Clonal diversity and polyphyletic origins of hybrid and spontaneous parthenogenetic Campeloma (Gastropoda: Viviparidae) from the south-eastern United States

Some theories for the maintenance of sexual reproduction indicate that parthenogens may persist if there is high clonal diversity and high dispersal rates. Using allozymic variation, we report on the origin, clonal diversity and population structure of hybrid and spontaneous parthenogens from south-eastern United States populations of the freshwater snail Campeloma. Independent origins of triploid hybrid parthenogens in the Florida panhandle occurred by hybridization between an Atlantic coastal species (C. limum or C. floridense) and the Florida sexual species (C. geniculum). Allozyme genotypic diversity is similar between these hybrid parthenogens and sexuals. Diploid spontaneous parthenogens originated multiple times from nonlocal C.limum sexual populations in Atlantic coastal rivers, and levels of genotypic diversity are significantly higher in sexual C. limum. How parthenogens originate, the degree of clonal diversity, and their subsequent dispersal influence whether basic assumptions of evolution-of-sex models are met.     

Predominance of clonal reproduction, but recombinant origins of new genotypes in the free-floating aquatic bladderwort Utricularia australis f. tenuicaulis (Lentibulariaceae)

Aquatic plants are a biological group sharing several adaptations to aquatic conditions. The most striking evolutionary convergence in this group is the extensive reliance on clonal reproduction, which largely determines the patterns and process of evolution in aquatic plants. Utricularia australis f. tenuicaulis is a free-floating aquatic bladderwort that reproduces both sexually via seeds and clonally via turions and shoot fragments. Amplified fragment length polymorphism analysis was conducted on 267 ramets collected from 30 populations in Japan. The genotypic diversity within populations was extremely low, regardless of the geographical distribution range: the mean number of genotypes per population (G) was 1.4 and the mean genotypic diversity (D), including monoclonal populations, was 0.17. In contrast to the predominance of a few clones within populations, many of the populations investigated had different genotypes; a large portion of the genetic variation was explained by variation among populations. Character compatibility analysis clearly revealed that somatic mutations did not contribute to the origin of genotypic diversity in this aquatic bladderwort; instead, rare-to-sporadic sexual reproduction probably generated new genotypes. Thus, future studies should examine the role of sexual reproduction in this species from the viewpoint of long-term evolutionary benefits.     

THE GENETIC STRUCTURE OF LARGE-LAKE DAPHNIA POPULATIONS

Cyclical parthenogenesis allows study of the genetic and evolutionary characteristics of groups exhibiting both asexual and sexual reproduction. The cladoceran genus Daphnia contains species which vary with respect to the relative incidence of sexual reproduction; pond species tend to undergo sexual reproduction more regularly than species found in large lakes. Previous genetic studies have focused on pond populations, generating expectations about large-lake populations that have not been fully met by recent studies. The present study of the Palearctic species Daphnia galeata further examines the genetic structure of large-lake populations. Nine local populations, from lakes in northern Germany, are examined for genetic variation at seven enzyme loci. Populations exhibit similar allelic arrays and often similar allele frequencies at the five polymorphic loci; values of Nei's genetic distance (D) ranged from 0.002 to 0.239, with a mean of 0.084. F_ST values range from 0.012 to 0.257, and spatial autocorrelation coefficients range from -0.533 to 0.551, for the eight alleles analyzed. With few exceptions, within-population genotypic frequencies were in Hardy-Weinberg equilibrium. There was, however, significant heterogeneity in genotypic frequencies among populations. The number of coexisting clonal groups, as determined by three locus genotypes, is high within populations. Clonal groups are widely distributed among localities. The amount of genetic divergence observed among these large-lake populations is smaller than that previously observed among pond populations and suggests that different processes may be important in determining the genetic structure and subsequent phenotypic divergence of lake versus pond populations.     

