Article

Patterns of Genetic Differentiation in the Slender Wild Oat Species Avena barbata

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Abstract

Allozyme frequencies at five enzyme loci were determined for 14 California populations of Avena barbata, a species introduced to California from the Mediterranean Basin during the colonization of North America. Allelic frequencies at these loci were also determined in Mediterranean collections of this species. The pattern of divergence of the California populations from the ancestral gene pool was not random and was strongly correlated with environment; thus, the pattern is not in accord with the hypothesis that most electrophoretically detectable variants are adaptively neutral. Rates of gene substitution in California were also not in accord with the neutrality hypothesis. The observations are, however, compatible with predictions of Neo-Darwinian evolutionary theory. We interpret these observations to indicate that natural selection plays a major role in determining the unique patterns of distribution of genetic variability in the slender wild oat in California.

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... Plant population biology has long been focused on patterns of genetic variation in non-native annual grass species in the western US and with particular emphasis on responses to abiotic gradients. An impressive body of literature helped develop the field of population biology (''ecogenetics,' ' Jain 1969) by documenting quantitative and qualitative measures of genetic variation in populations of annual grasses across habitats and climatic zones [e.g., Clegg and Allard (1972); Hamrick and Allard (1972); Jain (1969); Jain and Marshall (1967); Rai and Jain (1982); (1991a, b, c)]. Specific goals of many studies were to correlate plant traits to particular habitats and to reach conclusions concerning adaptability of the populations [e.g., Clegg and Allard (1972); Rai and Jain (1982); Rice and Mack (1991a, c)]. ...
... An impressive body of literature helped develop the field of population biology (''ecogenetics,' ' Jain 1969) by documenting quantitative and qualitative measures of genetic variation in populations of annual grasses across habitats and climatic zones [e.g., Clegg and Allard (1972); Hamrick and Allard (1972); Jain (1969); Jain and Marshall (1967); Rai and Jain (1982); (1991a, b, c)]. Specific goals of many studies were to correlate plant traits to particular habitats and to reach conclusions concerning adaptability of the populations [e.g., Clegg and Allard (1972); Rai and Jain (1982); Rice and Mack (1991a, c)]. Another objective was to investigate the rate of adaptation of invasive species in new habitats and several annual grass and forb species were useful because of their introduction to California during the Spanish mission period about 200 years earlier (Burcham 1957;Spira and Wagner 1983;Garcia et al. 1989). ...
... The history of the invasive genotypes or of the genetic diversity at the time of population spread could not be known and they attributed the presence of certain genotypes to selection related to regional rainfall patterns and correlated abiotic stresses. Clegg and Allard (1972) and Hamrick and Holden (1979) showed very specific patterns of genetic differentiation over very short distances in A. barbata and credited those differences to site-specific selection pressure, especially to competitive interactions and water availability, over very small spatial scales. In particular, Allard et al. (1972) concluded that genotypes occupying specific microhabitats represented co-adapted gene complexes. ...
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Non-native annual grasses in California are a functional group based on taxonomic similarity, but they may be dissimilar in terms of evolutionary potential and capacity for plastic response. The ecological character of each species is conditioned by their evolutionary history, the genetic circumstances of their introduction, and their subsequent interactions with the selective pressures in their new ranges. Early population ecology studies revealed regional monomorphism and population genetic uniformity as well as strong patterns of local adaptation. Recent studies have both challenged and verified those conclusions, and have also contributed useful new information regarding the genetic makeup of these non-native grasses. We used two very abundant and widespread species to test the prediction that non-native annual grasses would show similar adaptive responses under the same environmental conditions. Using six populations of two naturalized annual grass species collected from identical locations across a strong south-to-north rainfall gradient, we manipulated soil quality and soil moisture to test response in growth, reproduction, and phenology. Avena barbata populations showed strong response conditioned by resource availability and with a correlated shift in flowering phenology. In contrast, Hordeum murinum populations showed very uniform responses to changes in resource availability independent of their position on the rainfall gradient and with no correlated phenological shifts. Several genetic and historical factors may contribute to idiosyncratic responses and general adaptive patterns in these and other non-native species, and a better understanding of these factors will help us understand their current distributions.
... It does not spread vegetatively. A. barbata has a well-documented polymorphism with two ecotypes: the mesic ecotype associated with moister microhabitats and regions is pubescent, while the xeric ecotype grows in drier environments and is glabrous (Marshall and Jain 1969;Clegg and Allard 1972;Latta 2009). Pubescent individuals exhibit leaf hairs that point toward the leaf tips. ...
... Common gardens were planted into pastures at Hopland Research and Extension Center and the Sierra Foothills Research and Extension Center. These sites are typical of the regions occupied by the mesic and xeric ecotypes, respectively (surveyed by Clegg and Allard 1972). We planted three to five individuals per genotype from each of 166 RILs of known pubescence, and five accessions of each of the two ecotypes. ...
