B chromosome polymorphism in populations of Apodemus flavicollis in Yugoslavia

Four populations of yellow necked mice Apodemus flavicollis were karyologically studied for the presence of B chromosomes. One or two B chromosomes were found in all populations except one, in frequencies which differ significantly. The Bs cannot be accurately distinguished from chromosomes of normal complement even after application of differential staining.     

Does habitat fragmentation reduce genetic diversity and subpopulation connectivity?

The estimation of levels of genetic variation has received considerable attention because it is generally thought to be indicative of overall species vitality and the potential for evolutionary responses to environmental changes. Here, we use allozymes markers and two distinct collections of Cakile maritima, an annual species from sandy coastal habitats (2000 generation and 2005 generation collected from 9 populations in their natural habitats), to assess the magnitude of expected genetic change. We compared genetic diversity between generations (all populations combined), and then between populations at each generation. Based on 13 loci scored from the eight enzymes examined, a high genetic diversity was detected at both the population and generation level as compared to other herbaceous species. However, allelic richness reduction in the 2005 generation suggested restricted gene flow and a high risk of future genetic bottlenecks, if larger tracts of coastal areas disappear. Most loci showed deviation from Hardy‐Weinberg equilibrium due to excess of heterozygotes in all populations suggesting that this species has an allogamic mode of reproduction. It appears most likely that this species has experienced a recent decrease in population size, and that genetic drift in small populations has resulted in a loss of alleles occurring at low frequency. Despite the deterioration process, maintenance of high genetic diversity suggests that there are some ecological factors determining population structure.     

Drifting fronds and drifting alleles: range dynamics,local dispersal and habitat isolation shape the population structure of the estuarine seaweed Fucus ceranoides

Aim The seaweed Fucus ceranoides is restricted to spatially discrete estuarine habitats and lacks planktonic dispersal phases; it is therefore expected to exhibit strong population differentiation. Its cold‐temperate affinities and mtDNA variation imply that the northern part of the species’ range, where F. ceranoides is now ubiquitous, was recently colonized after the onset of the last deglaciation, potentially resulting in areas with greater genetic homogeneity. Here we examine the population structure of F. ceranoides to test these predictions, emphasizing the contrasting genetic signatures of limited dispersal in refugial versus recently colonized regions. Location North‐eastern Atlantic estuaries from Portugal to Norway. Methods A total of 504 individuals from 21 estuarine sites spanning the entire range of F. ceranoides were sampled and genotyped for nine microsatellite loci. Population structure was inferred from several genotypic and allele‐frequency analyses. Geographical patterns of genetic diversity were used to reconstruct the historical biogeography of the species. Results Genetic diversity and regional population differentiation showed a consistent decline with increasing latitude. Southernmost populations harboured most of the endemic variation, whereas the northern populations (> 55° N) were almost fixed for the same alleles across loci. In southern and central regions of its distribution, F. ceranoides showed striking population subdivision, with many of the sampled estuaries corresponding to coherent genetic units that were easily discriminated from one another with standard clustering methods. Main conclusions The geographical pattern of genetic diversity supports the long‐term refugial status of Iberia and a post‐glacial range expansion of F. ceranoides into previously glaciated latitudes. Despite the species’ capacity to colonize newly available habitats, the genetic structure of F. ceranoides outside the recently (re)colonized range reveals that gene flow between populations is extremely low. This study provides a remarkable example of how infrequent and spatially limited dispersal can have contrasting effects at the scales of meta‐population (connectivity) versus range dynamics (habitat tracking), and of how dispersal restrictions can result in either genetic divergence or homogeneity depending on the maturity and demographic conditions of the populations.     

