Population genetic structure of the gynodioecious Thymus vulgaris L. (Labiatae) in southern France

In a self-compatible gynodioecious species, the abundance of female plants (which are obligate outcrossers) relative to hermaphrodites (which may self and outcross) may be a critical factor influencing genetic diversity and population structure. In the gynodioecious Thymus vulgaris L., female frequency varies from 5 to 95%, providing a suitable model to examine this issue. In this study, we use allozyme markers to (1) evaluate the relationship between female frequency, genetic diversity and population structure, (2) determine whether females and hermaphrodites vary in heterozygote deficiency and (3) examine whether other factors such as plant density are related to heterozygote deficiency. Twenty three natural populations, with female frequencies ranging from 11 to 92%, were sampled in and around the St-Martin-de-Londres basin in southern France. Based on four polymorphic allozyme loci, we found no significant correlation between female frequency and heterozygote deficiency. A significant (P < 0.05) F_IS value over loci and over populations of 0.11 was detected. The F_IS value per population showed a significant heterozygote deficiency in 11 of the 23 populations. However, no significant difference between female and hermaphrodite F_IS values was found. A significant heterozygote deficiency only occurred in populations of intermediate density. There was little differentiation among populations (F_ST = 0.038) nor among subpopulations within each population. The significant F_IS values are thus mostly due to inbreeding effects. The lack of a correlation between F_IS values and female frequency may be due to outcrossing in hermaphrodites and/or restoration of male fertility which may occur to a greater extent at low female frequency. The similarity of female and hermaphrodite F_IS values indicates that females may occasion high levels of biparental inbreeding.     

A one‐locus model of androdioecy with two homomorphic self‐incompatibility groups: expected vs. observed male frequencies

Androdioecy, the occurrence of males and hermaphrodites in a single population, is a rare breeding system because the conditions for maintenance of males are restrictive. In the androdioecious shrub Phillyrea angustifolia, high male frequencies are observed in some populations. The species has a sporophytic self‐incompatibility (SI) system with two self‐incompatibility groups, which ensures that two groups of hermaphrodites can each mate only with the other group, whereas males can fertilize hermaphrodites of both groups. Here, we analyse a population genetic model to investigate the dynamics of such an androdioecious species, assuming that self‐incompatibility and sex phenotypes are determined by a single locus. Our model confirms a previous prediction that a slight reproductive advantage of males relative to hermaphrodites allows the maintenance of males at high equilibrium frequencies. The model predicts different equilibria between hermaphrodites of the two SI groups and males, depending on the male advantage, the initial composition of the population and the population size, whose effect is studied through stochastic simulations. Although the model can generate high male frequencies, observed frequencies are considerably higher than the model predicts. We finally discuss how this model may help explain the large male frequency variation observed in other androdioecious species of Oleaceae: some species show only androdioecious populations, as P. angustifolia, whereas others show populations either completely hermaphrodite or androdioecious.     

Inbreeding variability and population structure in the invasive haplodiploid palm‐seed borer (Coccotrypes dactyliperda)

We investigated the mating system and population genetic structure of the invasive haplodiploid palm‐seed borer Coccotrypes dactyliperda in California. We focused on whether these primarily inbreeding beetles have a ‘mixed‐breeding’ system that includes occasional outbreeding, and whether local inbreeding coefficients (F_IS) varied with dominant environmental factors. We also analysed the genetic structure of C. dactyliperda populations across local and regional scales. Based on the analysis of genetic variation at seven microsatellite loci in 1034 individual beetles from 59 populations, we found both high rates of inbreeding and plentiful evidence of mixed‐breeding. F_IS ranged from ?0.56 to 0.90, the highest variability reported within any animal species. There was a negative correlation between F_IS and latitude, suggesting that some latitude‐associated factor affecting mating decisions influenced inbreeding rates. Multiple regressions suggested that precipitation, but not temperature, may be an important correlate. Finally, we found highly significant genetic differentiation among sites, even over short geographic distances (< 1000 m).     