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Grasses are major agricultural products worldwide and they are critical to ecosystem function in many terrestrial habitats. Despite their global importance, we know relatively little about their defenses against herbivory. Grasses tend to be tolerant of leaf loss because their valuable meristems are located underground, out of reach for above ground herbivores. Many grasses have unidirectional leaf hairs, prickles, and spines that make moving up the leaf blade easy, but make moving down, toward the meristem, difficult. We tested the hypothesis that unidirectional grass hairs direct small arthropod herbivores away from the meristems. In a field survey of the distribution of herbivore damage, we found that leaf tips received five times more damage than leaf bases for Avena barbata. Early-instar grasshoppers fed three times as often on leaf tops as on leaf bases of pubescent individuals in a common garden laboratory experiment. This effect was not observed for glabrous individuals where grasshoppers damaged leaf bases as often as leaf tops. A common generalist caterpillar, Heliothus virescens, was more than twice as likely to turn in the direction of the hairs, away from the meristems, when it encountered pubescent leaves of A. barbata. However, larger caterpillars of the generalist feeder Arctia virginalis showed no directional bias when they encountered pubescent leaves. In common garden experiments, selection on pubescence was weak and inconsistent over space and time. Under some circumstances, individuals of A. barbata with pubescent leaves were more likely to produce seeds than were individuals with fewer hairs. The surveys, behavioral experiments with small insects, and estimates of lifetime reproduction all support the hypothesis that unidirectional leaf hairs on A. barbata, and perhaps other grasses, serve as an unstudied defense that direct small herbivores away from the meristems.
... Gene pools of inbreeding plant species are likely to be especially impoverished because selfing usually increases levels of homozygosity within a population (Brown 1979). Several studies on self-pollinating species have illustrated that introduced populations of these species retain only a small proportion of the variation existing within the original gene pool (Clegg and Allard 1972;Brown and Marshall 1981; Barrett and Richardson 1986). ...
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An understanding of how genetic differentiation and phenotypic plasticity may interact to promote the spread of an introduced species requires information on the hierarchical distribution of genetic variation within the species in its new range. For example, a lack of genetic variation within marginal populations of an introduced species may slow its rate of spread into new habitats. In a glasshouse study, we examined the phenotypic variation among populations, among families, within families, and the homogeneity of variances within families for morphological and life history characters of an alien, self-pollinating annual grass, Bromus tectorum. The populations of B. tectorum studied were collected from both large, central populations in steppe habitats and small, peripheral populations in forest sites along a broad temperature-moisture gradient in western North America. Most variation in average flowering time was due to differences among seed source populations; among family variation contributed less than 20% to the total variance for flowering time. Populations from arid steppe habitats were the earliest flowering while the population from the most mesic forest habitat was last to flower. In contrast, the within-family variance was a major contributor to the total variance for plant dry weight, seed number per plant, total seed weight, and individual seed weight. The amount of total variation explained by among-family differences ranged between 18% for average seed weight to 30% for total plant dry weight. There was no consistent difference in within-population genetic variability between large, central populations in steppe habitats and smaller, potentially more isolated populations in forest habitats. Significant heterogeneity in within-family variance in some of the source populations suggest that families differ in the capacity for phenotypic response to environmental variation. Considered independently from source population, there was no consistent trend linking a particular trait to increased heterogeneity of within-family variances
... Natural or low management systems are hypothesized to generate stronger rhizosphere effects (Philippot et al. 2013). The current study was situated in grasslands that have been uncultivated for the previous 50-60 yr; Avena has been a resident of these communities throughout this time period (Clegg and Allard 1972). Avena also has a number of selective mechanisms by which it could influence microbial community assembly. ...
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The interface between roots and soil, known as the rhizosphere, is a dynamic habitat in the soil ecosystem. Unraveling the factors that control rhizosphere community assembly is a key starting point for understanding the diversity of plant-microbial interactions that occur in soil. The goals of this study were to determine how environmental factors shape rhizosphere microbial communities, such as local soil characteristics and the regional climate, and to determine the relative influence of the rhizosphere on microbial community assembly compared to the pressures imposed by the local and regional environment. We identified the bacteria present in the soil immediately adjacent to the roots of wild oat (Avena spp.) in three California grasslands using deep Illumina 16S sequencing. Rhizosphere communities were more similar to each other than to the surrounding soil communities from which they were derived, despite the fact that the grasslands studied were separated by hundreds of kilometers. The rhizosphere was the dominant factor structuring bacterial community composition (38% variance explained), and was comparable in magnitude to the combined local and regional effects (22% and 21%, respectively). Rhizosphere communities were most influenced by factors related to the regional climate (soil moisture and temperature), while background soil communities were more influenced by soil characteristics (pH, CEC, exchangeable cations, clay content). The Avena core microbiome was strongly phylogenetically clustered according to the metrics NRI and NTI, which indicates that selective processes likely shaped these communities. Furthermore, 17% of these taxa were not detectable in the background soil, even with a robust sequencing depth of approximately 70,000 sequences per sample. These results support the hypothesis that roots select less abundant or possibly rare populations in the soil microbial community, which appear to be lineages of bacteria that have made a physiological tradeoff for rhizosphere competence at the expense of their competitiveness in non-rhizosphere soil.
... Both self-fertilization and laying an unfertilized egg for a male to fertilize carry inherent benefits and costs. Selfing offers reproductive assurance and preservation of favorable phenotypes; i.e. favorable genes related to high fitness will not be diluted by an outside genetic contribution of a potentially less fit male (Baker, 1955;Clegg and Allard, 1972). Outcrossing, however, introduces genetic diversity and guards against inbreeding depression (Charlesworth et al., 1993). ...