Low Levels of Genetic Diversity within Populations of Hosta clausa (Liliaceae)

Abstract Genetic diversity of Korean populations in Hosta clausa was investigated using starch gel electrophoresis. Hosta clausa is widespread, grows only along streamsides, and has both sexual and asexual reproduction. Populations of the species are small and isolated. Thirty-two percent of the loci examined were polymorphic, and mean genetic diversity within populations (He_p=0.082) was lower than mean estimates for species with very similar life history characteristics (0.131), particularly for its congener H. yingeri (0.250). The mean number of multilocus genotypes per population was 8.7, and genotypic diversity index (D_G) was 0.84. Significant differences in allele frequencies among populations were found in all seven polymorphic loci (P < 0.001). About one-fifth of the total allozyme variation was among populations (G_ST=0.192). Indirect estimate of the number of migrants per generation (Nm=0.48, calculated from mean G_ST) and nine private alleles found indicate that gene movement among populations was low. The low levels of genetic diversity within populations and the relatively high levels of genetic diversity among populations suggest that strong moist habitat preferences, clonal reproduction, low level of gene flow among populations, genetic drift, and historical events may have played roles in the genetic structuring of the species.     

Population genetic analysis of Elliottiaracemosa (Ericaceae), a rare Georgia shrub

Genetic and genotypic diversity found within populations of threatened plant species can have important implications for their conservation and management. In this study we describe genetic and genotypic diversity found within 10 populations of the endemic shrub Elliottiaracemosa (Ericaceae), the Georgia plume. E. racemosa is a threatened species known from fewer than 50 locations, all within the state of Georgia, USA. Seedset is limited to nonexistent in some E. racemosa populations and sexual recruitment has not been documented. However, the species is known to spread vegetatively via root-sprouts. Twenty-one allozyme loci were resolved for E. racemosa, nine of which were polymorphic. Compared with other woody taxa, E. racemosa has low genetic (i.e. allelic) diversity within populations (H_ep = 0.063) and at the species level (H_es = 0.091). Most of the genetic variation (82%) was found within populations, and genetic identities between populations were high (mean I = 0.96). However, genotypic diversity (i.e. the number of multilocus genotypes) differed markedly among populations. Two of the 10 populations consisted almost entirely of single multilocus genotypes, whereas more than 20 multilocus genotypes (in samples of 48 stems) were detected at three sites. Sites in which few multilocus genotypes were detected have low seedset, suggesting that the lack of clonal diversity limits reproduction in some populations of this reportedly self-incompatible species.     

Reduced recombination in gynodioecious populations of a facultative apomictic orchid

Many plants combine sexual reproduction with some form of asexual reproduction to different degrees, and lower genetic diversity is expected with asexuality. Moreover, the ratios of sexual morphs in species with gender dimorphism are expected to vary in proportion to the reproductive success of the sexual process. Hence, sex ratios can directly influence the genetic structure and diversity of a population. We investigated genotypic diversity in 23 populations of a facultative, apomictic gynodioecious orchid, Satyrium ciliatum, to examine the effect on genotypic diversity of variation in the frequency of females and in the amount of sexual reproduction. The study involved one pure female, seven gynodioecious (both females and hermaphrodites present) and 15 hermaphroditic populations. Pollinia receipt was higher in hermaphroditic than in gynodioecious populations. Analyses of variation in ISSRs demonstrated that genotypic diversity was high in all populations and was not significantly different between hermaphroditic and gynodioecious populations. We used character compatibility analysis to determine the extent to which recombination by sexual reproduction contributed to genotypic diversity. The results indicate that the contribution of recombination to genotypic diversity is higher in hermaphroditic than in gynodioecious populations, consistent with the finding that hermaphroditic populations received higher amounts of pollinia. Our finding of reduced recombination in gynodioecious populations suggests that maintenance of sex in hermaphrodites plays an important role in generating genotypic diversity in this apomictic orchid.     

