rare auxosporulation. The two isolates differed in their minimum and maximum sizes, and the threshold size for the induction of sperm formation. However, the two sets of isolated sub-clones displayed comparable relationships between growth rate, sperm inducibility, and cell size relative to the minimum, maximum, and threshold sizes. Growth rate increased as cell size decreased during vegetative divisions until the threshold for sperm inducibility was crossed. Below the size threshold for sperm inducibility, growth rate declined as cell size continued to decrease. Smaller cells were susceptible to failure of normal cytokinesis and valve deposition, resulting in the formation of abnormally long and often multinucleate cells. Culture conditions may select against restoration of cell size via auxosporulation due to the relationship between growth rate and cell size.
The monograph concerns items related to reproductive biology of diatoms including such sections as sex and principles of sex determination in diatoms, their life cycles, patterns of sexual reproduction, sex distribution and mating systems, factors inducing sexual reproduction, inheritance coupled with sex, reproductive boundaries and distribution of diatoms, phylogeny of diatoms in the light of reproductive characteristics, etc. The book is dedicated for specialists in algology, hydrobiolody, reproductive biology; for students and graduate students in the relevant areas of study. (in Russian)
This chapter contains sections titled:IntroductionCentric DiatomsPennate Diatom Life Cycles and ReproductionAuxospore Development and StructureInduction of Sexual ReproductionAcknowledgments
The inter- and intraclonal variability of the morphology and ultrastructure of the frustule of Nitzschia rectilonga Takano, 1983 was investigated. This study showed a wider intraclonal variability than in the diagnosis of the species. An emended diagnosis of the species is provided; a twofold reduction in the density of striae that are visible under a light microscope was found compared to electron-microscope images.
The LIFEHAB workshop was conceived as a forum for discussion among specialists on life histories of microalgal species. The main goals were to: 1) review current knowledge on the life-cycles of phytoplanktonic organisms, focusing on HAB species; 2) identify the role of heteromorphic life cycles in population dynamics; 3) define future HAB research directions to fill existing gaps in knowledge; 4) debate the most appropriate approaches and methods; and 5) promote the development of cooperative scientific initiatives.
The workshop included 25 presentations covering the most relevant aspects of life cycle studies in phytoplankton, ranging from an updated review of the information available for the major groups, to the role played by different life history stages in population dynamics, to the new tools available for addressing research questions. Discussion sessions focussed on the main gaps of knowledge and research needs. Complex, heteromorphic life cycles are a general feature of the large majority of
phytoplanktonic algae. Diatoms, dinoflagellates, haptophytes and raphidophytes are characterized by different life cycles, including stages with different ploidy levels, linked together by syngamy and meiotic events. There is increasing evidence that complex mating systems are a common feature among phytoplankton organisms. Understanding these is crucial for the taxonomic circumscription of species and for gaining insights into speciation mechanisms in these organisms.
Most taxa are characterized by an alternation between dormant/quiescent phases and growth phases. It is recognized that life history strategies (sensu latu, including formation of resistant/defense stages, colonies, encystment-excystment rates, vertical migrations, modulation of growth capabilities, etc), play a key role in the occurrence and dynamics of Harmful Algae Blooms (HABs). The alternation between different life history stages and their relationships with the physical, chemical and biological environment are very important for the success of harmful species.
In the course of the Workshop, the main gaps in knowledge and research priorities in this field were identified and discussed.
Diatoms are the most species-rich group of algae. They are ecologically widespread and have global significance in the carbon and silicon cycles, and are used increasingly in ecological monitoring, paleoecological reconstruction, and stratigraphic correlation. Despite this, the species taxonomy of diatoms is messy and lacks a satisfactory practical or conceptual basis, hindering further advances in all aspects of diatom biology. Several model systems have provided valuable insights into the nature of diatom species. A consilience of evidence (the 'Waltonian species concept') from morphology, genetic data, mating systems, physiology, ecology, and crossing behavior suggests that species boundaries have traditionally been drawn too broadly; many species probably contain several reproductively isolated entities that are worth taxonomic recognition at species level. Phenotypic plasticity, although present, is not a serious problem for diatom taxonomy. However, although good data are now available for demes living in sympatry, we have barely begun to extend studies to take into account variation between allopatric demes, which is necessary if a global taxonomy is to be bulk. Endemism has been seriously underestimated among diatoms, but biogeographical and stratigraphic patterns are difficult to discern, because of a lack of trustworthy data and because the taxonomic concepts of many authors are undocumented. Morphological diversity may often be a largely accidental consequence of physiological differentiation, as a result of the peculiarities of diatom cell division and the life cycle.
While sex requires two parents, there is no obvious need for them to be differentiated into distinct mating types or sexes. Yet this is the predominate state of nature. Here, we argue that mating types could play a decisive role because they prevent the apparent inevitability of self-stimulation during sexual signalling. We rigorously assess this hypothesis by developing a model for signaller-detector dynamics based on chemical diffusion, chemotaxis and cell movement. Our model examines the conditions under which chemotaxis improves partner finding. Varying parameter values within ranges typical of protists and their environments, we show that simultaneous secretion and detection of a single chemoattractant can cause a multifold movement impediment and severely hinder mate finding. Mutually exclusive roles result in faster pair formation, even when cells conferring the same roles cannot pair up. This arrangement also allows the separate mating types to optimize their signalling or detecting roles, which is effectively impossible for cells that are both secretors and detectors. Our findings suggest that asymmetric roles in sexual chemotaxis (and possibly other forms of sexual signalling) are crucial, even without morphological differences, and may underlie the evolution of gametic differentiation among both mating types and sexes.