Article
The mangrove rivulus, Kryptolebias marmoratus, is one of two known vertebrate species with preferentially self-fertilizing hermaphrodites. Males also exist, and can outcross with hermaphrodites. Outcrossing events vary across wild populations and occur infrequently in laboratory settings. This study sought to add dimension to our understanding of mangrove rivulus reproductive habits by probing the effects of male presence on hermaphroditic unfertilized egg production. Specifically, we quantified egg production of solitary hermaphrodites compared to hermaphrodites exposed to males and exposed to other hermaphrodites. Hermaphrodites tended to produce more fertilized eggs in the presence of males but unfertilized eggs were produced relatively rarely and did not vary significantly among treatments. The probability that hermaphrodites would produce eggs changed as a function of genetic dissimilarity with their partner and in a season-dependent manner. In the fall, the probability of laying eggs decreased as a function of increased genetic dissimilarity, regardless of the sex of the partner. In the winter/spring, however, the probability of laying eggs increased markedly with increased genetic dissimilarity, regardless of the sex of the partner. Our findings indicate that reproductive decisions are modulated by factors beyond male presence, and we discuss a number of alternative hypotheses that should be tested in future studies.
... A general understanding of the direction, magnitude, and constancy of selection acting on the phenotypic variation in natural populations remains an unrealized goal in evolutionary biology. Studies ofgenetic differences among populations or sub-populations in morphological, phenological, and electrophoretic traits (Jain and Bradshaw, 1966;Clegg and Allard, 1972;Hamrick and Allard, 1972;Wu and Antonovics, 1976;Law et al., 1977;Primack and Antonovics, 1981;Schemske, 1984) have been used to exemplify the results of the evolutionary processes. Generally, these differences are attributed to past differences in natural selection that might have acted to create the observed distributions of phenotypic variation. ...
Article
Natural selection on the timing of seed germination was investigated in a natural population of the winter annual Collinsia verna (Scrophulariaceae) for two years. The goal was to quantify 1) the importance of the timing of seed germination to life history evolution in this population and 2) variation in selection in time and space. During fall germination, seedlings were assigned to cohorts on the basis of their dates of germination. Growth, survivorship, and reproduction were censused throughout both years. Selection on the timing of germination was quantified using linear and quadratic regressions during three ecologically important periods in the life cycle, using the techniques of Lande and Arnold (1983) and Arnold and Wade (1984a, 1984b). Comparisons were made between years and on two spatial scales within years. Overall, selection favored early-germinating plants in the first year. The primary determinant of the relationship of the timing of germination to fitness was fecundity selection, rather than viability selection on seedlings. Fecundity selection was respondible for from 54% to 80% of the change in the mean time of germination. Significant disruptive selection characterized the second field season, again mediated mainly through fecundity selection. There was also temporal and spatial heterogeneity in selection on this character. Transects and quadrats differed significantly in the direction and magnitude of natural selection. In addition, the direction of selection changed between episodes for the transects. The results illustrate the importance of the timing of germination to life-history evolution in this annual plant and the complex action of natural selection on this character.
... Clarkia Ungulata and C. biloba also have two or three alleles at four polymorphic genes which have been studied and both have at least five alleles for an esterase gene (Gottlieb, 1974). Populations of Avena barbata and A. fatua have two or three alleles at polymorphic gene loci (Marshall and Allard, 1970;Clegg and Allard, 1972;N. Berding, pers. ...
... As to selective importance of the h allele, there is very good reason to believe that it is not neutral. This allele has never been found in regions which are completely or largely monomorphic for the same alleles at Bib, Lsfls and at five isoenzyme loci (Clegg and Allard, 1972). A preliminary analysis of the Californian climate in relation to polymorphism in A vena barbata indicates that precipitation and late spring-early summer temperatures are two major environmental factors dictating monomorphism vs. polymorphism in this species (Rai and Jain, in prep.), ...
... That this is not the case argues that deterministic forces are acting either on the allozyme genes scored in this study or on closely linked genes. The high rate of self-fertilization in H. jubatum may also be an im-portant factor in this pattern, since one of its consequences is believed to be a potential for close, rapid adaptation to local environments (Allard et al., 1968;Clegg and Allard, 1972;Hamrick and Allard, 1972;Nyberg, 1974). Of additional interest is the observation that one of the loci which shows strong differentiation between the two habitats and hence indicates the action of natural selection (CPX-6, see Table 2) also shows strong clinal differentiation among populations sampled throughout the United States with allele "33" in high frequency in northern populations and at high elevations (Babbel, unpubl.). ...
... In a recent analysis of genetic variability in the slender wild oat species, A vena barbata, in California, Clegg and Allard ( 1972) described two complementary fivelocus allozyme complexes whose distributions are closely associated with environment. Populations in the semi-arid Mediterranean warm-summer region of California are fixed for one specific combination of alleles whereas populations in the most mesic portions of the Mediterranean cool-summer region are monomorphic for a specific balanced opposite set of alleles. ...
... Conspecific populations of the highly self-fertilizing annual grass Avena barbata in California have a mean genetic identity of .715 (Clegg and Allard, 1972). Consubspecific populations of the obligate outcrosser Stephanomeria exigua ssp. ...
... Kahler et al. (1980) found isozyme phenotypes to be significantly correlated with environmental variables for one of nine enzymes among populations of Avena barbata in Israel (experiment-wise error adjusted to 0.05). Introduced populations of A. barbata in California, on the other hand, are segregated into two ecotypes which are strongly differentiated at five enzyme loci (Cleggand Allard, 1972;Allard et al., 1978;Hamrick and Holden, 1979). Similar studies have been conducted in Israel on Hordeum spontaneum (Nevo et al., 1979) and Triticum dicoccoides (Nevo et al., 1982), but it is difficult to assess the statistical significance of their results since they took the circular approach of generating the best model using stepwise regression and then testing the model using the same data set. ...