High levels of genetic divergence between Tasmanian and Victorian platypuses, Ornithorhynchus anatinus, as revealed by microsatellite loci

The platypus, Ornithorhynchus anatinus is a unique, iconic mammal endemic to Australia. Despite being listed as ‘common’ throughout its range, platypus abundance is poorly understood. Dependence on aquatic habitats in Australia renders this species potentially vulnerable to a variety of processes including drought, climate change and habitat loss. To assist with understanding population processes, 180 individuals from Tasmania and Victoria were characterised across thirteen microsatellite loci. Large genetic differences were evident between Tasmanian and Victorian O. anatinus. Within Tasmania, high levels of allelic diversity were detected with genetic differentiation identified among some populations. Similarly, allelic diversity was high within Victorian platypuses, along with significant genetic differentiation among populations. The large genetic differences found between Tasmanian and mainland platypuses indicate long-term isolation and it is likely that the lack of past/present catchment connectedness contributes to differentiation found between populations within these regions. Understanding patterns of genetic differentiation within and between catchments will help guide future conservation management decisions for platypus.     

Genetic Variation and Population Structure of Oriental Migratory Locust, Locusta migratoria manilensis, in China by Allozyme, SSRP-PCR, and AFLP Markers

Allozyme analysis, microsatellite primer PCR (SSRP-PCR), and amplified fragment length polymorphism (AFLP) techniques were used to assess genetic diversity and population structure of the Chinese oriental migratory locust, Locusta migratoria manilensis. A total of 299 PCR markers (67 SSRPs and 232 AFLPs) were detected in eight populations, of which 98.7% were polymorphic markers. The proportion of polymorphic loci (95.5–98.8%) by SSRP+AFLP markers indicated no significant differences between populations, and all populations exhibited a similar level of variability; results of the allozyme analysis demonstrated that 19 loci gave rise to a lower level of polymorphism (55.6–66.7%). The genetic distances between the populations were relatively low. Shannon’s index and Nei’s gene diversity showed low differentiation among the populations. Allozyme analysis, however, reflected greater similarity and smaller differentiation between the populations than those shown by SSRP and AFLP markers. Neighbor-joining dendrograms derived from both the allozyme and SSRP+AFLP markers showed that the genetic distances among Chinese oriental migratory locust populations were not greatly influenced by geographic distance and breeding habitats.     

Genetic variability, population size and reproduction potential in Ligularia sibirica (L.) populations in Estonia

Ligularia sibirica (L.) Cass. (Asteraceae) is a EU Habitats Directive Annex II plant species that has suffered a lot from human-caused major changes in quality and availability of habitats in Estonia. The aim of this study was to find out if the observed decline in population size is reflected in the amount of genetic variation and fertility in remnant populations of this species. AFLP technique was used for that purpose. Genetic diversity within populations was assessed as the percentage of polymorphic loci in a given population and average gene diversity over loci. The degree of genetic differentiation among populations and genetic differentiation between pairs of populations was estimated. The amount of viable seeds per flower stem was compared among populations and between years (2007 and 2008). Average genetic diversity over loci and proportion of polymorphic loci in L. sibirica populations were significantly correlated with population size, suggesting the action of genetic drift and/or inbreeding. No correlation was found between genetic and geographic distances. Natural barriers like forests may have been efficiently preventing seed migration even between geographically closer populations. Results of this study suggest that genetic erosion could be partially responsible for the lower fitness in smaller populations of this species.     

Variability of Minisatellite Loci and mtDNA in Individuals with and without B Chromosomes from Populations of the Grasshopper <Emphasis Type="Italic">Dichroplus elongatus</Emphasis>