The role of hybridization in the evolution of sexual system diversity in a clonal,aquatic plant

The stable coexistence within populations of females, males, and hermaphrodites (subdioecy) is enigmatic because theoretical models indicate that maintenance of this sexual system involves highly restricted conditions. Subdioecy is more commonly interpreted as a transitory stage along the gynodioecious pathway from hermaphroditism to dioecy. The widespread, North American, aquatic plant Sagittaria latifolia is largely composed of monoecious or dioecious populations; however, subdioecious populations with high frequencies of hermaphrodites (mean frequency = 0.50) characterize the northern range boundary of dioecy in eastern North America. We investigated two hypotheses for the origin of subdioecy in this region. Using polymorphic microsatellite loci, we evaluated whether subdioecy arises through selection on standing genetic variation for male sex inconstancy in dioecious populations, or results from hybridization between monoecious and dioecious populations. We found evidence for both pathways to subdioecy, although hybridization was the more common mechanism, with genetic evidence of admixture in nine of 14 subdioecious populations examined. Hybridization has also played a role in the origin of androdioecious populations in S. latifolia, a mechanism not often considered in the evolution of this rare sexual system. Our study demonstrates how hybridization has the potential to play a role in the diversification of plant sexual systems.     

Genetic Structure of Blastocerus dichotomus Populations in the Paraná River Basin (Brazil) Based on Protein Variability

The population structure of 147 marsh deer (Blastocerus dichotomus) from three areas in the Paraná River basin, Brazil, was studied by observing protein polymorphism at 17 loci. Six loci were polymorphic and 11 monomorphic. The proportion of polymorphic loci (P) was 35.29% and the average heterozygosity (H) was 6.31%. Wright's F_ST indicated that only 4.9% of the total variation in allelic frequencies was due to genetic differences between the three groups. The high value of F_IS (0.246) indicated inbreeding in the marsh deer. Genetic distance values (D = 0.014–0.051) showed little divergence between the three areas. We suggest that probable mechanisms accounting for the genetic structure are female phylopatry and polygyny and also that inbreeding has resulted from decreasing areas of wetland leading to isolation, overhunting, and diseases transmitted by cattle.     

Genetic diversity of Cryptomeria japonica using co-dominant DNA markers based on sequenced-tagged sites

 We have investigated the genetic diversity of 11 natural populations of C. japonica using 13 polymorphic STS markers. The average unbiased heterozygosities (H _e ), the average number of alleles per locus (N _a ) and the proportion of polymorphic loci (Pl) were 0.281, 1.93 and 76.92%, respectively. Coefficients of linkage disequilibrium were calculated, and no significant deviation was found except in four combinations – which might have occurred by chance alone. The fixation index (F _IS ) for 3 loci showed statistically significant values at the 1% level. The genetic differentiation between populations was only 0.047, and there were no clear geographical tendencies in the allele frequencies or the heterozygosities among populations. Consequently, the results from STS-based co-dominant DNA marker analysis were very similar to those from a previous allozyme study. However, the resolution of the technique is greater than allozyme analysis because many loci with high heterozygosities can be evaluated, and it is very simple. Therefore, the STS-based marker approach is very useful and convenient for population genetics and genome mapping of C. japonica. Received: 18 July 1998 / Accepted: 13 August 1998     

Mixed mating in androdioecious Mercurialis annua inferred using progeny arrays and diploid-acting microsatellite loci in a hexaploid background

Background and Aims

The frequency at which males can be maintained with hermaphrodites in androdioecious populations is predicted to depend on the selfing rate, because self-fertilization by hermaphrodites reduces prospective siring opportunities for males. In particular, high selfing rates by hermaphrodites are expected to exclude males from a population. Here, the first estimates are provided of the mating system from two wild hexaploid populations of the androdioecious European wind-pollinated plant M. annua with contrasting male frequencies.

Methods

Four diploid microsatellite loci were used to genotype 19–20 progeny arrays from two populations of M. annua, one with males and one without. Mating-system parameters were estimated using the program MLTR.