Article
Selection favoring different alleles in different environments frequently has been suggested as an explanation for allozyme variation within and among populations. This hypothesis predicts that allozyme frequencies will be correlated with environmental variables. Previous studies on allozyme frequency-environment covariation in plants often have relied on qualitative assessments of the environment and have emphasized highly autogamous species. We have examined allozyme frequency-soil associations in Gaillardia pulchella, an obligately outcrossed annual plant, by regressing the frequencies of 15 common allozymes representing six polymorphic enzyme loci on principal components from a set of 20 quantitative soil variables. Fifty-one populations, representing four taxonomic varieties, were included in the analysis. Among the 26 populations representing the var. pulchella, allozymes Adh-2f and Pgm-1c were significantly associated with a block of highly inter-correlated soil characteristics which serve to discriminate between soils derived from calcareous vs. non-calcareous rock types. This geographically complex pattern of allozyme frequency-soil covariation is not likely to be spurious and, thus, indicates the presence of adaptively differentiated soil races, or ecotypes. However, these results are not sufficient to conclude that the allozyme frequency divergence between ecotypes was mediated by selection, either directly or through genetic hitchhiking. The pattern of allozyme frequency-soil covariation within var. pulchella was not found among the other taxonomic varieties. Patterns of genotype-environment covariation often may be recognizable only within geographically or environmentally restricted groups of populations because of the confounding influences of other environmental variables.
... 120 species for such a population structure Clegg and Allard, 1972;Allard, 1975). ...
... while that for Mediterranean populations was .666 (Clegg and Allard, 1972). The mean genetic identity of L. lucidulum populations more closely approaches that of species experiencing little or no self-fertilization. ...
... Electrophoretic studies of protein polymorphisms in plants have focused upon herbaceous species, primarily inbreeding annuals, in efforts to characterize the levels and patterns of genic variation within and between populations (Clegg and Allard, 1972;Gottlieb, 1973Gottlieb, , 1975Levin, 1975Levin, , 1978Levy and Levin, 1975;Schaal, 1975;Roose and Gottlieb, 1976;Brown et al., 1978;and others). These studies have indicated that predominantly outbreeding species maintain higher levels of intrapopulation variation than predominantly inbreeding species, while inbreeders exhibit a greater degree of population differentiation than outbreeders (Brown, 1979;Hamrick et al., 1979). ...
... However, the elinal variation is not statistically significant in this population and therefore may be coincidental. The results of this study provide an interesting comparison with similar studies on the two edaphic ecotypes of the autogamous annual species Avena barbata in California (Clegg and Allard, 1972;Allard et al., 1978). The ecotypes of A. barbata frequently meet in zones of contact which are necessarily secondary because of the introduced status of the species. ...
Article
The variety pulchella of the outcrossing annual plant species Gaillardia pulchella consists of two edaphic races in central Texas which are divergent for one morphological and four electrophoretic characters. Reduced pollen stainability in F1 hybrids suggests the races are also divergent in chromosome structure. The recent proliferation of this species on roadsides and in pastures has led to hybridization between these races. An analysis of character variation in three hybrid populations revealed significant clinal variation associated with edaphic ecotones, and the width of these clines was found to vary among characters in a consistent pattern. It is argued that this pattern is the result of different characters experiencing different effective selection regimes, with narrower clines reflecting greater differentials in effective selection. Several mechanisms are discussed by which selection may impede the transgression of alleles across the ecotones in these populations. The results of this study are compared to the results of parallel studies on the autogamous annual species Avena barbata in California, and it is suggested that the difference between these two species in the width of clines separating edaphic ecotypes may be accounted for by their different breeding systems.
... The clusters observed in E. coli differ from races in typical outcrossing eukaryotes in that they are not geographically separated, and there appears to be greater diversity among clusters than among most races. Perhaps the type of eukaryotic population structure most readily compared with that found in E. coli is the population structure of sexual but predominantly self-fertilizing species (Clegg and Allard, 1972;Allard et aI., 1978;McCracken and Selander, 1980;Price and Kahler, 1983). ...
Article
A. reference collection of 72 natural isolates of Escherichia coli (the ECOR collection) has been examined with respect to eight metabolic capabilities (biotype characters) plus motility and resistance or sensitivity to five common antibiotics. Data from biotype characters were analyzed by means of unweighted pair-group cluster analysis, and the genetic variation among the strains defines three major clusters of strains with substantial variation within each cluster but greater genetic similarity of strains within a cluster than between clusters. These clusters define an infraspecific population structure in E. coli, which reflects the predominantly clonal mode of reproduction in this organism. The clusters identified by the biotype characters are in good agreement with those resulting from an analysis of 11 enzyme polymorphisms (allozymes) among the strains, and these are in good agreement with the infraspecific structure detected by factor analysis of allozyme data. The clusters of strains also differ in several genetic characteristics that are independent of those used in making the classification.
... A number of studies analysed population genetic pattern in xeric vs. mesic environments when the latter represented the interior and edge conditions of the species range, respectively (Clegg & Allard 1972;Hamrick & Allard 1972;Comes & Abbott 1999;Volis et al. 2002aVolis et al. , 2014Volis 2011). Organisms inhabiting a species' edge usually experience more extreme and less predictable environmental conditions, and lower availability of suitable habitats which results in lower and more fluctuating population densities, smaller population sizes and reduced population connectivity than in the species interior (Hengeveld & Haeck 1982;Lawton 1993;Gaston 2003;Vucetich & Waite 2003). ...