Genetic structure of Polytrichum formosum in relation to the breeding system as revealed by microsatellites

Microsatellite variation was determined for three Danish and three Dutch populations of the haploid moss species Polytrichum formosum to gain insight into the relative importance of sexual vs. asexual reproduction for the amount and structure of genetic variation. In general, low levels of microsatellite variation were observed within this species. Even when estimated for polymorphic loci only, the levels of microsatellite variability (P=90.6, A=4.3 and H_S=0.468) within populations were on average lower than those reported for most other plant species. In contrast, genotypic diversity was high within each of the examined populations, indicating that sexual reproduction is a very important determinant of the genetic structure of P. formosum within populations. In agreement with previous findings for allozyme data, no significant genetic differentiation (F_ST=0.028, R_ST=0.015) was observed neither between populations nor between regions approximately 450 km apart (Denmark vs. the Netherlands). These low levels of population differentiation observed for both types of genetic markers are probably best explained by a high level of effective spore dispersal (gene flow) between populations. Therefore, also on a large geographical scale sexual reproduction is the most important determinant of the genetic structure of P. formosum, despite the high potential to reproduce clonally.     

The effect of herbivores on genotypic diversity in a clonal aquatic plant

In clonal plants, vegetative parts may outcompete seeds in the absence of disturbance, limiting the build‐up of genotypic diversity through repeated seedling recruitment (RSR). Herbivory may provide disturbance and trigger establishment of strong colonizers (seeds) at the expense of strong competitors (clonal propagules). In the clonal aquatic fennel pondweed Potamogeton pectinatus, two distinct herbivore guilds may modify the dynamics of propagation. In winter, Bewick's swans may deplete patches of tubers, promoting seedling establishment in spring. In summer, seed consumption by waterfowl can reduce the density of viable seeds but grazing may also reduce tuber production and hence facilitate seedling establishment. This study is among the first to experimentally test herbivore impact on plant genotypic diversity. We assess the separate and combined effects of both herbivore guilds on genotypic diversity and structure of fennel pondweed beds. Using microsatellites, we genotyped P. pectinatus from an exclosure experiment and assessed the contribution of herbivory, dispersal and sexual reproduction to the population genetic structure. Despite the predominance of clonal propagation in P. pectinatus, we found considerable genotypic diversity. Within the experimental blocks, kinship among genets decreased with geographic distance, clearly identifying a role for RSR in the maintenance of genotypic diversity within the fennel pondweed beds. However, over a period of five years, none of the herbivory treatments affected genotypic diversity. Hence, sexual reproduction on a local scale is important in this putatively clonal plant and possibly sufficient to ensure a relatively high genotypic diversity even in the absence of herbivores. Although we cannot preclude a role of herbivory in shaping genotypic diversity of a clonal plant, after five years of exclusion of the two investigated herbivore guilds no measurable effect on genotypic diversity was detected.     

Genetic and clonal diversity in Korean populations ofVitex rotundifolia (Verbenaceae)

Vitex rotundifolia L.f. is a woody perennial and has sexual and asexual modes of reproduction. Allozyme study was conducted on 550 plants in 13 Korean populations. The levels of genetic variability and divergence within and among populations, respectively, are considerably lower and higher than the mean values for woody plants with similar life history tralts. Mean percentage of polymorphic loci (P _P), mean number of alleles per locus (A _P), and mean genetic diversity (He _P) within populations ofV. rotundifolia were: 16.7%, 1.21, and 0.047. On average, about 79% of the total variation inV. rotundifolia was common to all populations (meanG _ST=0.208). In addition, significant differences in allele frequencies among populations were found in all polymorphic loci examined (P<0.001). On the other hand, levels of genotypic diversity within and among populations were moderate. About 44% (18/41) of multilocus genotypes were “local genotypes” (genotypes occurring in only one population), whereas only one “widespread genotype” (genotypes occurring in more than 75% of the populations) were detected. The mean number of multilocus genotypes per population (G) and mean genotypic diversity index (D _G) were 8.4 and 0.74, respectively. Most common multilocus genotypes found in populations were homozygous for five polymorphic loci. The abundance of ramets of these genets is responsible for the low levels of expected heterozygosity within populations. The results indicate that clonal reproduction may act as an enhancer of genetic drift by reducing effective size of local populations ofV. rotundifolia.     