© 2015 The Authors.
Diatoms are the most species-rich group of algae. They are ecologically widespread and have global significance in the carbon and silicon cycles, and are used increasingly in ecological monitoring, paleoecological reconstruction, and stratigraphic correlation. Despite this, the species taxonomy of diatoms is messy and lacks a satisfactory practical or conceptual basis, hindering further advances in all aspects of diatom biology. Several model systems have provided valuable insights into the nature of diatom species. A consilience of evidence (the 'Waltonian species concept') from morphology, genetic data, mating systems,
physiology, ecology, and crossing behavior suggests that species boundaries have traditionally been drawn too broadly; many species probably contain several reproductively isolated entities that are worth taxonomic recognition at species level. Phenotypic plasticity, although present, is not a serious problem for diatom taxonomy. However, although good data are now available for demes living in sympatry, we have barely begun to extend studies to take into account variation between allopatric demes, which is necessary if a global taxonomy is to be built. Endemism has been seriously underestimated among diatoms, but biogeographical and stratigraphic patterns are difficult to discern, because of a lack of trustwOlthy data and because the
taxonomic concepts of many authors are undocumented. Morphological diversity may often be a largely accidental consequence of physiological differentiation, as a result of the peculiarities of diatom cell division and the life cycle
Diatoms are unicellular heterokonts characterised by a bipartite cell wall made of amorphous silica. Their ecological importance is proved by the fact that they yearly produce one fifth of the oxygen on Earth, while their economic interest resides in that they are used in filters for water and beer and for biofuel production. This class of algae has been studied for long time but our knowledge about their reproductive biology is still a new field and needs more intense investigation, especially from a molecular point of view. Here I review what is known about sexual reproduction in pennate and centric diatoms from both a morphologic/behavioural and an ecologic standpoint.
Problems on reproductive biology, the trend in biology that concerns such fundamental subjects as sexual reproduction, life cycles, conjugation systems, regularities in sex determination and inheritance, are elucidated with reference to diatoms. Tendencies in development of reproductive biology are shown and attention is focused on unsolved problems. The role of procedures and methods of reproductive biology in determination of interspecific borders and expediency of employing the conjugation methods in combination with molecular-biology procedures are emphasized. Publications on reproductive biology for the last 150 years are analyzed.
We investigated male clones of the heterothallic (dioecious) pennate diatomaceous alga Nitzschia longissima (Bréb.) Ralfs, capable of infrequent intraclonal reproduction. Assuming the diplgenotypic nature of sex inheritance, we experimentally checked the sex of the intraclonal progeny. In the offspring we discovered both male and female clones, which proved to be viable and fertile. Male descendants, in turn, also revealed capability for intraclonal reproduction. Two chosen clones of the second inbred generation proved to be male. During intraclonal reproduction, the sexual process is isogamous, while interclonal reproduction is anisogamous (heterogamous). It is established that the shape and behavior of the gametes are inherited characteristics coupled with sex. The possibility of alternating from "monoecious" to "dioecious" generations in the life cycles of diatoms is discussed.
The first laboratory cultures of diatoms were made in the mid nineteenth century and involved incubating samples of mixed natural populations. Clonal cultures were first reported in 1892. Since then, rough, clonal and axenic cultures have provided many insights into taxonomically relevant aspects of diatom biology, although diatom taxonomy has remained obstinately linked to the study of cleaned silica valves. Relevant discoveries using cultures include diplonty, control of sexuality by cell size, specific and generalized patterns of shape change during size reduction, mechanisms of sexual reproduction (e. g. oogamy v. isogamy), heterothally (dioecy), reproductive isolation between morphologically similar demes (semicryptic species), aspects of phenotypic plasticity, patterns of cell wall and protoplast ontogeny, evolutionary relationships (from molecular sequence data), and population genetics. There is considerable scope for further contributions by culture-based studies in each of these subject areas but we suggest that increased attention should be paid to examining mechanisms of dispersal and gene flow, intrinsic reproductive isolation between allopatric demes, discovery of cryptic species, mate choice, evolution of the karyotype, 'evo-devo' (using molecular phylogenies to reveal how changes in wall morphology and protoplast structure have been brought about through modifications of developmental pathways), and girdle development and function. In addition, there is a great need for simple observations ('natural history') of e. g. growth form, motility, secretions, protoplast structure and dynamics, and mechanisms of sexual reproduction. Without such basic information about diatom structure and function, molecular phylogenies will be largely bald and uninteresting. Major impediments to the development and maintenance of diatom culture collections are (1) the finite life of clones (which has consequences for validation of research) because of size reduction and obligate sexual reproduction, (2) absence of dormant stages and lack of cryopreservation methods, and (3) recalcitrance in culture of many medium- and large-celled benthic species.