Article
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Evaluating the relative importance of neutral and adaptive processes as determinants of population differentiation across environments is a central theme of evolutionary biology. We applied the QST -FST comparison flanked by a direct test for local adaptation to infer the role of climate-driven selection and gene flow in population differentiation of an annual grass Avena sterilis in two distinct parts of the species range, edge and interior, which represent two globally different climates, desert and Mediterranean. In a multi-year reciprocal transplant experiment the plants of desert and Mediterranean origin demonstrated home advantage, and population differentiation in several phenotypic traits related to reproduction exceeded neutral predictions, as determined by comparisons of QST values with theoretical FST distributions. Thus variation in these traits likely resulted from local adaptation to desert and Mediterranean environments. The two separate common garden experiments conducted with different experimental design revealed that two population comparisons are likely to detect population differences in virtually every trait, but many of these differences reflect effects of local rather than regional environment. We detected a general reduction in neutral (SSR) genetic variation but not in adaptive quantitative trait variation in peripheral desert as compared with Mediterranean core populations. On the other hand, the molecular data indicated intensive gene flow from the Mediterranean core towards desert periphery. Although species range position in our study (edge vs. interior) was confounded with climate (desert vs. Mediterranean), the results suggest that the gene flow from the species core does not have negative consequences for either performance of the peripheral plants or their adaptive potential. This article is protected by copyright. All rights reserved.
... ., allributes, e.g., Trifolium hirtum All. (Jain and Martins 1979); Trifolium subrerraneum L. (Cocks and Phillips 1979); Avena barbata (Clegg and Allard 1972 M . T. Clegg and A . ...
... We also used an existing A. barbata mapping population 39 , derived from a cross between Californian ecotypes associated with moist and dry habitats 41 . Of the initial 196 RILs, 180 F7 lines were available for GBS. ...
Article
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The genus Avena (oats) contains diploid, tetraploid and hexaploid species that evolved through hybridization and polyploidization. Four genome types (named A through D) are generally recognized. We used GBS markers to construct linkage maps of A genome diploid (Avena strigosa x A. wiestii, 2n = 14), and AB genome tetraploid (A. barbata 2n = 28) oats. These maps greatly improve coverage from older marker systems. Seven linkage groups in the tetraploid showed much stronger homology and synteny with the A genome diploids than did the other seven, implying an allopolyploid hybrid origin of A. barbata from distinct A and B genome diploid ancestors. Inferred homeologies within A. barbata revealed that the A and B genomes are differentiated by several translocations between chromosomes within each subgenome. However, no translocation exchanges were observed between A and B genomes. Comparison to a consensus map of ACD hexaploid A. sativa (2n = 42) revealed that the A and D genomes of A. sativa show parallel rearrangements when compared to the A genomes of the diploids and tetraploids. While intergenomic translocations are well known in polyploid Avena, our results are most parsimoniously explained if translocations also occurred in the A, B and D genome diploid ancestors of polyploid Avena.
... The increased genetic diversity expected in larger founder populations may act via several different mechanisms to enhance colonization success. Increased diversity will often reduce the expression of genetic load in species that are prone to inbreeding depression (Firestone and Jasieniuk 2013, Hufbauer et al. 2013) and promote increased rates of adaptation post-introduction (Clegg and Allard 1972, Reznick and Ghalambor 2001, Novak and Mack 2005. These "evolutionary" mechanisms are closely related conceptually to the conservation genetics of small populations and, as such, have received a great deal of attention in the literature (Newman andPilson 1997, Elam et al. 2007). ...
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Colonization is a critical filter, setting the stage for short‐term and long‐term population success. Increased propagule pressure (e.g., more founding individuals) usually enhances colonization; however, this pattern may be driven by purely numeric effects, population genetic diversity effects, or both. To determine the independent and interactive effects of propagule pressure and genetic diversity, we conducted a seed addition experiment in the field using the ruderal annual Arabidopsis thaliana. Propagule pressure treatments spanned five levels, from 32 to 960 seeds per 0.25‐m2 plot. Founder populations were composed of one, four, or eight genotypes and exposed to ambient or reduced levels of interspecific competition. Genotype monocultures were included to quantify additive vs. non‐additive effects. Populations were followed for three generations, with abundance, population persistence and genotype retention (the proportion of introduced genotypes persisting over time) as the major response variables. Increased propagule pressure enhanced abundance immediately following introduction, particularly where nutrient availability was high and competition reduced. Greater propagule pressure also increased the likelihood of population persistence and genotype retention through three generations. However, most populations experienced rapid abundance declines over time, yielding no relationship between propagule pressure and third‐generation abundance across persisting populations. Under reduced competition, increased genetic diversity led to a marginal increase in persistence through the third generation that was more pronounced, and statistically significant, in low nutrient conditions. Genetic diversity did not affect persistence through the first generation, thus indicating that genetic diversity effects strengthened over time. Nevertheless, genotypic mixture populations fell short of expectations based on performance in monocultures (negative non‐additive effects). Increased genetic diversity was also associated with abundance declines, largely due to one particularly high‐performing genotype in the lowest diversity treatments (i.e., genotypic identity effects). Overall, our results indicate that increases in both propagule pressure and genetic diversity can enhance colonization success but are highly context dependent. They also highlight novel ways in which both factors can impact the retention of introduced genetic diversity over time. Our findings pinpoint the determinants of a fundamental population process and have key implications for applications where enhanced or suppressed colonization is desired, including ecological restoration and invasive species management.
... For instance, the genetic variability of A. canariensis Baum & al. was studied by means of isoenzymes and it was determined that there were unique populations in areas with different soil properties [26]. Furthermore, it has been recorded that there was a clear discrepancy among 16 A. barbata Pott ex Link populations, in relation to climatic conditions [27]. ...
