Overall Levels Of Genetic Diversity Research Articles

Inferring the demographic history of Japanese eel (Anguilla japonica) from genomic data: Insights for conservation and fisheries management

Abstract Assessing the status or population size of species is a key task for wildlife conservation and the sustainable management of harvested species. In particular, assessing historical changes in population size provides an evolutionary perspective on current population dynamics. Japanese eel (Anguilla japonica) is an endangered yet commercially important catadromous fish species. This article assesses the demographic history of Japanese eel using the pairwise and multiple sequentially Markovian coalescent methods. The analyses indicate a reduction in effective population size Ne from 38,000 to 10,000 individuals between 4 and 1 Mya, followed by an increase to 80,000 individuals between 1 Mya and 22–30 kya. Approximately 22–30 kya there is evidence for a reduction in Ne to approximately 60,000 individuals. These events may be related to changes in environmental conditions, especially around the last glacial maximum (19–33 kya). The results of this study suggest that Japanese eel has experienced at least two population bottlenecks, interspersed by a period of population growth. The overall level of genetic diversity is relatively low, although there is no evidence for inbreeding. Data from this study will be used to help model the extinction risk of Japanese eel.

Characterization of reproductive gene diversity in the endangered Tasmanian devil.

Interindividual variation at genes known to play a role in reproduction may impact reproductive fitness. The Tasmanian devil is an endangered Australian marsupial with low genetic diversity. Recent work has shown concerning declines in productivity in both wild and captive populations over time. Understanding whether functional diversity exists at reproductive genes in the Tasmanian devil is a key first step in identifying genes that may influence productivity. We characterized single nucleotide polymorphisms (SNPs) at 214 genes involved in reproduction in 37 Tasmanian devils. Twenty genes contained nonsynonymous substitutions, with genes involved in embryogenesis, fertilization and hormonal regulation of reproduction displaying greater numbers of nonsynonymous SNPs than synonymous SNPs. Two genes, ADAMTS9 and NANOG, showed putative signatures of balancing selection indicating that natural selection is maintaining diversity at these genes despite the species exhibiting low overall levels of genetic diversity. We will use this information in future to examine the interplay between reproductive gene variation and reproductive fitness in Tasmanian devil populations.

Effects of landscape heterogeneity on population genetic structure and demography of Amazonian phyllostomid bats

Anthropogenic landscape changes resulting in habitat degradation, loss and fragmentation might decrease connectivity and effective sizes of wild populations, threatening local biodiversity. Assumed large distributions of both species and habitats have probably discouraged studies correlating altered landscapes’ structure and composition to patterns of genetic diversity in the Neotropics. Yet, considering the increasingly high rates of deforestation and habitat conversion in Amazonia, investigating local and regional-scale effects of habitat conversion on Amazonian mammals is of paramount importance. Here, we use widespread, non-threatened, phyllostomid bats to understand how current forest dynamics are influencing genetic diversity patterns and functional connectivity of eastern Amazonian volant mammals. Four frugivorous species, Carollia perspicillata, Rhinophylla pumilio, Dermanura gnoma and Artibeus obscurus, were sampled across a heterogeneously fragmented region. Twelve more or less isolated rainforest remnants surrounded by urban areas, pastures, crops, secondary forests and other land uses comprise the studied landscapes, and are compared using a landscape genetics approach applied to mitochondrial DNA loci. Here, we report species-specific consequences of habitat fragmentation. Although the overall levels of genetic diversity were high, our data show that (i) population structure is heterogeneous across the altered landscapes; (ii) landscapes with higher habitat availability harbour populations with higher genetic diversity; and (iii) the populations assessed might be demographically declining.

Genetic Diversity Within and Among Populations of the Endangered Southeastern North American Plant Species, Tiedemannia canbyi (Apiaceae) and its More Common Congener, T. filiformis

Genetic analyses can provide important insights for the successful reestablishment or augmentation of populations of endangered plant species. Tiedemannia canbyi (Apiaceae) is an endangered southeastern US coastal plain species whose natural range extends from southwestern Georgia to southeastern North Carolina with a disjunct (approximately 600 km) population in Maryland. T. canbyi currently has only approximately 40 known populations. Thirty nuclear allozyme loci were employed to determine the levels and distribution of neutral genetic diversity within and among 14 populations of T. canbyi and eight populations of its more common and widespread congener, T. filiformis. Both species exhibited relatively high levels of genetic diversity (He = 0.185 and 0.284 for T. canbyi and T. filiformis, respectively). The Maryland population of T. canbyi had the lowest overall level of genetic diversity (He = 0.089). Genetic differentiation among populations of both species was similar (GST = 0.250 and 0.254). Populations of T. canbyi formed two genetically distinct groups, southwest and south-central Georgia vs. eastern Georgia, South Carolina, and Maryland. Six T. filiformis populations from southern and coastal Georgia and coastal South Carolina formed one genetically similar group while an inland population from Georgia and an inland South Carolina population were genetically distinct from each other and from the six coastal populations. Patterns of genetic variation observed for these two Tiedemannia species are most consistent with the different groups of populations having originated from genetically discrete glacial refugia. The value of these results for the restoration and/ or augmentation of T. canbyi populations is discussed.