CONTRIBUTIONS OF SEXUAL AND ASEXUAL REPRODUCTION TO POPULATION STRUCTURE IN THE CLONAL SOFT CORAL,ALCYONIUM RUDYI

Numerous studies of population structure in sessile clonal marine invertebrates have demonstrated low genotypic diversity and nonequilibrium genotype frequencies within local populations that are monopolized by relatively few, highly replicated genets. All of the species studied to date produce planktonic sexual propagules capable of dispersing long distances; despite local genotypic disequilibria, populations are often panmictic over large geographic areas. The population structure paradigm these species represent may not be typical of the majority of clonal invertebrate groups, however, which are believed to produce highly philopatric sexual propagules. I used allozyme variation to examine the population structure of the temperate soft coral, Alcyonium rudyi, a typical clonal species whose sexually produced larvae and asexually produced ramets both have very low dispersal capabilities. Like other clonal plants and invertebrates, the local population dynamics of A. rudyi are dominated by asexual reproduction, and recruitment of new sexually produced genets occurs infrequently. As expected from its philopatric larval stage, estimates of genetic differentiation among populations of A. rudyi were highly significant at all spatial scales examined (mean θ = 0.300 among 20 populations spanning a 1100-km range), suggesting that genetic exchange seldom occurs among populations separated by as little as a few hundred meters. Mapping of multilocus allozyme genotypes within a dense aggregation of A. rudyi ramets confirmed that dispersal of asexual propagules is also very limited: members of the same genet usually remain within < 50 cm of one another on the same rock surface. Unlike most previously studied clonal invertebrates, populations of A. rudyi do not appear to be dominated by a few widespread genets: estimates of genotypic diversity (G_o) within 20 geographically distinct populations did not differ from expectations for outcrossing, sexual populations. Despite theoretical suggestions that philopatric dispersal combined with typically small effective population sizes should promote inbreeding in clonal species, inbreeding does not appear to contribute significantly to the population structure of A. rudyi. Genet genotype frequencies conformed to Hardy-Weinberg expectations in all populations, and inbreeding coefficients (f) were close to zero. In general, the population structure of A. rudyi did not differ significantly from that observed among outcrossing sexual species with philopatric larval dispersal. Age estimates suggest, however, that genets of A. rudyi live for many decades. Genet longevity may promote high genotypic diversity within A. rudyi populations and may be the most important evolutionary consequence of clonal reproduction in this species and the many others that share its dispersal characteristics.     

Genetic variation in Pueraria lobata (Fabaceae), an introduced, clonal, invasive plant of the southeastern United States

Pueraria lobata (kudzu), a clonal, leguminous vine, is invading the southeastern United States at a rate of 50 000 ha per year. Genetic variability and clonal diversity were measured in 20 southeastern U.S. populations using 14 allozyme loci. Within its U.S. range, 92.9% of the loci were polymorphic and overall genetic diversity was 0.290. Such high levels of genetic diversity are consistent with its history of multiple introductions over an extended period of time. The average proportions of polymorphic loci and genetic diversity within populations were 55.7% (range = 28.6–85.7%) and 0.213 (range = 0.114–0.317), respectively. The proportion of total genetic diversity found among populations was similar to species with equivalent life history characters (G_ST = 0.199). No regional patterns of variation were seen. The number of putative genotypes in each population ranged from 2 to 26. Mean genotypic diversity was 0.694, ranging from 0.223 to 0.955. Such high levels of genotypic diversity indicate that local sites are often colonized by several propagules (most likely seeds) and/or that sexual reproduction occurs within populations after establishment. An excess of heterozygosity was observed in populations with few unique genets, implying that selection for highly heterozygous individuals may occur in populations of P. lobata.     

Allozymic diversity of Allium tricoccum (Ait.) Solander var. burdickii Hanes in isolated populations of Nova Scotia (Canada)

 Allium tricoccumvar. burdickii is a rare species in Nova Scotia, growing in isolated populations. Genetic variation was investigated in three populations using cellulose acetate gel electrophoresis. The results showed that all 13 enzyme loci were polymorphic. An excess of heterozygotes was found in all populations. A total of 29 genotypes were detected in the populations. Very few genotypes were common to two populations and none to the three populations. Genetic diversity among populations was compared with geographic distance and previous work. The comparison of the genetic distance matrix to the geographic distance matrix indicated no relationship. F-statistics suggested that the populations are predominantly maintained through vegetative propagation. Occasional sexual reproduction and gene flow might occur in order to maintain high level of variation among populations. Environmental conditions could also influence population genetic structure as they occur in highly different habitats. Received November 13, 2000 Accepted February 26, 2001     