Achnanthes was described by Bory to include three species, A. adnata, A. dubia and A. baccillarioides, formerly included under Echinella stipitata Lyngbye. Boyer typified the genus Achnanthes using A. adnata and this typification has been accepted by subsequent authors. However no extant material of A. adnata is known. Since this species was based on E. stipitata, Lyngbye material was sought and examined. This material contains specimens of Achnanthes sensu stricto and can be used as the generitype of that genus. Light and electron microscopical observations of this material are presented.C. A. Agardh created the name Achnanthes brevipes as an avowed substitution for A. adnata and this is the name that has generally been used. There is little doubt that E. stipitata, A. adnata and A. brevipes refer to the same species. Since A. brevipes has been used most extensively in the literature there is a case for conserving this name. Based on our observations we propose that Lyngbye's material be accepted as the type for Achnanthes but that the name A. brevipes be considered for conservation against E. stipitata and A. adnata.
The availability of extensive experimental data and remarkable intra- and interspecific variation in breeding behaviour make Achnanthes Bory sensu stricto an especially good model for studying the reproductive and population biology of pennate diatoms. In most Achnanthes species studied, auxospore formation is accompanied by biparental sexual reproduction, but we found uniparental auxosporulation in Achnanthes cf. subsessilis. Auxosporulation appears to be apomictic and follows contraction of the contents of unpaired cells and then a mitotic division, which is normally acytokinetic: one nucleus aborts before the cell develops into an auxospore. Rarely, both daughter nuclei survive and cytokinesis produces two auxospores (two auxospores per mother cell is highly unusual in pennate diatoms); one may abort. Expansion of auxospores is not accompanied by deposition of a transverse perizonium, but a longitudinal perizonium is produced and consists of a wide central strip (structurally similar to the araphid valve) and at least one narrow lateral strip. This newly discovered asexual lineage in Achnanthes is discussed in relation to other reproductive systems found in the genus, and also in relation to the ‘sex clock’ hypothesis concerning the adaptive significance of the diatom life cycle. Brief information on chloroplast division and nuclear dynamics over the cell cycle is also presented.
This study clarifies the fine structure of the vegetative and initial valves of Achnanthes yaquinensis and briefly compares them to other Achnanthes species. It also elucidates the structure of the perizonium, based on auxospore development in short-term cultures. The araphid valve has marginal ridges and terminal spines that allow connecting valves to form a chain. The terminal spines develop from the rapheless sternum. The complete cingulum consists of 3–5 split bands with two rows of areolae. These features can be used to discriminate species within the genus. Sexual reproduction is isogamous with two mother cells producing two auxospores, which are enclosed in mucilage. The perizonium develops on one side of an auxospore only, comprising one large central longitudinal band and four closed bands. There are no transverse perizonial bands. The raphid valve of the initial cell forms first, underneath the longitudinal perizonium, followed by the araphid valve, which is not covered by any perizonial bands. The araphid valve of the initial cells lacks a marginal spine, and the rapheless sternum lies more centrally than in the vegetative cell. The relationship of the genus Achnanthes to other monoraphid diatoms is discussed briefly.
The genus Ceratoneis Ehrenberg, as listed in the online Catalogue of Diatom Names
(2007), has 103 names if redundancy of duplicated entries is ignored. It seems quite possible
that most, if not all, of these taxa (and associated names) will end up in different genera, if
they have not already been transferred. Many of the freshwater species of Ceratoneis are
better placed in Hannaea R. Patr. (although some understand Hannaea as synonymous with
Fragilaria Lyngb., e.g. Krammer & Lange-Bertalot 1991, 2000), the marine species better
placed in Cylindrothecea Rabenh. (Medlin & Mann 2007). In short, Ceratoneis has been used
as a ‘dumping ground’ for many unrelated diatom species that happen to have curved valves
and, usually, a central area on the concave side. To make matters worse, typification of
Ceratoneis is controversial and still being debated (Jahn & Kusber 2005, Medlin & Mann
2007). This paper can (mercifully) avoid these contentious issues as we discuss only
specimens named as the species Ceratoneis iyengarii Gonzalves & Gandhi, a taxon described
some 50 plus years ago from a brackish water locality in Mahim Creek, Mumbai (then
Bombay), India (Gonzalves & Gandhi 1952). The original Latin description provided by
Gonzalves & Gandhi is given in Figure 1 ; the English translation provided is as follows:
“Frustules solitary, free-floating. Valves vary strongly arcuate with a prominent gibbosity in
the middle of the concave side, tapering very slightly from the middle towards the poles
which are broadly rounded. Raphe absent on both valves. Pseudoraphe uniformly broad and
distinct, somewhat excentrical. Unilateral central area present, but marked with indistinct
scattered puncta. Striae radial and distinctly punctuate” (Gonzalves & Gandhi 1952, p. 123).
Given the characters usually associated with species of Ceratoneis (curved valves and,
usually, a central area on the concave side) it is no surprise these specimens ended up there.