Collective effects of common single nucleotide polymorphisms and genetic risk prediction in type 1 diabetes.

Type 1 diabetes (T1D) is a common autoimmune disease and may be related to multiple genetic and environmental risk factors. Previous genetic studies have focused on looking for individual polymorphic risk variants. Here, we studied the overall levels of genetic diversity in T1D patients by making use of a previously published study including 1865 cases and 2828 reference samples with genotyping data for 500 K common single nucleotide polymorphisms (SNPs). We determined the minor allele (MA) status of each SNP in the reference samples and calculated the total number of MAs or minor allele contents (MAC) of each individual. We found the average MAC of cases to be greater than that of the reference samples. By focusing on MAs with strong linkage to cases, we further identified a set of 112 SNPs that could predict 19.19% of cases. These results suggest that overall genetic variation over a threshold level may be a risk factor in T1D and provide a new genetic method for predicting the disorder.

Seasonal cycle of inbreeding and recombination of the parasitic mite Varroa destructor in honeybee colonies and its implications for the selection of acaricide resistance

Varroa destructor is the most devastating parasite of the Western honeybee, Apis mellifera. In the light of the arm race opposing the host and its parasite, the population dynamics and genetic diversity of these organisms are key parameters. However, the life cycle of V. destructor is characterized by extreme inbreeding due to full sibling mating in the host brood cells. We here present an equation reflecting the evolution of inbreeding in such a clonal system, and compare our predictions with empirical data based on the analysis of seven microsatellite markers. This comparison revealed that the mites perform essentially incestuous mating in the beginning of the brood season. However, this pattern changes with the development of mite infestation. Despite the fact that the overall level of genetic diversity of the mites remained low through the season, multiple inbred lineages were identified in the mites we sampled in June. As a response to the decrease of brood availability and the increase of the parasite population in parallel in the colonies, these lineages recombined towards the end of the season as mites co-infest brood cells. Our results suggest that the ratio of the number of mite per brood cell in the colony determines the genetic structure of the populations of V. destructor. This intracolonial population dynamics has great relevance for the selection of acaricide resistance in V. destructor. If chemical treatments occur before the recombination phase, inbreeding will greatly enhance the fixation of resistance alleles at the colony level.

Distribution and population genetic variation of cryptic species of the Alpine mayfly Baetis alpinus (Ephemeroptera: Baetidae) in the Central Alps.

BackgroundMany species contain evolutionarily distinct groups that are genetically highly differentiated but morphologically difficult to distinguish (i.e., cryptic species). The presence of cryptic species poses significant challenges for the accurate assessment of biodiversity and, if unrecognized, may lead to erroneous inferences in many fields of biological research and conservation.ResultsWe tested for cryptic genetic variation within the broadly distributed alpine mayfly Baetis alpinus across several major European drainages in the central Alps. Bayesian clustering and multivariate analyses of nuclear microsatellite loci, combined with phylogenetic analyses of mitochondrial DNA, were used to assess population genetic structure and diversity. We identified two genetically highly differentiated lineages (A and B) that had no obvious differences in regional distribution patterns, and occurred in local sympatry. Furthermore, the two lineages differed in relative abundance, overall levels of genetic diversity as well as patterns of population structure: lineage A was abundant, widely distributed and had a higher level of genetic variation, whereas lineage B was less abundant, more prevalent in spring-fed tributaries than glacier-fed streams and restricted to high elevations. Subsequent morphological analyses revealed that traits previously acknowledged as intraspecific variation of B. alpinus in fact segregated these two lineages.ConclusionsTaken together, our findings indicate that even common and apparently ecologically well-studied species may consist of reproductively isolated units, with distinct evolutionary histories and likely different ecology and evolutionary potential. These findings emphasize the need to investigate hidden diversity even in well-known species to allow for appropriate assessment of biological diversity and conservation measures.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0643-y) contains supplementary material, which is available to authorized users.