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Avena ventricosa Balansa ex Coss. is considered the C-genome donor of the cultivated hexaploid oat and is a 'priority' species for conservation, since it has limited geographic distribution and the only recorded populations in Europe are present in Cyprus. The current study attempts to characterize the genetic structure and fragmentation of the species via the application of genotypic markers. It was revealed that the genetic variety was mainly allocated among the populations collected, since clustering obtained was according to the geographic origin of the samples and the habitat. Species distribution modeling showed that the most important climatic variable defining A. ventricosa distribution is the mean diurnal temperature. Furthermore, significant association of the genetic structure to environmental variables was detected; overall, a negative association to precipitation was confirmed, while significant correlations of genetic structure and the temperature at the time of anthesis and germination were established. The safeguarding of this valuable genetic resource is discussed.
... For example, in Avena barbata there is a strong association between reproductive morphology and genetic markers, and both of these are geographically structured and correlated with environmental variables (de la Vega et al. 1991). Allele frequencies and the color and hairiness of the lemma are correlated regionally with aridity across Californian Mediterranean cool and warm summer zones (Clegg and Allard 1972) and along microgeographic gradients in intermediate zones (Hamrick and Allard 1972). In Mimulus there is geographic structure of isozymes that correlates with morphological traits at large geographic scales, that is, the whole continent of America, but no significant structure at smaller geographic scales (Vickery 1990). ...
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Adaptive geographic differentiation is documented for seed morphology of 36 populations of Heterosperma pinnatum Cav. (Asteraceae), a seed heteromorphic annual plant in the central highlands of Mexico. Achenes (single-seeded fruits) vary continuously within heads but are classified by shape and position as central, intermediate, or peripheral morphs and as having adhesive awns or not. Here we quantify shape as a principal component score contrasting log length and width of achenes. Heads and their variation among populations are described in terms of maximum, minimum, and range of shape scores; the number of achenes per head; quantitative indices of the abruptness of shape shift; where in the head the most abrupt change in shape occurs; and what achene shapes have awns. First and second principal components of these descriptors summarize 86% of among-population variation in achene and head morphology and correlate strongly with percent central achenes per head (%C) and percent of achenes with awns (%A), respectively. Awns are associated with greater dispersibility and achene shape is correlated with speed of dormancy loss. We hypothesized that dispersal morphology would be associated with vegetation attributes indicative of population ephemerality and that dormancy morphology would be associated with precipitation patterns during the early germination season. Morphological distance matrices were calculated using Euclidean distances among populations in %A and %C. Geographic distances among populations were calculated, as were genetic distances based on isozyme frequencies from 29 bands of six enzymes. Vegetation was classified as open or closed and early spring (germination season) and summer precipitation means were determined for each site. Closed vegetation was assumed to provide only ephemeral habitats for H. pinnatum. Partial matrix correlations between morphology and environment controlled for geographic but not genetic distance among sites, since the latter was not significantly correlated with either morphology or geography. A significant relationship was found between %A and closed vegetation, lower spring, and higher summer precipitation. %C was only correlated with lower spring precipitation. Independence of isozyme and morphological traits is interpreted in terms of selection on the latter but not the former.
... Isozymes as detectable protein markers which are under the direct influence of specific gene loci have been intensively used during the last three decades for examining the role of the environment in maintaining genetic variation (reviewed in Eanes 1999;Hedrick 1986;Hedrick et al. 1976;Lewontin 1991). However, despite large efforts invested in allozyme population genetics, no study except a classic work by Allard and others on Avena barbata Clegg and Allard 1972;Allard 1972, 1975) could unequivocally relate the observed allozymic variation to environmentally specific adaptive process. It has been common for studies examining the association of genetic variation with discrete environmental heterogeneity (soil type, sun versus shade, vegetative community type, etc.) to assume a priori that natural selection was occurring. ...
... Natural or low management systems are hypothesized to generate stronger rhizosphere effects (Philippot et al. 2013). The current study was situated in grasslands that have been uncultivated for the previous 50-60 yr; Avena has been a resident of these communities throughout this time period (Clegg and Allard 1972). Avena also has a number of selective mechanisms by which it could influence microbial community assembly. ...
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... In focusing upon the pattern of variation, however, less attention has been given to the fact that many organisms with large populations, particularly insects, have considerably less variation than predicted by the neutral model and that the level of variation is remarkably consistent among widely divergent organisms. Thus, putatively inbred organisms as Avena and Mus (Selander et al., 1969;Marshall and Allard, 1970;Clegg and Allard, 1972), organisms living in highly uniform environments (Gooch and Schlopf, 1972;Selander et al., 1974;Selander, 1976), parthenogenetic organisms (Suomalainen and Saura, 1973;Parker et al., 1977) and organisms as diverse as the horseshoe crab and man (Selander et al., 1970;Harris and Hopkinson, 1972) all exhibit similar levels of variability, in spite of varied evolutionary histories, contemporary ecologies, and population structures. ...
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Pinus densiflora is a widespread montane conifer that ranges from Japan to Manchuria via Korea. Enzyme electrophoresis was employed to assess genetic diversity and divergence in the populations of the species. Fourteen of the 19 loci (74.0%) were polymorphic in at least one of: the 20 populations. Genetic diversity at the species level (Hes) and at the population level (Hep) were 0.232 and 0.147. respectively. The proportion of total genetic diversity due to difference among populations (GST) was 0.279. An indirect estimate of gene flow (Nm = 0.65) indicates genetic drift is not seemed to play a major role in shaping genetic structure. P. densiflora maintains as much or perhaps more variation within their populations than other tree species in temperate regions.