Only one figure was provided (Gonzalves & Gandhi 1952, p. 124, fig. 1 l), reproduced here as
Figure 2, rotated through 180’. Gonzalves & Gandhi (1952) cite no particular specimens in the
protologue, none that might be interpreted as types (Gonzalves & Gandhi 1952, p. 123
describe it as ‘rare’). 1 Ftustula solitaria Iiberc Ructuantia Valve fortissirna arcuatae,
prominenta tumcscentes ad lateres concavi medium, tenuissinie decr escentcs
a medio in utrumque apicem qui obtuse rotundus est Raphe
in utraque valva abest. Psefidoraphc miformiter lata et distincta,
aliqnantum ex medio exorbitam Area centralis unilateralis adest,
sed indistinctisdinis punctis signatz. Stria la&&! et distincta punc.
tat= Zongi 63-4711.; latit 12-6-13p; S h k 8 in l o p
Gonzalves & Gandhi
6347 pm 69 pm
12.6-13 pm
8 in 10 pm
Specimen on CESH-5-92
Poles: 10.5 p; Centre: 12.3 pm
10 in 10 pm
Fig. 1. Reproduction ofthe Latin description of Cerufoneis iyengarii (Gonzalves & Gandhi 1952, p. 123).
In 2006 Gandhi’s entire diatom collection was donated to the Energy and Wetlands
Research Group at the Centre for Ecological Sciences, Indian Institute of Science (IISc),
Bangalore, India and a programme of collection development is currently being implemented,
which will include typifying Gandhi’s taxa as well as digitizing the specimens and many
drawings. Among the slides, a specimen of Ceratoneis iyengarii was found on Slide No.
CESH-5-92 (Dharavi rd [road], [Mahim Creek], brackish water, 23-12-45) that resembled the
illustration in Gonzalves & Gandhi (1952, fig. 1 1).
Gonzalves & Gandhi (1952, p. 123) provided some basic dimensions, which were
compared to the specimen found on No. CESH-5-92 suggesting that they are of similar
dimensions:
This specimen is best considered a lectotype. It is clear that the specimen is neither a
species of Hannaea nor Cylindrothecea; while both valves of the frustule are curved, they
differ in structure from each other: the image is actually of a developing frustule of a species
of Achnanthes Bory. The frustule illustrated in Figures 3 4 , the lectotype, is of an initial
araphid valve (Fig. 3a) coupled with the perizonium of the raphid valve (Fig. 3b) -the raphid
initial cell is not yet formed. Further specimens, from slide no CESH-5-94 ((Dharavi rd [road],
[Mahim Creek], brackish water, 23- 12-45), also show what appears to be a possibly deformed
araphid valve (Fig. 7; maybe a vegetative cell?) and an initial araphid valve also from a
species of Achnanthes (Fig. 8).
Comparison of our Figures 3-6 with Sabbe et al. (2004, fig. 42), who illustrate the
central and lateral bands of the longitudinal perizonium of Achnanthes cf. subsessilis, show a
certain identity in structure and position (see also Roshchin & Chepurnov 1993, for
Achnanthes brevipes var. intermedia (Kiitz.) Cleve; Mizuno, 1994 for Achnanthes javanica f.
subconstricta (Meister) Hust.; Toyoda et al. 2005, for Achnanthes yaquinensis McIntire &
Reimer; and Toyoda et al. 2006, figs 2628 for illustrations of the perizonium and initial
valves of Achnanthes crenulata Grunow). As for the deformed araphid valve in our Figure 7, a
similar valve for Achnanthes longipes C.A. Agardh was illustrated in Chepurnov & Mann
(1999, p. 3), who note “Even during expansion, it was not uncommon for one auxospore to
abort in each pair. The contents of such auxospores looked abnormal (Fig. 1) and they never
developed into initial cells” (for hrther details see Roshchin & Chepumov 1992 and
Chepumov & Mann 1997). Thus, we conclude that Ceratoneis iyengarii is a name that refers to the early part of the
life-cycle of an as yet unidentified species of Achnanthes and its record should be adjusted
accordingly for the diatom flora of India.
The diatom genus Eunotia is unusual among raphid diatoms in having a raphe system consisting of two short slits that are not integrated into the primary pattern center. This and other characteristics, particularly the presence of rimoportulae, are consistent with the hypothesis that Eunotia is a basal lineage within the raphid group. We studied auxosporulation in E. bilunaris (Ehrenberg) Mills and E. tropica Hustedt for comparison with other raphid pennate diatoms and with araphid pennates; E. bilunaris was studied in parental and F1 generations. Like araphid pennates, E. bilunaris and E. tropica are heterothallic. Clones of the same mating type did not interact sexually, and intraclonal sexual reproduction was absent or very rare. Clones retained the same sex throughout the life cycle, as shown by experiments using abrupt size reduction to produce clones of similar age but different size and using subclones derived from a single initial cell within six mitotic generations. Unlike in araphid pennate diatoms, in the Eunotia species the gametes are not visibly or behaviorally differentiated. Gametogenesis is merogenous, because the gametangium formed a supernumerary cell as well as a single gametic cell, both undergoing meiosis II to form a surviving functional nucleus and a nucleus that quickly degenerated. Plasmogamy is via papillae that grew out toward each other from the ends of the gametangia to create a copulation canal. After plasmogamy, the gametes moves bodily into the copulation canal, producing an elongate zygote, which expands to form a curved sausage-like auxospore.