Mitochondrial DNA evidences reflect an incipient population structure in Atlantic goliath grouper (<em>Epinephelus itajara</em>, Epinephelidae) in Brazil

The Atlantic goliath grouper is a critically endangered species that inhabits estuarine and reef environments and is threatened primarily by fishing activities and habitat destruction. Despite the urgent need for protection, its genetic conservation status remains unknown. The aim of the present study was to evaluate the gene flow among the populations of the species along the coast of Brazil based on the control region of the mitochondrial DNA. The results indicate low haplotype diversity (0.40-0.86) and very low nucleotide diversity (0.1-0.5%). They also show that the genetic diversity of the species varies considerably along the coast and that this finding may be especially important for the identification of priority areas for its conservation. The population analyses indicate a low but significant degree of genetic structuring (Φ ST =0.111), probably due to the occurrence of rare haplotypes at some locations, although the genetic differentiation between sites was not correlated with geographic distance (r=0.0501; p=0.7719), and the shared haplotypes indicate that gene flow occurs among all locations along the Brazilian coast. The results of the pairwise FST indicate a high degree of genetic differentiation between locations. The incipient population structuring detected in the present study is not related systematically to the geological or physical features of the Brazilian coast. The complex interaction of fluctuations in sea level, marine currents, and the reproductive characteristics of the species hampers the identification of the specific role of each of these processes in the gene flow dynamics of the population units of the Atlantic goliath grouper. The low overall levels of genetic diversity, the pairwise FST values and the significant population structuring among groups (Φ CT ) identified in the present study all reinforce the critically endangered status of the species and are inconsistent with the presence of a single, panmictic population of groupers on the Brazilian coast. The results of this study suggest that, though it may be incipient, the observed genetic structuring must be taken into account in order to prevent potential problems, such as outbreeding depression, in the management of wild stocks.

Genetic diversity and host alternation of the egg parasitoid Ooencyrtus pityocampae between the pine processionary moth and the caper bug.

The increased use of molecular tools for species identification in recent decades revealed that each of many apparently generalist parasitoids are actually a complex of morphologically similar congeners, most of which have a rather narrow host range. Ooencyrtus pityocampae (OP), an important egg parasitoid of the pine processionary moth (PPM), is considered a generalist parasitoid. OP emerges from PPM eggs after winter hibernation, mainly in spring and early summer, long before the eggs of the next PPM generation occurs. The occurrence of OP in eggs of the variegated caper bug (CB) Stenozygum coloratum in spring and summer suggests that OP populations alternate seasonally between PPM and CB. However, the identity of OP population on CB eggs seemed uncertain; unlike OP-PPM populations, the former displayed apparently high male/female ratios and lack of attraction to the PPM sex pheromone. We studied the molecular identities of the two populations since the morphological identification of the genus Ooencyrtus, and OP in particular, is difficult. Sequencing of COI and ITS2 DNA fragments and AFLP analysis of individuals from both hosts revealed no apparent differences between the OP-PPM and the OP-CB populations for both the Israeli and the Turkish OPs, which therefore supported the possibility of host alternation. Sequencing data extended our knowledge of the genetic structure of OP populations in the Mediterranean area, and revealed clear separation between East and West Mediterranean populations. The overall level of genetic diversity was rather small, with the Israeli population much less diverse than all others; possible explanations for this finding are discussed. The findings support the possibility of utilizing the CB and other hosts for enhancing biological control of the PPM.

Population history, gene flow, and bottlenecks in island populations of a secondary seed disperser, the southern grey shrike (Lanius meridionalis koenigi).

Studying the population history and demography of organisms with important ecological roles can aid understanding of evolutionary processes at the community level and inform conservation. We screened genetic variation (mtDNA and microsatellite) across the populations of the southern grey shrike (Lanius meridionalis koenigi) in the Canary Islands, where it is an endemic subspecies and an important secondary seed disperser. We show that the Canarian subspecies is polyphyletic with L. meridionalis elegans from North Africa and that shrikes have colonized the Canary Islands from North Africa multiple times. Substantial differences in genetic diversity exist across islands, which are most likely the product of a combination of historical colonization events and recent bottlenecks. The Eastern Canary Islands had the highest overall levels of genetic diversity and have probably been most recently and/or frequently colonized from Africa. Recent or ongoing bottlenecks were detected in three of the islands and are consistent with anecdotal evidence of population declines due to human disturbance. These findings are troubling given the shrike's key ecological role in the Canary Islands, and further research is needed to understand the community-level consequences of declines in shrike populations. Finally, we found moderate genetic differentiation among populations, which largely reflected the shrike's bottleneck history; however, a significant pattern of isolation-by-distance indicated that some gene flow occurs between islands. This study is a useful first step toward understanding how secondary seed dispersal operates over broad spatial scales.