Dichroplus elongatus is an extensively distributed South American grasshopper considered a pest of major crops. Argentinean populations show a widespread B-chromosome polymorphism which could be maintained as the result trade-offs among opposite selective effects and interactions with their mitotic instability. The main objective of this study was to evaluate the relationships between B chromosomes and mtDNA sequences coupled with minisatellites loci, and verify the genotype/karyotype covariation in 12 populations located at both sides of Paraná River (Eastern and Western Regions). B carrier individuals showed significantly higher genetic diversity (H_E and X) respect to standard individuals. AMOVAs based on nuclear loci and mtDNA sequence datasets showed statistically significant levels of differentiation among karyotypes in the Eastern Region. Cluster analysis through Bayesian procedure considering nuclear loci splits B carriers and standard individuals into different genetic clusters in some Eastern populations. The Bayesian phylogenetic analysis showed two divergent mtDNA clades. Haplogroup 1 is composed exclusively of standard individuals, however all B chromosome carriers are included in haplogroup 2. There is an association between some haplotypes and B chromosomes and a strong effect of phylogenetic signal on B chromosome population structure. Genetic differentiation between karyotypes at Eastern Region revealed by AMOVA, Bayesian approaches and clustering analysis based on uniparental and biparental inherited markers may be due to the inherent nature of the B chromosome, to karyotype biased dispersal or to difference tolerance of B chromosomes on different genetic background. The combination of molecular and chromosome analysis performed in this study indicated that B chromosomes in D. elongatus is an important factor in explaining the genetic population structure at minisatellite and mitochondrial DNA levels.     

Sequential loss of genetic variation in flea beetle Agasicles hygrophila (Coleoptera: Chrysomelidae) following introduction into China

The flea beetle (Agasicles hygrophila) was imported to Florida, USA and then introduced from Florida into China in 1987 as a biological control agent for the invasive plant alligator weed (Alternanthera philoxeroides). The initial beetle population was subsequently used for sequential introductions in other areas of China, but little is known about the genetic consequences of the introductions. In this study, the genetic diversity and population structure of five beetle populations, the source Florida population, three intentionally introduced China populations and one accidentally dispersed China population, were examined using amplified fragment length polymorphisms. The results showed a clear pattern of decreasing genetic diversity with the sequential introductions. The diversity was highest in the Florida population followed by the first introduction to Chongqing and then in Kunming and Fuzhou. The lowest diversity was found in the accidentally dispersed Guangzhou population that was first recorded in 1996. Both loci parameters and Nei's genetic diversity showed a high variation among these populations. Genetic differentiation among populations was further verified by the G_ST statistic (0.136–0.432). Beetles in Kunming had the highest gene flow with those in Guangzhou, and therefore lowest differentiation and closest genetic distance. These data show that sequential introduction influenced the genetic diversity of populations in China. Genetic diversity should be considered in planning introduction and long‐term maintenance of populations.     

濒危灌木长叶红砂遗传多样性的RAPD分析

应用RAPD分子标记对濒危灌木长叶红砂(Reaumuria trigyna)种群遗传多样性进行了分析.应用18条随机引物对长叶红砂5个种群的95个个体进行扩增,检测到118个位点,其中多态位点105个.结果表明:长叶红砂种群的多态位点比率(P)为88.98%,显示出长叶红砂种群存在较高的遗传多样性.Shannon多样性指数(0.4966)、Nei基因多样性指数(0.3303)和基因分化系数(Gst=0.1425)的分析结果显示,长叶红砂种群遗传变异大多存在于种群内,种群间的遗传分化占14.25%.聚类分析表明,长叶红砂种群遗传距离与地理距离之间无直接相关关系.遗传多样性水平与物种特性和所处不同群落有关,濒危植物并不一定表现为遗传变异水平的降低.     

Allozymic differentiation in the land snail Arianta arbustorum (Stylommatophora, Helicidae): historical inferences