Examination of Grammatophora marina from rough and clonal cultures showed that cell size changes were more flexible than is generally reported for diatoms. Allogamous sexual auxosporulation took place through copulation between small male cells and larger female cells, but only in mixed rough culture and never in clonal cultures. Auxospores were also formed without copulation in clonal cultures ('uniparental auxosporulation') and these, like sexual auxospores, developed through formation of a perizonium, which consisted of a series of transverse bands. All of these bands, including the primary band, were open. Circular scales were present in the auxospore wall before initiation of perizonium formation and irregular, elongate structures lined the suture of the transverse perizonium. Perizonium and scales resembled those of another araphid pennate diatom, Gephyria media. Initial cells were formed within the perizonium and consisted of an initial epivalve with a simplified structure, an initial hypovalve (formed beneath the perizonium suture) and a third, normally structured valve formed beneath the epivalve; the epivalve was then sloughed off. Initial cells of similar configuration but often aberrant morphology could also be formed through expansion from vegetative cells, without involvement of a perizonium. Vegetative cells were also capable of limited enlargement through simple expansion without formation of an initial cell, and abrupt size reduction. Cell size ranges in populations from different regions suggest that G. marina may contain pseudocryptic species.
Pseudo-nitzschia delicatissima (Cleve) Heiden is a very common pennate planktonic diatom found in temperate marine waters, where it is often responsible for blooms. Recently, three distinct internal transcribed spacer types have been recorded during a P. delicatissima bloom in the Gulf of Naples (Mediterranean Sea, Italy), which suggests the existence of cryptic diversity. We carried out mating experiments with clonal strains belonging to the most abundant internal transcribed spacer type. Pseudo-nitzschia delicatissima is heterothallic and produces two functional anisogametes per gametangium. The elongated auxospore possesses a transverse and a longitudinal perizonium. The sexual phase was observed to occur over a wide size spectrum, spanning 19–80 μm and corresponding to almost the whole range of cell length observed for P. delicatissima. We also investigated cell morphology, valve ultrastructure and morphometry of parental, F1-generation strains, and the progeny of crosses between parental and F1 strains. Although ultrastructural features match those described for P. delicatissima, variability in cell shape was recorded in the largest cells of the F1 generation as well as in valves with an abnormal arrangement of poroids. As many other diatoms, P. delicatissima undergoes size reduction over its life cycle, and cells of different size showed differences in growth rates and the amount of size reduction per cell cycle. Cells between 60 and 30 μm in length showed the fastest growth and the slowest rates of size reduction per generation. In culture, P. delicatissima cells can decrease to 8 μm in length; however, such small cells (≤30 μm) are not recorded in the sea, and this raises interesting questions about the factors that control their survival in the natural environment.
Diatoms are becoming increasingly attractive for scientists and important for various industrial applications. Diatoms are
diploid sexual organisms and it is generally accepted that controlled genetic manipulations via sexual reproduction are instrumental
for basic diatom research. For more practical aims, e.g., selection and “improvement” of economically relevant diatom strains,
the classical breeding approach still remains entirely unexplored. Here, we discuss the potential of this approach for both
scientific and practical goals. Our analysis is largely comparative and builds on the knowledge of plant reproductive systems,
with emphasis on the experience of manipulating them for applied purposes (plant breeding). We believe this comparison is
relevant because of the striking similarity between flowering plants and diatoms in some important biological characteristics
including sexuality. The main topics chosen for the evaluation and the comparative analysis are mating systems, breeding procedures,
methods of strain selection, potential genetic resources, and consequences of inbreeding.
Keywords: Auxospore-Auxosporulation-Classical breeding-Crossing-Diatoms-Genetic engineering-Genetic resources-Hybridization-Inbreeding-Life cycle-Mating systems-Microalgae-Sexual reproduction-Strain selection
Domoic acid (DA), a potent neurotoxin, is synthesized by certain members of the ubiquitous
marine diatom genus Pseudo-nitzschia. We recently detected elevated concentrations of DA in
phytoplankton field samples from the northern Gulf of Mexico. In searching for a possible source of
the toxin, we used a receptor-binding assay to detect DA activity in cultures of P. sp. cf. pseudodelicatissima
(Hasle) isolated from this region and confirmed its presence in 2 of 7 clones using liquid
chromatography coupled with tandem mass-spectrometric detection (LC-MS/MS). Unlike other toxic
Pseudo-nitzschia species examined previously (e.g., P. multiseries, P. australis), cellular levels and net
production of DA in these clones were highest in the early exponential phase, while the population
growth rate was high and cell concentration was low. There was a negative correlation between cellular
DA and cell concentration. The maximum cellular DA activity in cultures was 36 fg DA equiv.
cell–1. No net toxin production was evident in the stationary phase, yet extracellular DA levels
increased markedly during this period to as much as 88% of the total DA in the cultures. Interestingly,
these 2 toxic clones were able to enlarge their cell size after the apical axes declined to 15 to
25 μm, and these larger cells had considerably higher levels of DA than the original small cells. This
study unequivocally establishes P. sp. cf. pseudodelicatissima as a source of DA in the northern Gulf
of Mexico. Moreover, our work suggests that rapidly growing, rather than nutrient-limited, populations
of this diatom should yield maximum net DA production rates and DA cell quotas. Thus, the
presence of P. sp. cf. pseudodelicatissima cells, even at the low levels of early, rapidly growing bloom
stages, can potentially lead to toxic events.