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There are scarcely any characters in systematics that have generated more controversy during their long period of application than that of the proteins. On the one hand their high a priori significance tempts an experimentator to extend, often inadmissibly, the a posteriori significance of his results. While, on the other hand, exaggerated criticism may arise when results from comparative protein experiments are at odds with the current concepts of relationships. An example of this comes from amino acid sequencing, where initial reflections (BOULTER et al., 1970) have led to a too optimistic view, some a posteriori considerations (CRONQUIST, 1976), however, to an underestimation of its significance in systematics.
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Robert W. Allard, population geneticist and agronomist;population geneticist and agronomist;early plant breeding work;disease resistance gene identification - work on backcross method of breeding;quantitative genetics, as major advance in scientific crop improvement;experimental plant population genetics;genetic parameter estimation;genetic diversity in self-fertilizing species and ecological genetics of plant populations;plant population genetics - focusing on influence of gene interactions
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Migration patterns in wild emmer wheat, Triticum dicoccoides, were inferred from single-locus and multilocus genetic distributions based primarily on expectations derived from single-locus and multilocus cline theory. Populations from five collections displayed a large amount of multilocus structuring, as indicated by high values of gametic-phase disequilibrium between pairs of loci and by high values of multilocus associations. Analyses of the distributions of individual alleles, however, indicated that alleles had apparent independent centers of origin and that at least some independent dispersal within regions occupied by ecotypes or races of wild emmer had occurred. The distribution of the degrees of multilocus association suggests that there has been a net migration of the northern or Qazrin race south and west into the pocket occupied by the Yehudiyya race. The results suggest that ecotypic differentiation may be independent and may antedate electrophoretically determined differentiation in these populations. There is no convincing evidence that the multilocus associations represent coadapted complexes; rather they appear to involve associations of mutant alleles that have been accentuated and preserved by low recombination rates and gene-flow barriers within previously differentiated ecotypes.
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Artificially founded colonies of Avena barbata were utilized in two experiments in order to examine the role of natural selection in the evolution of regional differentiation in Central California. One experiment involved a total of 41 colonies founded in three different areas and scored over a 10-yr period; these colonies, started with known genotypes, showed that although both Valley and Bay region genotypes establish successful colonies, their relative survivorship and fecundity suggests weak selective forces consistent with those predicted from the observed patterns in natural populations. The second experiment involved two localities, representing two climatic regimes of temperature, in each of which ten colonies were started from identical seed sources. These colonies also showed relatively higher fitness of the genotypes matching with those sampled from the Valley and Coastal regions, respectively, again in line with their regional pattern of distribution. Colonization experiments appeared to be useful even as short-term in situ tests of certain specific evolutionary hypotheses on the role and intensity of natural selection.
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Deschampsia caespitosa is a widespread grass common in moist areas of the alpine tundra of the Rocky Mountains. Enzyme electrophoresis was used to examine population genetic structure along two soil moisture gradients in Rocky Mountain National Park, Colorado. Introduced plants used in a revegetation project were also sampled at one of the sites. At both sites, there were significant differences among subpopulations in allele frequencies, but these differences were distributed in a patchy fashion and were not correlated with the apparent soil moisture gradients. The degree of genetic subdivision differed between the two sites. At one site, gene flow appeared to be high and differences in allele frequencies are attributed to selection in a mosaic environment. At the other site, gene flow appeared more restricted and differences in allele frequencies between subpopulations are attributed to selection and limited gene flow acting simultaneously. Overall, 15% of the genetic variability is between subpopulations and gene flow is high, even between subpopulations separated by up to 1.5 km, but local conditions can apparently limit gene flow and increase the degree of genetic subdivision. The mean genetic distance between introduced plants and the native subpopulations was significantly higher than the mean genetic distance between all other subpopulations. Despite the high gene flow apparent in alpine tundra subpopulations of Deschampsia caespitosa, significant genetic structuring of these subpopulations has developed.
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Allelic variation in seedlings from 60 North American populations of the alien annual grass Bromus tectorum was determined at 25 loci using starch gel electrophoresis. Populations were collected from four regions; east of the Rocky Mountains, Nevada and California, the Intermountain West, and British Columbia. Compared to other diploid seed plants, genetic variation within these populations of B. tectorum is low: 4.60% of loci are polymorphic per population, with an average of 1.05 alleles per locus and a mean expected heterozygosity of 0.012. Although 2,141 individuals were analyzed, no heterozygous individuals were detected, and consequently, mean observed heterozygosity is 0.000. Extensive deviations from Hardy-Weinberg expectations were observed at every polymorphic locus due to heterozygote deficiencies. The mean genetic identity (Nei's I) between population pairs was 0.980 and indicates a high level of overall genetic similarity among populations. The among-population component of the total gene diversity is high (GST= 0.478), indicating substantial genetic differentiation among populations. These results are consistent with previous reports for highly self-pollinating plants of low genetic variation and substantial genetic differentiation among populations. Despite the lack of genetic variation as measured by enzyme electrophoresis, this weedy grass has become exceedingly abundant in a diverse array of arid environments throughout much of western North America, perhaps due to phenotypic plasticity.