The intraspecific genetic differentiation depends to a large part on historical factors. The mode of dispersal is an important determinant of the genetic diversity especially of newly founded populations. Fast long-distance colonization of formerly unsuitable habitats results in reduced genetic diversity and lineages originating from different glacial refugia form well-defined hybrid zones, where they meet. Slow dispersers, on the other hand, preserve their refugial genetic diversity. Paleontological findings and a previous analysis of mtDNA suggest that the land snail Arianta arbustorum, which today is widely spread across Europe, has survived the Pleistocene glaciations within the boundaries of permafrost. In the present paper, we asked, whether the differentiation of nuclear markers, i.e. allozymes, supports these results. We investigated the allozymic differentiation at 15 polymorphic loci in 14 Alpine and one English populations of the land snail A. arbustorum. Various tests of genetic population differentiation yielded ambiguous results. Allele frequencies differed significantly across populations and all but five pairwise F_ST values were significant. Genetic distances between populations were comparatively high. On the other hand, an exact test of population differentiation found no differences and the assignment of individuals to their populations of origin was poor. This indicates that allozymic differentiation is constrained. Whether these constraints are of natural or methodological nature remains open. Matrices of genetic and geographical distances were correlated only after removal of the English population from the analysis. Similarly, with one exception F_ST values increased as the number of regions included in the calculations increased. Thus, on a regional scale, we largely observed a pattern of isolation by distance, which does no longer hold on a larger scale. This is well in accordance with the previous findings based on mtDNA. Mitochondrial lineages were widely distributed across Europe and overlapped extensively. Considering the low potential for dispersal of a land snail, the patchy distribution of lineages, and the high genetic diversity also found in the present study suggests that A. arbustorum has survived the Pleistocene within the boundaries of permafrost.     

Comprehensive analysis of population genetics of Phoxinus phoxinus ujmonensis in the Irtysh River: Abiotic and biotic factors

As a widely distributed species along the Irtysh River, Phoxinus phoxinus ujmonensis (Kaschtschenko, 1899) was used as a model to investigate genetic diversity and population structure as well as the influence of environmental factors on population genetics. In this study, we specifically developed 12 polymorphic microsatellite loci. The analysis of microsatellite and mtDNA markers revealed a high and a moderate genetic diversity across seven populations, respectively. Moderate differentiation was also detected among several populations, indicating the impact of habitat fragmentation and divergence. The absence of isolation by distance implied an extensive gene flow, while the presence of isolation by adaptation implied that these populations might be in the process of adapting to divergent habitats. Correlation analysis showed that abiotic factors like dissolved oxygen, pH, total dissolved solids, and conductivity in water as well as biotic factors like plankton diversity and fish species diversity had impact on genetic diversity and divergence in P. phoxinus ujmonensis populations. The results of this study will provide an insight into the effect of environmental factors on genetic diversity and contribute to the study of population genetics of sympatric species.     

Population genetic structure of Bellamya aeruginosa (Mollusca: Gastropoda: Viviparidae) in China: weak divergence across large geographic distances

Bellamya aeruginosa is a widely distributed Chinese freshwater snail that is heavily harvested, and its natural habitats are under severe threat due to fragmentation and loss. We were interested whether the large geographic distances between populations and habitat fragmentation have led to population differentiation and reduced genetic diversity in the species. To estimate the genetic diversity and population structure of B. aeruginosa, 277 individuals from 12 populations throughout its distribution range across China were sampled: two populations were sampled from the Yellow River system, eight populations from the Yangtze River system, and two populations from isolated plateau lakes. We used seven microsatellite loci and mitochondrial cytochrome oxidase I sequences to estimate population genetic parameters and test for demographic fluctuations. Our results showed that (1) the genetic diversity of B. aeruginosa was high for both markers in most of the studied populations and effective population sizes appear to be large, (2) only very low and mostly nonsignificant levels of genetic differentiation existed among the 12 populations, gene flow was generally high, and (3) relatively weak geographic structure was detected despite large geographic distances between populations. Further, no isolation by linear or stream distance was found among populations within the Yangtze River system and no signs of population bottlenecks were detected. Gene flow occurred even between far distant populations, possibly as a result of passive dispersal during flooding events, zoochoric dispersal, and/or anthropogenic translocations explaining the lack of stronger differentiation across large geographic distances. The high genetic diversity of B. aeruginosa and the weak population differentiation are likely the results of strong gene flow facilitated by passive dispersal and large population sizes suggesting that the species currently is not of conservation concern.     