Isozyme and morphological diversity within apomictic and sexual taxa of the Ranunculus auricomus complex

 In 17 populations of the tetraploid agamospecies R. variabilis, 10 enzyme systems have been studied by horizontal starch gel electrophoresis, and compared to previous data of 3 populations of the diploid sexual species R. notabilis. Allozymic and genotypic diversity of 12 polymorphic loci, and multilocus genotype diversity in R. variabilis compares to other apomictic taxa, confirming the assumed apomictic mode of reproduction previously found in 3 populations. A cluster analysis based on genetic distance values separated the two taxa, the populations of southeastern Austria sympatric with R. notabilis showing no closer relationships to the sexual species than the allopatric ones. The R. variabilis populations of the circumalpine regions are separated as a rather uniform group, those of the Bohemian massif as another, more heterogeneous group. This geographic differentiation is mainly due to genotypic variation, which is higher in the Bohemian massif than in the circumalpine group, and lower in areas where R. variabilis is sympatric with R. notabilis. Proportion of multilocus genotypes within populations (G/N), genotype diversity (D), and genotype evenness (E) measures indicate facultative recombination events with fixation and clonal reproduction of new recombined genotypes, which is regarded as the most important factor for the evolution of new lineages in goldilocks. Morphometric data of the R. variabilis and R. notabilis populations studied by cluster and principal coordinate analyses clearly separated the two taxa, and indicated a slight geographic differentiation with in R. variabilis corresponding to the results of isozyme data. A principal coordinate analysis of R. variabilis individuals based on stem and fruit characters showed a clinal variation, but no clear separation of groups. The variation found in this PCoA is almost completely covered from the most widespread clone (genotype I), and also from individual genotypes. The implications of the results on different species concepts for apomicts are discussed. For R. variabilis, no taxonomic conclusions can be drawn from the present data set. Received December 15, 1999 Accepted November 15, 2000     

Genetic structure and gene flow in an endangered perennial grass, <Emphasis Type="Italic">Arctophila fulva</Emphasis> var. <Emphasis Type="Italic">pendulina</Emphasis>

Arctophila fulva var. pendulina is a rare endemic perennial grass confined to seashore and riverbank meadows around the Bothnian Bay, the northernmost part of the Baltic Sea. The number of A. fulva populations has decreased during the last few decades in Finland and Sweden, and nowadays there are only eight populations left in the drainage area of the Bothnian Bay. We investigated the distribution of genetic variation within and between six subpopulations in the largest remaining population at Liminka Bay, Finland, using amplified fragment length polymorphism (AFLP) markers. Relatively high amounts of variation were found in the subpopulations, the mean Nei’s expected heterozygosity being typical (0.267) for an outcrossing species. Despite the fact that no seedlings or viable seeds of A. fulva have been found in the previous field studies, the observed high genotypic diversity suggested that sexual reproduction has played an important role at some time during the history of the studied A. fulva population. Analysis of population structure revealed a low level of genotypic differentiation (Φ_ST=0.046) between subpopulations, and also significant sub-structuring within subpopulations. Isolation-by-distance between subpopulations was present on scales larger than 1 km. The overall pattern of genetic variation within and between subpopulations suggest that the population has characters of both stepping-stone and metapopulation models. Because our results suggested that subpopulations are more or less ephemeral, the conservation and management effort in this species should be targeted to conservation of the required habitat of the species instead of extant subpopulations.     

Reproductive strategies and isolation‐by‐demography in a marine clonal plant along an eutrophication gradient