Most pennate diatoms are allogamous, and various types of mating systems have been described. In Pseudo-nitzschia, reproductive stages have been identified in some species, and it is generally accepted that the genus is mainly heterothallic. Here we report homothallic auxosporulation of Pseudo-nitzschia brasiliana Lundholm, Hasle et G. A. Fryxell. To our knowledge, this is the first verified description of homothallic sexual reproduction in the genus. Auxospore formation was observed in all 16 subclones derived from three initial clonal cultures of P. brasiliana. Pairing was followed by production of two gametes per gametangium, which fused to give two zygotes. Each zygote (early auxospore) was initially spherical and adhered to one girdle band of the parental frustule. The two auxospores tended to expand parallel to each other and perpendicular to the parental frustule. Elongation was synchronous, slightly asynchronous, or totally asynchronous. The entire process of sexual reproduction, from gamete formation to the appearance of the initial vegetative cells, took 2-4 d. The occurrence of sex in a homothallic species seems an advantageous life strategy for this species in that any encounter between cells of the right size class is potentially sexual. © 2009 Phycological Society of America.
The diatoms are the most speciose group of algae, having global ecological significance in the carbon and silicon cycles. They are almost unique among algae in being diplontic, and sexual reproduction is an obligate stage in the life cycle of most diatom species. It is unclear which are the principal factors that have fostered the evolutionary success of diatoms, but the unique life cycle (which is correlated with a curious wall structure and cell division mechanism) and size-dependent control of sexuality must have played an important part. Progress in understanding life cycle dynamics and their interrelationships with population biology and evolution will depend on how successfully sex can be initiated and manipulated experimentally, and our review provides a foundation for such work. Relevant data are scattered in time and come mostly from non-English publications, producing a false impression of diatoms as recalcitrant with respect to sexualization. Recent advances dependent on experimental cultures include the discovery of widespread heterothallism (including some complex types of behavior) in pennate diatoms, sexual diversity among clones of centric diatoms, more flexible size restitution strategies in centric diatoms than had been suspected, and use of reproductive isolation as a criterion in diatom taxonomy. We identify unsolved problems in the life history of diatoms, including aspects of sexualization, cell-cell recognition, sexual reproduction, and the development of the special expanding cell (the auxospore), which is crucial to morphogenesis in this group. Some of these problems are being addressed using modern molecular genetic tools, and progress will be facilitated when whole-genome sequences are published (e.g., for Thalassiosira pseudonana). Problems of culture maintenance and methods for manipulating the life cycle are discussed.
Examination of the geographic distributions of sexual organisms and
their asexual, or parthenogenetic, competitors reveals certain
consistent patterns. These patterns are called geographic parthenogenes
is. For example, if we compare sexual organisms with closely related
asexuals, we find that, in the Northern Hemisphere, there is a strong
tendency for the asexuals to occur further to the north. One researcher
to document this pattern is Bierzychudek, who examined 43 cases (drawn
from 10 genera) where the geographic distributions of a sexual plant and
a closely related asexual are known. In 76% of these cases, the asexual
plant's range was more northerly than the range of the sexual. Some of
the remaining cases probably fit with this pattern, but more data must
be obtained before this suggestion can be confirmed. Asexuals also tend
to occur at high altitudes, and in marginal, resource-poor environments.
We have constructed a mathematical model of a habitat that stretches
from south to north in the Northern Hemisphere. Our computer simulations
based on this model support the idea that a single basic process may
account for much of what is known about geographic parthenogenesis. This
process involves the movement of individuals from areas in which they
are well adapted to areas where they are poorly adapted.
The evolutionary dynamics of recessive or slightly dominant lethal mutations in partially self-fertilizing plants are analyzed using two models. In the identity-equilibrium model, lethals occur at a finite number of unlinked loci among which genotype frequencies are independent in mature plants. In the Kondrashov model, lethals occur at an infinite number of unlinked loci with identity disequilibrium produced by partial selfing. If the genomic mutation rate to (nearly) recessive lethal alleles is sufficiently high, such that the mean number of lethals (or lethal equivalents) per mature plant maintained at equilibrium under complete outcrossing exceeds 10, selective interference among loci creates a sharp discontinuity in the mean number of lethals maintained as a function of the selfing rate. Virtually no purging of the lethals occurs unless the selfing rate closely approaches or exceeds a threshold selfing rate, at which there is a precipitous drop in the mean number of lethals maintained. Identity disequilibrium lowers the threshold selfing rate by increasing the ratio of variance to mean number of lethals per plant, increasing the opportunity for selection. This theory helps to explain observations on plant species that display very high inbreeding depression despite intermediate selfing rates.