Key Results

Both populations had similar, intermediate outcrossing rates (t_m = 0·64 and 0·52 for the population with and without males, respectively). The population without males showed a lower level of correlated paternity and biparental inbreeding and higher allelic richness and gene diversity than the population with males.

Conclusions

The results demonstrate the utility of new diploid microsatellite loci for mating system analysis in a hexaploid plant. It would appear that androdioecious M. annua has a mixed-mating system in the wild, an uncommon finding for wind-pollinated species. This study sets a foundation for future research to assess the relative importance of the sexual system, plant-density variation and stochastic processes for the regulation of male frequencies in M. annua over space and time.     

High male reproductive success of hermaphrodites in the androdioecious Phillyrea angustifolia

Androdioecy, the coexistence of males and hermaphrodites within a population, is a rare breeding system, often considered as unlikely to evolve because of restrictive conditions for its maintenance. Phillyrea angustifolia, a wind-pollinated shrub, is one of the handful species reported to be androdioecious. Our previous studies have shown that natural populations of this species in southern France exhibit higher male frequencies (approximately 50%) than predicted on theoretical grounds. Thus, the male functionality of hermaphrodites is still debated. To assess the functional breeding system of this species in the wild, a paternity analysis was performed with two highly polymorphic microsatellite loci on 729 seeds collected on 10 maternal shrubs in a natural population of 24 mature individuals of P. angustifolia. A large proportion of seeds were found to have been sired by pollen from outside the population. Analysis of seeds sired by individuals within the study population revealed a high male fertility of hermaphrodites resulting in a low male advantage in fertility for male plants. Intermate distances were found to have a strong impact on male reproductive success, whereas sexual morph had no effect, with males and hermaphrodites performing equivalently. This study is the first to unequivocally document the occurrence of a male function of hermaphrodites in a natural population of an androdioecious species.     

Estimation and adjustment of microsatellite null alleles in nonequilibrium populations

Nonamplified (null) alleles are a common feature of microsatellite genotyping and can bias estimates of allele and genotype frequencies, thereby hindering population genetic analyses. The frequency of microsatellite null alleles in diploid populations can be estimated for populations that are in Hardy–Weinberg equilibrium. However, many microsatellite data sets are from nonequilibrium populations, often with known inbreeding coefficients (F) or fixation indices (F_IS or F_ST). Here, we propose a novel null allele estimator that can be used to estimate the null allele frequency and adjust visible allele frequencies in populations for which independent estimates of F, F_IS or F_ST are available. The algorithm is currently available as an Excel macro that can be downloaded at no cost from http://www.microchecker.hull.ac.uk/ and will be incorporated into the software micro ‐checker .     

THE INFLUENCE OF DISPERSAL PATTERNS AND MATING SYSTEM ON GENETIC DIFFERENTIATION WITHIN AND BETWEEN POPULATIONS OF THE RED HOWLER MONKEY (ALOUATTA SENICULUS)

The relationship between social structure and partitioning of genetic variance was examined in two red howler monkey populations (W and G) in Venezuela, one of which (G) was undergoing rapid growth through colonization by new troops. Rates and patterns of gene flow had been determined through radiotelemetry and direct observation data on solitary migrants, and 10 years of troop censusing. Standard electrophoresis techniques were used to examine 29 loci in blood samples taken from 137 of the study animals. Analysis of genetic variance demonstrated: (1) a significantly high level of genetic variation among troops within populations (F_ST = 0.225 for W and 0.142 for G), and (2) a significant excess of heterozygosity within troops relative to expected (F_IS = -0.136 for W and -0.064 for G), despite relatively high levels of observed and inferred inbreeding in W. Differences between the populations in F_ST values conformed to those predicted based on differences in colonization rate. Comparison of partitioning of genetic variance among different genealogical subsets of troops demonstrated that the pattern of genetic differentiation observed among troops within populations was promoted by an essentially single-male harem breeding structure, a very low rate of random exchange of breeding males among troops, and a high degree of relatedness among troop females. Between-troop genetic differentiation (F_ST) was thereby increased relative to that expected from other types of social organization, while the correlation between uniting gametes within troops (F_IS) was decreased. Genetic differentiation between populations (2%) corresponded to that predicted from migration rates. Such a mosaic of genetic variation, combined with differences in reproductive success observed among troops and a high troop failure rate, create conditions in which interdemic selection could result in more rapid spread of advantageous gene combinations than would be expected in a panmictic population, particularly in a colonizing situation in which the founder population is small.     