Genetic diversity in clonal organisms includes two distinct components, (i) the diversity of genotypes or clones (i.e. genotypic richness) in a population and (ii) that of the alleles (i.e. allelic and gene diversity within populations, and differentiation between populations). We investigated how population differentiation and genotypic components are associated across a gradient of eutrophication in a clonal marine plant. To that end, we combined direct measurements of sexual allocation (i.e. flower and seed counts) and genotypic analyses, which are used as an estimator of effective sexual reproduction across multiple generations. Genetic differentiation across sites was also modelled according to a hypothesis here defined as isolation‐by‐demography, in which we use population‐specific factors, genotypic richness and eutrophication that are hypothesized to affect the source‐sink dynamics and thus influence the genetic differentiation between a pair of populations. Eutrophic populations exhibited lower genotypic richness, in agreement with lower direct measurements of sexual allocation and contemporaneous gene flow. Genetic differentiation, while not explained by distance, was best predicted by genotypic richness and habitat quality. A multiple regression model using these two predictors was considered the best model (R² = 0.43). In this study, the relationship between environment and effective sexual–asexual balance is not simply (linearly) predicted by direct measurements of sexual allocation. Our results indicate that population‐specific factors and the isolation‐by‐demography model should be used more often to understand genetic differentiation.     

Clonal reproduction and patterns of genotypic diversity in Decodon verticillatus (Lythraceae)

Most perennial plants combine sexual reproduction with some form of clonal propagation. These mixed strategies may produce considerable variation among populations in levels of clonal diversity in response to ecological factors limiting one or other reproductive mode. Surveys of style morph frequencies in 163 populations of the eastern North American, clonal, tristylous aquatic, Decodon verticillatus (L.) Ell. (Lythraceae) suggested a wide range of clonal diversity among populations. Populations consisting of a single style morph were most common at the northern margin of the species' range and could have arisen through severe founder events followed by exclusive clonal propagation. Here, we test this hypothesis by comparing allozyme variation in populations monomorphic and polymorphic for style length located in Ontario and Michigan. Each of the four populations monomorphic for style length were fixed for a single three-locus allozyme genotype while the seven trimorphic and five dimorphic populations contained an average of 26 multilocus genotypes each. Measures of genotypic diversity were high in polymorphic populations (average D = 0.93 ± 0.02 standard error; D = 0.00 for all populations monomorphic for style length). Three of the populations monomorphic for style length were fixed for a heterozygous genotype at one of the loci surveyed, suggesting that each consists of a single clone. In contrast, genotype frequencies in polymorphic populations conformed to Hardy-Weinberg proportions indicative of sexual reproduction. The range of clonal diversity found in D. verticillatus is the largest reported for a clonal plant species, although the literature is too limited to determine whether this is truly unusual. Clonal diversity in D. verticillatus is likely to be regulated largely by ecological factors affecting seed production and establishment. However, genetically based sexual sterility also occurs in some populations.     

Patterns of genotypic diversity suggest a long history of clonality and population isolation in the Australian arid zone shrub Acacia carneorum

For plants capable of both sexual and clonal reproduction, the relative frequency of these reproductive modes is influenced by genetic and ecological factors. Acacia carneorum is a threatened shrub from the Australian arid zone that occurs as a set of small, spatially isolated populations. Sexual reproduction appears to be very rare: despite regular flowering, only two populations set seed. It is not known whether this reflects an ancient pattern, or results from rapid land use changes following arrival of Europeans in the region 150 years ago. We assessed genotypic variation throughout the range of A. carneorum using AFLP markers, to elucidate the relative importance of clonal and sexual reproduction in this species’ history. Clonal diversity (CD) within populations ranged from 0 to 0.820 (mean CD = 0.270, SE = 0.094), but the relative abundances of genets were typically highly skewed. On average, the two fruiting populations had higher CD (mean CD = 0.590, SE = 0.265) than non-fruiting populations (mean CD = 0.179, SE = 0.077) (t = 2.315, p = 0.049), but most populations contained multiple genets. All genets were population-specific, and there was substantial divergence among populations (Φ _ST = 0.690), implying a long history of isolation. We conclude that clonality has predominated in A. carneorum populations, with occasional sexual recruitment, and that current failure of most populations to set seed likely reflects both a long history of asexual reproduction and effects of habitat disturbance. Conservation of this species may benefit from translocations to increase genotypic diversity within populations.     