Analysis of genetic diversity in Glyptosternum maculatum (Sisoridae, Siluriformes) populations with AFLP markers

We determined the genetic diversity of geographic populations from three spawning grounds (Nyang River, Lhasa River, Shetongmon Reach of Yarlung Zangbo River) of Glyptosternum maculatum with amplified fragment length polymorphism (AFLP) markers. Five primer combinations detected 332 products, 51 of them (15.4%) were polymorphic in at least one population. The Shetongmon population was found to be the richest in genetic diversity as was indicated by the percentage of polymorphic loci and heterozygosity, followed by the Nyang population and the Lhasa population. The pair-wise genetic distance between populations were all very close, ranging from 0.0015 to 0.0042 with an average of 0.0024. The genetic distance was not proportional to the geographic distance. The analysis of molecular variance demonstrated that all variation occurred within populations. The average estimated fixation index (F _st) of three populations across all polymorphic loci was −0.0184, indicating the absence of genetic differences among the three sampled populations. The differentiation among populations was not significant, and population structure was weak. Our observations will help identify the genetic relationship among populations as the first approach to understand the genetic diversity of Glyptosternum maculatum.     

Genetic variation and island biogreography: Microsatellite and mitochondrial DNA variation in island populations of the Australian bush rat, Rattus fuscipes greyii

To understand the impact of various factors on the maintenance of genetic variation in natural populations, we need to focus on situations where at least some of these factors are removed or controlled. In this study, we used highly variable, presumably neutral, microsatellite and mtDNA markers to assess the nature of genetic variation in 14 island and two mainland populations of the Australian bush rat, where there is no migration between islands. Thus we are controlling for selection and gene flow. Both marker sets revealed low levels of diversity within the small island populations and extreme differentiation between populations. For six microsatellite loci, all of the small island populations had less genetic variation than the mainland populations; reduction in allelic diversity was more pronounced than loss of heterozygosity. Kangaroo Island, the large island population, had similar levels of diversity to the mainland populations. A 442 base pair (bp) section of the mtDNA control region was screened for variation by outgroup heteroduplex analysis/temperature gradient gel electrophoresis (OHA/TGGE). Only three of the 13 small island populations showed haplotypic diversity: Gambier (2), Waldegrave (2), and Eyere (3). The level of haplotypic diversity in the small island populations was similar to that on the mainland, most likely reflecting a recent population bottleneck on the mainland. In contrast, Kangaroo Island had 9 mtDNA haplotypes. The dominant factor influencing genetic diversity on the islands was island size. No correlation was detected between genetic diversity and the time since isolation or distance form the mainland. The combination of genetic drift within and complete isolation among the small island populations has resulted in rapid and extreme population divergence. Population pair-wise comparisons of allele frequency distributions showed significant differences for all populations for all loci (F _st = 0.11–0.84, R _st = 0.07–0.99). For the mtDNA control region, 92.6% of variation was apportioned between populations; only the Pearson islands shared a haplotype. Mantel tests of pair-wise genetic distance with pair-wise geographic distance showed no significant geographical clustering of haplotypes. However, population substructuring was detected within populations where sampling was conducted over a broader geographical range, as indicated by departures from Hardy-Weinberg equilibrium. Thus substructuring in the ancestral population cannot be ruled out. The dominant evolutionary forces on the islands, after the initial founder event, are stochastic population processes such as genetic drift and mutation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic variation and population structure of oriental migratory locust, Locusta migratoria manilensis, in China by allozyme, SSRP-PCR, and AFLP markers