Partial asexual reproduction was introduced into a model of inbreeding depression due to nearly recessive lethal mutations in a partially selfing population. The frequencies of asexuality, selfing, and outcrossing were either constant or occurred in cycles of a single sexual generation followed by one or more asexual generations. We found that increasing the degree of asexuality generally increases the inbreeding depression maintained in an equilibrium population with a given selfing rate. This is due to the increase in the number of mutations relative to sexual generations during which selfing-induced purging of mutations may take place. For very high genomic mutation rates, sufficient to produce a threshold rate of self-fertilization for purging recessive lethal mutations, asexuality can have the opposite effect, decreasing equilibrium inbreeding depression, because of an increase in the efficiency of selection against mutations in heterozygotes with asexuality.
The amounts of inbreeding depression upon selfing and of heterosis upon outcrossing determine the strength of selection on the selfing rate in a population when this evolves polygenically by small steps. Genetic models are constructed which allow inbreeding depression to change with the mean selfing rate in a population by incorporating both mutation to recessive and partially dominant lethal and sublethal alleles at many loci and mutation in quantitative characters under stabilizing selection. The models help to explain observations of high inbreeding depression (>50%) upon selfing in primarily outcrossing populations, as well as considerable heterosis upon outcrossing in primarily selfing populations. Predominant selfing and predominant outcrossing are found to be alternative stable states of the mating system in most plant populations. Which of these stable states a species approaches depends on the history of its population structure and the magnitude of effect of genes influencing the selfing rate.
A bimodal distribution of outcrossing rates was observed for natural plant populations, with more primarily selfing and primarily outcrossing species, and fewer species with intermediate outcrossing rate than expected by chance. This distrbution is argued to result from selection for the maintenance of outcrossing in historically large, outcrossing populations with substantial inbreeding depression, and from selection for selfing when increased inbreeding, due to pollinator failure or population bottlenecks, reduces the level of inbreeding depression. Few species or populations are fixed at complete selfing or complete outcrossing. A low level of selfing in primarily outcrossing species is unlikely to be selectively advantageous, but will not reduce inbreeding depression to the level where selfing is selectively favored particularly if accompanied by reproductive compensation. Similarly, occasional outcrossing in primarily selfing species is unlikely to regularly provide sufficient heterosis to maintain selection for outcrossing through individual selection. Genetic, morphological and ecological constraints may limit the potential for outcrossing rates in selfers to be reduced below some minimum level. -Authors
Automixis in the form of paedogamy or more rarely of autogamy is widely distributed among pennate diatoms.
Presents a phenotypic model of the selection of self-fertilization that applies to any mode of self-pollination. A novel factor, seed discounting (loss of outcrossing maternal fitness accompanying an increase in self-fertilization) is introduced, and pollen discounting is reformulated. The model incorporates the effects of five principal factors - the cost of meiosis, inbreeding depression, reproductive assurance, and seed and pollen discounting. Either complete cross- or self-fertilization is selected, depending on the values of the parameters, but a number of additional factors can lead to intermediate frequencies of self-fertilization. Specific conditions for the selection of each mode of chasmogamous self-pollination and the effects of some special circumstances are derived by substituting the appropriate values of seed and pollen discounting in the general model. Geitonogamy and facilitated selfing are primarily nonadaptive by-products of mechanisms for outcrossing. Delayed selfing is most advantageous, but competing and prior selfing may be selected when structural constraints prevent delayed selfing or the prepotency of outcrossed pollen reduces seed discounting. The model predicts environmental modification of the frequency of self-fertilization and can explain the observed associations between frequent self-fertilization and both the annual habitat and poor conditions for cross-pollination. -Author
Cymbella aspera bildet in isolierten Mutterzellen zwei Gameten in der gleichen Weise wie nächstverwandte Arten in Kopulationspaaren. Die Gameten kopulieren pädogam—es handelt sich um Tochterprotoplasten mit Enkelkernen—, sie sind also verschiedengeschlechtig. Aus einer Mutterzelle entsteht eine Zygote bzw. Auxospore. Als sehr seltene Ausnahme erfolgt Paarung der Mutterzellen; mit welchem Erfolg, konnte nicht festgestellt werden, da nur Prophasen beobachtet wurden. Ebenfalls nur ausnahmsweise kommt Bildung von Azygoten unter Ablauf einer Pseudomeiose vor. Die Gründe für und gegen die Verallgemeinerung des Verhaltens auf allogame Arten werden besprochen, ohne daß sich zwingende Schlüsse ergeben. Für die Beantwortung der Frage nach den Ursachen des Zugrundegehens je eines Gonenkerns in jedem Gameten lassen sich bestimmte Gesichtspunkte gewinnen. Das Pyknotischwerden je eines Tochterkerns bei den Mitosen, die anläßlich der Bildung der Erstlingsschalen ablaufen, läßt sich daraus erklärne, daß sich der Protoplast nicht in einem prophasischen Zustand befindet. Die Zentralspindel der I. meiotischen Teilung besitzt aus räumlichmechanischen Gründen keinen kreisrunden, sondern einen flach elliptischen Querschnitt. Die Tochterchromosomengruppen in der I. Anaphase erleiden aus den gleichen Gründen charakteristische Verschiebungen.