Population structure and pattern of geographic variation in Muscari comosum along its range of distribution

Genetic variation for six loci in 37 populations of Muscari comosum L. (Liliaeeae) is surveyed. One locus is monomorphic and identical in all the populations. The remaining loci are polymorphic. Although the GOT-1 and GOT-3 loci show a pronounced heterozygote deficit explained by selection acting upon these loci (or on genes linked to them), the remaining loci nearly conform to Hardy-Weinberg proportions. The overall pattern shows a low level of heterozygote deficit (F_IS=0.08) explained by the mixed mating system. The organization of genetic variation shows a low level of interpopulation differentiation (F_ST or G_ST=0.04). At the same time, autocorrelation analysis shows no pattern of geographical variation. It is concluded that gene flow and selection interact to produce the overall pattern of genetic variation.     

Genetic variability in white-tailed deer (Odocoileus virginianus) and its relationship to environmental parameters and herd origin (Cervidae)

Allozyme variation was examined in 1571 white-tailed deer (Odocoileus virginianus) from 29 localities in Tennessee by starch gel electrophoresis. For 11 polymorphic loci, sex-related, age-related and temporal differences were minimal. However, significant spatial hererogeneity was evident in genotypes (contingency table results), allele frequencies (F _ST=0.057) and heterozygosity. Heterozygosity ranged from 16.9% to 26.8% with a mean of 22.9%. The spatial pattern of allele frequencies determined from Rogers' coefficients of genetic similarity indicated associations based on geographic proximity and stocking history. In hierarchial analyses, physiographic regions accounted for more of the total gene diversity than herd origin groups (populations of similar origin) but less than individual populations. For five loci, physiographic regions accounted for more of the gene diversity than populations, suggesting a selection role in the observed genetic variability. Bivariate and canonical correlation analyses revealed significant associations between environmental and genetic variables. Temperature variables and allele frequencies for three loci (alcohol dehydrogenase, alpha-glycerophosphate dehydrogenase, sorbitol dehydrogenase) had the prominent roles in the multivariate association between environmental and genetic variables. Herd origin, gene flow and selection appear to be involved in the gene diversity in deer from Tennesee.     

Isolation and characterization of 13 polymorphic microsatellite loci from the clam shrimp Eulimnadia texana (Crustacea: Spinicaudata)

Thirteen polymorphic microsatellite loci were isolated and characterized from the clam shrimp Eulimnadia texana. In analyses of 20–50 individuals from two populations the number of alleles ranged from two to seven with observed heterozygosity ranging between 0.00 and 0.37. The low values for heterozygosity were not unexpected for a group characterized by its unusual androdioecious mating system, in which males compete with self‐compatible hermaphrodites for offspring production. These microsatellites are likely to be useful for further evolutionary investigations of this rare mating system in these crustaceans.     

Interspecific Genetic Variability of the Oyster Mushroom Pleurotus ostreatusAs Revealed by Allozyme Gene Analysis

Allozyme variation in natural populations of basidiomycete fungus Pleurotus ostreatus (88 individuals) from three regions of central Russia was studied. The species was shown to have 92.86% of polymorphic allozyme loci and expected heterozygosity H _e = 0.49. The mean number of alleles per locus was 3.5. The genetic differences among populations were supported by F-statistics (F _ST = 0.750). The low level of inbreeding (F _IS = 0.018) suggests that the P. ostreatus populations are panmictic, and the main reproduction mode involves basidiospores dispersing at long distances. Using cluster analysis, geographically isolated populations and intersterile groups were differentiated within the complex P. ostreatus species.     