Allozyme analysis, microsatellite primer PCR (SSRP-PCR), and amplified fragment length polymorphism (AFLP) techniques were used to assess genetic diversity and population structure of the Chinese oriental migratory locust, Locusta migratoria manilensis. A total of 299 PCR markers (67 SSRPs and 232 AFLPs) were detected in eight populations, of which 98.7% were polymorphic markers. The proportion of polymorphic loci (95.5-98.8%) by SSRP+AFLP markers indicated no significant differences between populations, and all populations exhibited a similar level of variability; results of the allozyme analysis demonstrated that 19 loci gave rise to a lower level of polymorphism (55.6-66.7%). The genetic distances between the populations were relatively low. Shannon's index and Nei's gene diversity showed low differentiation among the populations. Allozyme analysis, however, reflected greater similarity and smaller differentiation between the populations than those shown by SSRP and AFLP markers. Neighbor-joining dendrograms derived from both the allozyme and SSRP+AFLP markers showed that the genetic distances among Chinese oriental migratory locust populations were not greatly influenced by geographic distance and breeding habitats.     

Genetic structure among populations in the endemic Hawaiian Plantago lineage: insights from microsatellite variation

Endemic Hawaiian species in the genus Plantago show considerable morphological and ecological diversity. Despite their variation, a recent phylogenetic analysis based on DNA sequence data showed that the group is monophyletic and that sequence variation among species and morphotypes is low. This lack of sequence polymorphisms resulted in an inability to resolve species and population affinities within the most recently derived clade of this lineage. To assess species boundaries, population genetic structure and interpopulation connectivity among the morphologically and ecologically distinct populations within this clade, genetic variation was examined using eight microsatellite loci. Within‐population genetic diversity was found to be lowest in the Maunaiu, Hawai'i population of the endangered P. hawaiensis, and highest in the large P. pachyphylla population from 'Eke, West Maui. Isolation by distance across the range of populations was detected and indicated restricted dispersal. This result is likely to be attributable to few interisland dispersal events in the evolutionary history of this lineage. Genetic differentiation within islands tended to be higher among populations occurring in contrasting bog and woodland habitats, suggesting ecological barriers to gene flow and the potential role of ecological divergence in population diversification. Overall, these results are consistent with findings from phylogenetic analysis of the entire lineage. Our data bring new insights regarding patterns of dispersal and population genetic structure to this endemic and endangered group of island taxa. As island environments become increasingly fragmented, information of this type has important implications for the successful management of these fragile populations and habitats.     

Genetic Differentiation of the Tuva Population with Respect to the AluInsertions

Polymorphism of three rural populations of the Tuva Republic was examined using a set of five autosomal Aluinsertions at the ACE, PLAT, PV92, APOA1,and F13Bloci. The allele frequency distribution patterns revealed in Tuvinians were typical to Mongoloid populations of Asia and were characterized by relatively high frequency of the Alu-repeat insertion at the PV92and F13Bloci along with relatively low insertion frequency at the APOA1locus. With respect to the test systems used, Tuvinian populations examined displayed high levels of genetic diversity. The mean expected heterozygosity values in the populations of Kungurtug, Toora-Khem, and Teeli were 0.433, 0.407, and 0.437, respectively. The level of genetic diversity in the pooled Tuvinian sample was 0.432. The coefficient of genetic differentiation in the three populations studied was 1.4% pointing to relatively low level of genetic subdivision of the indigenous Tuvinian populations. However, estimates of genetic differentiation of the Tuvinian gene pool made by use of the Alu-repeat system were higher compared to those performed using classical protein systems, mtDNA, or Y-chromosomal haplotypes. Even though Tuvinian populations were characterized by common gene pool, some features specific to Western Tuvinian population could be distinguished. These features could be associated with higher contribution of the Caucasian component to the gene pool of this population. Phylogenetic analysis demonstrated close genetic relationships between the Tuvinian and Altaic ethnic populations.     