Clonality and recombination in the arctic plant Saxifraga cernua

The circumarctic clonal plant Saxifraga cernua reproduces efficiently via bulbils, largely depends on insects for pollination and appears to set seed very rarely. However, high levels of genotypic variation observed at small spatial scales in the arctic archipelago of Svalbard have been taken as evidence of occasional sexual reproduction. Here we assess the relative contributions of mutation and recombination to random amplified polymorphic DNA variation in four populations in East Greenland and re-analyse the Svalbard data. Greater variation due to recombination was predicted in Greenland than in Svalbard, because the higher summer temperatures and longer growing season likely increase the chances for sexual reproduction. Although we observed higher levels of genotypic diversity in Greenland than in Svalbard, matrix incompatibility and linkage disequilibrium measures provided no evidence of more sexuality, suggesting differences in glacial/postglacial history. The genetic structure and spatial distribution of clones suggest that clonal migration may increase variability in local populations, which is consistent with frequent large-scale migration in this species inferred from a circumarctic analysis of chloroplast DNA haplotypes. We conclude that a combination of somatic mutations and sexual reproduction has contributed to the observed patterns of genotypic diversity in the Greenland and Svalbard populations of S. cernua , and that sexual reproduction is important in maintaining genotypic diversity, despite the rarity of observations of seed setting.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 152 , 209–217.     

Polyploidy and microsatellite variation in the relict tree Prunus lusitanica L.: how effective are refugia in preserving genotypic diversity of clonal taxa?

Refugia are expected to preserve genetic variation of relict taxa, especially in polyploids, because high gene dosages could prevent genetic erosion in small isolated populations. However, other attributes linked to polyploidy, such as asexual reproduction, may strongly limit the levels of genetic variability in relict populations. Here, ploidy levels and patterns of genetic variation at nuclear microsatellite loci were analysed in Prunus lusitanica, a polyploid species with clonal reproduction that is considered a paradigmatic example of a Tertiary relict. Sampling in this study considered a total of 20 populations of three subspecies: mainland lusitanica (Iberian Peninsula and Morocco), and island azorica (Azores) and hixa (Canary Islands and Madeira). Flow cytometry results supported an octoploid genome for lusitanica and hixa, whereas a 16‐ploid level was inferred for azorica. Fixed heterozygosity of a few allele variants at most microsatellite loci resulted in levels of allelic diversity much lower than those expected for a high‐order polyploid. Islands as a whole did not contain higher levels of genetic variation (allelic or genotypic) than mainland refuges, but island populations displayed more private alleles and higher genotypic diversity in old volcanic areas. Patterns of microsatellite variation were compatible with the occurrence of clonal individuals in all but two island populations, and the incidence of clonality within populations negatively correlated with the estimated timing of colonization. Our results also suggest that gene flow has been very rare among populations, and thus population growth following founder events was apparently mediated by clonality rather than seed recruitment, especially in mainland areas. This study extends to clonal taxa the idea of oceanic islands as important refugia for biodiversity, since the conditions for generation and maintenance of clonal diversity (i.e. occasional events of sexual reproduction, mutation and/or seed immigration) appear to have been more frequent in these enclaves than in mainland areas.     

Extreme genetic diversity in asexual grass thrips populations

The continuous generation of genetic variation has been proposed as one of the main factors explaining the maintenance of sexual reproduction in nature. However, populations of asexual individuals may attain high levels of genetic diversity through within‐lineage diversification, replicate transitions to asexuality from sexual ancestors and migration. How these mechanisms affect genetic variation in populations of closely related sexual and asexual taxa can therefore provide insights into the role of genetic diversity for the maintenance of sexual reproduction. Here, we evaluate patterns of intra‐ and interpopulation genetic diversity in sexual and asexual populations of Aptinothrips rufus grass thrips. Asexual A. rufus populations are found throughout the world, whereas sexual populations appear to be confined to few locations in the Mediterranean region. We found that asexual A. rufus populations are characterized by extremely high levels of genetic diversity, both in comparison with their sexual relatives and in comparison with other asexual species. Migration is extensive among asexual populations over large geographic distances, whereas close sexual populations are strongly isolated from each other. The combination of extensive migration with replicate evolution of asexual lineages, and a past demographic expansion in at least one of them, generated high local clone diversities in A. rufus. These high clone diversities in asexual populations may mimic certain benefits conferred by sex via genetic diversity and could help explain the extreme success of asexual A. rufus populations.