Genetic variation and population structure in Korean populations of sand dune speciesSalsola komarovi (Chenopodiaceae)

Salsola komarovi lljin is a herbaceous annual native to the sand dunes and beaches of Japan, northern China, Sakhalln and Korea. Starch-gel electrophoresis was conducted on leaves and stems collected from 300 plants in eight Korean populations. The mean number of alleles per locus (A _p=1.51), mean expected heterozygosity (He _p=0.116), and total genetic diversity (H _T=0.279) were comparable with those for species with similar life history and ecological traits. A general conformance of genotype frequencies to Hardy-Weinberg expectations (meanF _IS=−0.030) indicates thatS. komarovi is an outcrossing species. Slightly more than 20% of the genetic variation was found among populations (F _ST=0.204). In addition, significant differences in allele frequency were detected between populations at all 11 polymorphic loci (P<0.001). Nei's genetic identities range from 0.885 to 0.985 with a mean of 0.942. However, indirect estimates of the number of migrant per generation (0.97, calculated fromF _ST and 0.31, calculated from seven private alleles) indicate that the levels of gene flow is low among Korean populations. Although the species maintains a moderate level of genetic variation within populations, the small, isolated natural populations of the species have been severely destructed by human activities, particularly in summer season. If this is true, conservation efforts should be focused on those populations that currently maintain the most genetic diversity (e.g., populations of Cheju Island and coast of the southwestern Korean Peninsula).     

Is Phillyrea angustifolia L. (Oleaceae) an androdioecious species?

LEPART, J. & DOMMEE B., 1992. Is Phillyrea angustifolia L. (Oleaceae) an androdioecious species? Observations during two or three years on three natural populations of Phillyrea angustifolia growing in contrasting environments in southern France show that two distinct phenotypes occur in approximately equal proportions: hermaphrodites, which produce pollen and seeds, and males, which produce pollen only. The populations studied are thus morphologically androdioecious. Biometric investigation of the two morphs shows that they are clearly distinguished by (1) the shape of the stigma and (2) the ovary which is well developed in hermaphrodites and aborted in males. Neither size of anthers and corolla, nor pollen germination, differ between the two sexes. In the Camargue, where P. angustifolia is presently an invader, controlled crosses between hermaphrodites (selfing and inter-individual) are partially successful. In contrast, such crosses are sterile in the well-established population of the La Gardiole. Hence the Camargue population appears to be functionally androdioecious and La Gardiole functionally dioecious. This breeding system is particularly puzzling, since the occurrence of anemogamy and possible selfing do not fit classical explanations of androdioecy. The ability of hermaphrodites to be functionally male and female could be particularly adaptive in founding populations. Since functional hermaphroditism occurs in expanding populations of the Camargue population, we consider that the androdioecy of P. angustifolia could be a kind of leaky dioecism.     

Genetic diversity and population structure of two important medicinal plant species <Emphasis Type="Italic">Schisandra chinensis</Emphasis> and <Emphasis Type="Italic">Schisandra sphenanthera</Emphasis> revealed by nuclear microsatellites

Seven polymorphic and transferable nuclear microsatellites were used to investigate the population structure of genetic diversity of Schisandra chinensis and Schisandra sphenanthera for facilitating their conservation and sustainable utilization. High levels of gene diversity were revealed in these two medicinal species, the majority of genetic diversity was harbored within populations, and population structure was might due to restricted gene flow among populations. Isolation by distance was close to significance in S. chinensis but not in S. sphenanthera. In S. chinensis, null alleles were identified as a cause for excess of homozygotes at loci G24 and WGA60, but inbreeding might also be partly responsible for the positive F _IS values in this species. In contrast, null allele frequencies were high at all the seven loci in S. sphenanthera and resulted in overestimation of fixation index. The strategy for ex situ conservation of these two medicinal species is discussed based on the genetic results.     