The mammalian model for population studies of B chromosomes: the wood mouse (Apodemus)

The presence of B chromosomes was reported in six species of the genus Apodemus (A. peninsulae, A. agrarius, A. sylvaticus, A. flavicollis, A. mystacinus, A. argenteus). High frequencies of Bs were recorded particularly in A. peninsulae and A. flavicollis. The origin of Bs in Apodemus seems to be rather ancient, and it is possible that the supernumerary elements, and/or a tendency for their appearance, were inherited from the common ancestor of the extant species. We have not found any correlated changes between frequencies of Bs and the level of protein polymorphism and/or heterozygosity assessed in electrophoretic studies. No measurable effect of Bs on overall genetic variability was thus revealed in studied populations. The pattern of evolutionary dynamics of Bs can be distinctly different between geographical populations, and both the parasitic and the heterotic models can be applied to explain the maintenance of Bs in different populations. Further studies are desirable to improve our understanding of the complicated evolutionary dynamics of Bs in the Apodemus species. An essential condition for success in this respect is much more detailed information on inheritance and the molecular structure of Bs.     

Habitat loss other than fragmentation per se decreased nuclear and chloroplast genetic diversity in a monoecious tree

Generally, effect of fragmentation per se on biodiversity has not been separated from the effect of habitat loss. In this paper, using nDNA and cpDNA SSRs, we studied genetic diversity of Castanopsis sclerophylla (Lindl. & Paxton) Schotty populations and decoupled the effects of habitat loss and fragmentation per se. We selected seven nuclear and six cpDNA microsatellite loci and genotyped 460 individuals from mainland and island populations, which were located in the impoundment created in 1959. Number of alleles per locus of populations in larger habitats was significantly higher than that in smaller habitats. There was a significant relationship between the number of alleles per locus and habitat size. Based on this relationship, the predicted genetic diversity of an imaginary population of size equaling the total area of the islands was lower than that of the global population on the islands. Re-sampling demonstrated that low genetic diversity of populations in small habitats was caused by unevenness in sample size. Fisher's α index was similar among habitat types. These results indicate that the decreased nuclear and chloroplast genetic diversity of populations in smaller habitats was mainly caused by habitat loss. For nuclear and chloroplast microsatellite loci, values of F(ST) were 0.066 and 0.893, respectively, and the calculated pollen/seed dispersal ratio was 162.2. When separated into pre-and post-fragmentation cohorts, pollen/seed ratios were 121.2 and 189.5, respectively. Our results suggest that habitat loss explains the early decrease in genetic diversity, while fragmentation per se may play a major role in inbreeding and differentiation among fragmented populations and later loss of genetic diversity.     

LOW GENIC DIFFERENTIATION AMONG ISOLATED POPULATIONS OF THE CALIFORNIA FAN PALM (WASHINGTONIA FILIFERA)

The purpose of this study was to assess the relative roles of population size and geographic isolation in determining population-genetic structure. Using electrophoretic techniques to quantify allozymic variation at 16 genetic loci, we measured genic variation within and among 16 natural populations of the California fan palm (Washingtonia filifera). Genotypes were determined for every individual in each population so that parametric values rather than sample estimates for measures of genic variability were obtained. Palm populations displayed low levels of within-population variability. The proportion of polymorphic loci and observed heterozygosity were 0.098 and 0.009 per population, respectively. Population size displayed a significant positive correlation with proportion of polymorphic loci, but not with observed heterozygosity. Low levels of genetic differentiation among populations were demonstrated by an F-statistic analysis and the computation of genetic similarity values. A hierarchical analysis of gene diversity revealed that only about 2% of the total gene diversity in W. filifera resides as among-population diversity. Climatic and geological changes since the Pliocene have eliminated widespread palm populations, and the species is presently restricted to isolated locations around the Colorado Desert. Existing populations in southern California are either relicts or recent recolonizations resulting from the dispersal of seeds from a refugium population in Baja California, Mexico. The observed patterns of low within- and low among-population genic diversity seem most consistent with a recent colonization by fan palms. It is hypothesized that stochastic processes reduced levels of genic variability in this refugium population during its formation. Dispersal of seeds from this refugium into suitable habitats in the Colorado Desert would produce populations with low variability and high genetic similarity because of their common ancestry. However, low intrapopulation variability and genetic homogeneity across populations could be the product of uniform selection pressures favoring a narrow array of specialized genotypes in either relict or colonizing populations.