SYNOPSIS. The morphology of the vegetative phase and the sexual process in Pandorina morum have been studied in detail under controlled laboratory conditions. A number of strains from various areas of the United States, all essentially indistinguishable on morphological grounds, were analyzed for sexual compatibility. Forty-seven heterothallic clones were found to represent 15 separate pairs of mating types, or a minimum of 15 syngens (sensu Sonneborn, 1957). Two clones proved to be homothallic. Nine additional clones, which were not observed to mate with any strain, can be classified only after further collecting. The separate heterothallic pairs of mating types are sexually isolated by factors acting at the stage of gamete production; incompatible mating types produce no gametes when they are mixed together. The geographical distribution of the sets of mating types is very incompletely defined as yet, but they are not strictly endemic forms. Thirty heterothallic strains, representing each of the 15 sexually isolated sets of mating types of Pandorina morum, have been deposited with the Culture Collection of Algae maintained at Indiana University, Bloomington, Indiana, U.S.A.
The genus Dickieia is re-established for Navicula ulvacea, N. subinflata and N. expecta. Dickieia cells contain one complex chloroplast per cell; this consists of two ‘K’-shaped halves, linked by an isthmus containing the single discoid pyrenoid. The pyrenoid lies near one side of the girdle at the centre, while the nucleus lies opposite. Neither the nucleus nor the chloroplast move significantly within the cell during the cell cycle. The valves contain round poroids with hymenate occlusions. The raphe-sternum lacks accessory ribs and the central internal raphe endings are simple and straight. The epitheca contains several narrow, porous girdle bands. Auxosporulation is unusually variable in D. ulvacea and can be allogamous (with isogamy) or automictic, though the gametangia are always paired. Plasmogamy is brought about principally through expansion of the gametes until they meet and fuse. The orientation of the auxospores bears no fixed relation to the gametangia. Haploid parthenogenesis and the formation of polyploid auxospores and ‘double’ gametes are reported and their significance discussed. Dickieia belongs within the Cymbellales and is closely related to the Rhoicospheniaceae and Anomoeoneidaceae.
In this paper, as a sequel of my survey appeared in 1969 in this Journal, supplements are given to the topics "Pairing,' "Gametogenesis," "Haploid Parthenogenesis," "Hybrids," "Automixis," and "Some Cytological Details" in pennate diatoms. The various kinds of peculiarities are facts-their interpretation in terms of physiology and biochemistry, indeed, rest completely open for future research. THE EMINENT successes of electron microscopic research make it easy to understand that special and general presentations of biological phenomena in textbooks, exclusively or predominantly, deal with the results of that research. This practice, however, often results in the omission of facts obtained by light microscopy, even if such information contributes to the biological understanding of the object in question, and even if they are not or only accomplished with difficulty by electron microscopy. Examples of that sort
While crossing inbred clones of Achnanthes longipes Ag., obtained in laboratory culture, various kinds of sexual processes took place that were not characteristic of clones isolated from natural populations. “Normal” and “reduced” reproductive types, intermediates between these, paedogamous gamete fusion and haploid parthenogenesis were found in mixed cultures of clones from the first inbred generation. Furthermore, polyploid and triradiate auxospores were observed when clones of the second inbred generation were crossed.
Sexual reproduction and auxospore formation of Achnanlhes javanica f. subconstricta have been observed in laboratory culture. This monoraphid diatom showed cis-type behavioural anisogamy and produced two auxospores in one of the paired gametangia. This diatom is unique in that the gametes become re-arranged only in the gametangium producing the stationary gametes, not in that producing the active gametes. Its position is thus obscure in the present classifications of sexual reproduction and auxospore formation. Some modification of these classifications is necessary.
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Sexual reproduction takes many forms within the diatoms. The variation has been classified by several authors, but in most cases the distinctions between their main categories have depended on the number of gametes produced per gametangium (and thus on how many zygotes per pair of copulating cells), and upon whether fusion is oogamous, anisogamous or isogamous. These classifications are not themselves an adequate basis for taxonomic comparison, which should be based on individual characteristics of the sexual process. Diatoms seem to be primitively oogamous. In araphid pennate diatoms and some raphid diatoms the gametes and gametangia are morphologically alike but physiologically distinct; one gametangium produces active gametes and the other passive ones. This may be the primitive condition in pennate diatoms, providing a link to the oogamy of centrics via the morphological anisogamy of Rhabdonema Ktz.
Plants exhibit complex mating patterns because of their immobility, hermaphroditism and reliance on vectors for pollen transfer. Research on plant mating attempts to determine who mates with whom in plant populations and how and why mating patterns become evolutionarily modified. Most theoretical models of mating-system evolution have focused on the fitness consequences of selling and outcrossing, stimulating considerable empirical work on the ecology and genetics of inbreeding depression. Less attention has been given to how the mechanics of pollen dispersal influence the transmission of self and outcross gametes. Recent work on the relation between pollen dispersal and mating suggests that many features of floral design traditionally interpreted as anti-selling mechanisms may function to reduce the mating costs associated with large floral displays.
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