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Background: Whereas the incidence or rate of polyploid speciation in flowering plants is modest, the production of polyploid individuals within local populations is widespread. Explanations for this disparity primarily have focused on properties or interactions of polyploids which limit their persistence. Hypothesis: The emergence of local polyploid populations within diploid populations is similar to the arrival of invasive species at new, suitable sites, with the exception that polyploids suffer interference from their progenitor(s). The most consistent predictor of successful colonization by invasive plants is propagule pressure, i.e., the number of seeds introduced. Therefore, insufficient propagule pressure, i.e. the formation of polyploid seeds within diploid populations, ostensibly is a prime factor limiting the establishment of newly emergent polyploids within local populations. Increasing propagule number reduces the effects of genetic, environmental, and demographic stochasticity, which thwart population survival. As with invasive species, insufficient seed production within polyploid populations limits seed export, and thus reduces the chance of polyploid expansion. Conclusion: The extent to which propagule pressure limits the establishment of local polyploid populations remains to be determined, because we know so little. The numbers of auto- or allopolyploid seed in diploid populations rarely have been ascertained, as have the numbers of newly emergent polyploid plants within diploid populations. Moreover, seed production by these polyploids has yet to be assessed.
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Most evolutionary change in proteins may be due to neutral mutations and genetic drift.
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Quantitative consideration of the operation and evolution of the information-processing system that constitutes the terrestrial biosphere indicates that non-Darwinian evolutionary changes cannot be expected to account for the increase in the amount of non-random biospheric structure that constitutes the information content of the biosphere. Changes that lead to such an increase must also be selectively advantageous and lead to preferential survival through natural selection.
There are a large number of different enzymes synthesized in the human organism, and many of these probably contain more than one structurally distinct polypeptide chain. If current theories about genes and proteins are correct we must suppose that the primary structure of each of these different polypeptides is determined by a separate gene locus, and that there are probably also other loci which are specifically concerned with regulating the rate of synthesis of particular polypeptides or groups of polypeptides. Furthermore, we may expect that genetical diversity in a human population will to a considerable extent be reflected in enzymic diversity. That is to say, in differences between individuals either in the qualitative characteristics of the enzymes they synthesize, or in differences in rates of synthesis. The work I am going to discuss was largely aimed at trying to get some idea of the extent and character of such genetically determined enzyme diversity among what may be regarded as normal individuals. When my colleagues and I started on this line of work about three years ago the information available about this aspect of the subject was very limited. It had of course been recognized for quite a long time that there are many rare metabolic disorders, the so-called 'inborn errors of metabolism', which are due to genetically determined deficiencies of specific enzymes (Harris I963). These conditions can in general be attributed to mutant genes which result either in the synthesis of an abnormal enzyme protein with defective catalytic properties, or in a gross reduction in rate of synthesis of a specific enzyme protein. By and large such genes appear to be relatively uncommon and have frequencies of between 0.01 and 0-001 in the general population. Heterozygotes often show a partial enzyme deficiency though they are usually in other respects quite healthy. A few cases are also known where a specific enzyme deficiency occurs quite commonly in certain populations. The most extensively studied example of this is glucose 6-phosphate dehydrogenase deficiency, and it seems likely that in this particular case the relatively high incidence in certain populations is attributable to a specific selective advantage which the deficiency may confer in situations where endemic malaria is an important selective agent (Motulsky 1964). Virtually all these enzyme deficiencies have been identified in the first instance because of some more or less striking clinical or metabolic disturbance of which they were the cause. They therefore represent a highly selected group of mutants,
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A comparative study of genic allozyme and chromosomal polymorphisms in four continental (South American) and six oceanic island (West Indies) populations of Drosophila willistoni has been made. The pattern of genic polymorphism is closely similar in all populations. Although regional and local differences in gene frequencies are found, generally the same alleles occur at high, intermediate, and low frequencies in all populations. An average individual is heterozygous at 18.4 and 16.2% of its loci in the continental and island populations, respectively. By contrast, chromosomal polymorphism is sharply reduced on the islands compared to most continental populations, and some chromosomal inversions are more frequent on some islands than on others. The observations are not compatible with the hypothesis that most of the gene variants are adaptively neutral. Balancing natural selection is responsible for most of the genic polymorphism in natural populations of D. willistoni.
Article
Calculating the rate of evolution in terms of nucleotide substitutions seems to give a value so high that many of the mutations involved must be neutral ones.
Article
S pointed out in the first paper of this series (HUBBY and LEWONTIN 1966), A no one knows at the present time the kinds and frequencies of variant alleles present in natural populations of any organism, with the exception of certain special classes of genes. For human populations we know a good deal about certain polymorphisms for blood cell antigens, serum proteins, and metabolic disorders of various kinds but we can hardly regard these, a priori, as typical of the genome as a whole. Clearly we need a method that will randomly sample the genome and detect a major proportion of the individual allelic substitutions that are segre- gating in a population. In our previous paper, we discussed a method for accom- plishing this end by means of a study of electrophoretic variants at a large number of loci and we showed that the variation picked up by this method behaves in a simple Mendelian fashion so that phenotypes can be equated to homozygous and heterozygous genotypes at single loci. It is the purpose of this second paper to show the results of an application of the method to a series of samples chosen from natural populations of Drosophila pseudoobscura. In particular, we will show that there is a considerable amount of genic variation segregating in all of the populations studied and that the real variation in these populations must be greater than we are able to demonstrate. This study does not make clear what balance of forces is responsible for the genetic variation observed, but it does make clear the kind and amount of varia- tion at the genic level that we need to explain. An exactly similar method has recently been applied by HARRIS (1966) for the enzymes of human blood. In a preliminary report on ten randomly chosen enzymes, HARRIS describes two as definitely polymorphic genetically and a third as phenotypically polymorphic but with insufficient genetic data so far. Clearly these methods are applicable to any organism of macroscopic dimensions.
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