High population differentiation and genetic variation in the endangered Mexican pine Pinus Rzedowskii (Pinaceae)

Pinus rzedowskii is an endangered pine species from Michoaca´n (central Me´xico), which has been previously reported from only three localities. Classified within the subgenus Strobus, it exhibits intermediate morphological characters between subgenera Strobus and Pinus. We analyzed genetic aspects that could shed light on the evolution and conservation of this species. The genetic structure of nine populations was examined using 14 isozyme loci. Pinus rzedowskii has a relatively high level of genetic variation with 46.8% of the loci assayed being polymorphic, a total of 35 alleles, and a mean heterozygosity per population of 0.219. We calculated Wright's F_ST statistic to estimate gene flow indirectly and to evaluate whether or not there was genetic structuring among populations. We found a marked differentiation among populations (F_ST = 0.175) and significant inbreeding (F_IS = 0.247). No pattern of isolation by distance was found. We also constructed a dendrogram based on a genetic distance matrix to obtain an overview of the possible historical relationships among populations. Finally, we found a convex relationship between the genetic distance among populations and the number of ancestral lineages, suggesting that demographically this species has not been at risk recently. Although endangered, with small and fragmented populations, P. rzedowskii shows higher levels of genetic variation than other conifer species with larger populations or similar conservation status.     

Isozyme variation among teak (Tectona grandis L.f.) provenances

Fourteen enzyme systems were analysed in leaf parenchyma of nine native and introduced populations of teak. These enzyme systems were encoded by 20 putative loci of which 18 were polymorphic. Populations showed a general lack of heterozygosity (average F_IS = 0.11). On average over the 18 polymorphic loci, the genetic differentiation among provenances varied according to the estimator: 0.09 for G_ST, 0.12 for F_ST and 0.19 for . The cluster analysis showed two main gene pools, the first consisted of the Indian provenances and the second of African, Indonesian and Thai provenances. Genetic distances among populations of the same group were similar, and lower than the genetic distances between populations from different groups. The factorial analysis on genotypes of seedlings also showed the same geographic differentiation into two major groups. The possible natural distribution of teak in Java is discussed.     

Discriminating between the Effects of Founding Events and Reproductive Mode on the Genetic Structure of Triops Populations (Branchiopoda: Notostraca)

Crustaceans that initially colonize a freshwater temporary pond can strongly bias the subsequent genetic composition of the population, causing nearby populations to be genetically distinct. In addition, these crustaceans have various reproductive modes that can influence genetic differentiation and diversity within and between populations. We report on two species of tadpole shrimp, Triops newberryi and Triops longicaudatus “short”, with different reproductive modes. Reproduction in the tadpole shrimp can occur clonally (parthenogenesis), with self fertilization (hermaphroditism), or through outcrossing of hermaphrodites with males (androdioecy). For all these reproductive modes, population genetic theory predicts decreased genetic diversity and increased population differentiation. Here we use mitochondrial control region (mtCR) sequences and nuclear microsatellite loci to determine if the difference in reproductive mode affects the high genetic structure typical of persistent founder effects. Previous authors indicated that T. newberryi is androdioecious because populations are composed of hermaphrodites and males, and T. longicaudatus “short” is hermaphroditic or parthenogenetic because males are absent. In our data, T. newberryi and T. longicaudatus “short” populations were highly structured genetically over short geographic distances for mtCR sequences and microsatellite loci (T. newberryi: Φ_ST = 0.644, F _ST = 0.252, respectively; T. l. “short”: invariant mtCR sequences, F _ST = 0.600). Differences between the two Triops species in a number of diversity measures were generally consistent with expectations from population genetic theory regarding reproductive mode; however, three of four comparisons were not statistically significant. We conclude the high genetic differentiation between populations is likely due to founder effects and results suggest both species are composed of selfing hermaphrodites with some level of outcrossing; the presence of males in T. newberryi does not appreciably reduce inbreeding. We cannot exclude the possibility that males in T. newberryi are non-reproductive individuals and the two species have the same mating system.