Microsatellite DNA markers in Shorea platyclados (Dipterocarpaceae): genetic diversity, size homoplasy and mother trees

Cross-specific amplification of microsatellite loci greatly enhances the effectiveness of this marker system. This shortcut would greatly enhance our examination of the gene flow and population structure of trees in diverse tropical rainforests. To explore the effectiveness and limitations of this approach, we examined allelic diversity at six microsatellite loci, originally developed in a congeneric species, in three populations of Shorea platyclados from Peninsular Malaysia. Fragment sizing was performed by an efficient and sensitive (1 bp resolution) technique using capillary electrophoresis, ethidium bromide detection, and minimal clean-up. Fragment size ranges were conserved between species and null allele frequencies were low. Higher overall levels of genetic diversity were detected in our study. Variation among populations was directly related to geographic distance. Fragment size class distributions suggest that each locus should be studied using different evolutionary models. Direct sequencing of SSR fragments revealed that size differences were due to changes in both the flanking regions and repeat motifs. Several clear examples of size homoplasy were observed, along with the disruption of perfect repeats, suggesting that cross-specific amplification of microsatellite loci requires an additional level of confirmation at the DNA sequence level before the influence of size homoplasy and changes in repeat structure can be assessed. Simulation studies demonstrate that the increasing intensity of timber harvest leads to higher variability in levels of potential heterozygosity and decreasing total number of alleles in the remnant "mother trees" The careful selection of "mother" trees can greatly enhance the future genetic diversity of populations.   

Analysis of genetic diversity of Piper spp. in Hainan Island (China) using inter-simple sequence repeat ISSR markers

Inter-simple sequence repeat (ISSR) analysis was used to evaluate the genetic variation of Piper spp. from Hainan, China. 247 polymorphic bands out of a total of 248 (99.60%) were generated from 74 individual plants of Piper spp. The overall level of genetic diversity among Piper spp. in Hainan was high, with the mean Shannon information index (I) of 0.2843 and the mean Nei’s genetic diversity (H) of 0.1904. The genetic similarity (GS) coefficient ranged from 0.548 to 0.976 within 74 individual plants of Piper spp., and the within-species genetic distance ranged from 0.104 to 0.28. Unweighted pair group method with arithmetic mean (UPGMA) dendrogram showed that P. kadsura is the most divergent and the most distant of the 11 species, and that P. hainanense and P. bonii , are closely related as well as P. sarmentosum and P. betle . The diversity analysis unambiguously distinguished all Piper spp. The high levels of genetic diversity in Jianfengling and Diaoluoshan demonstrate that conservation of wild resources of Piper in these two localities is more effective than that in Limushan, Wuzhishan, Xinglong Tropical Botanical Garden and Danzhou. Key words: Black pepper, Piper, genetic variation, molecular marker, inter-simple sequence repeat.

Evaluation of genetic diversity in Piper spp using RAPD and SRAP markers

Random amplified polymorphic DNA (RAPD) and sequence-related amplified polymorphism (SRAP) analysis were applied to 74 individual plants of Piper spp in Hainan Island. The results showed that the SRAP technique may be more informative and more efficient and effective for studying genetic diversity of Piper spp than the RAPD technique. The overall level of genetic diversity among Piper spp in Hainan was relatively high, with the mean Shannon diversity index being 0.2822 and 0.2909, and the mean Nei's genetic diversity being 0.1880 and 0.1947, calculated with RAPD and SRAP data, respectively. The ranges of the genetic similarity coefficient were 0.486-0.991 and 0.520-1.000 for 74 individual plants of Piper spp (the mean genetic distance was 0.505 and 0.480) and the within-species genetic distance ranged from 0.063 to 0.291 and from 0.096 to 0.234, estimated with RAPD and SRAP data, respectively. These genetic indices indicated that these species are closely related genetically. The dendrogram generated with the RAPD markers was topologically different from the dendrogram based on SRAP markers, but the SRAP technique clearly distinguished all Piper spp from each other. Evaluation of genetic variation levels of six populations showed that the effective number of alleles, Nei's gene diversity and the Shannon information index within Jianfengling and Diaoluoshan populations are higher than those elsewhere; consequently conservation of wild resources of Piper in these two regions should have priority.

Clonal expansion of the BelgianPhytophthora ramorumpopulations based on new microsatellite markers

Co-existence of both mating types A1 and A2 within the EU1 lineage of Phytophthora ramorum has only been observed in Belgium, which begs the question whether sexual reproduction is occurring. A collection of 411 Belgian P. ramorum isolates was established during a 7-year survey. Our main objectives were genetic characterization of this population to test for sexual reproduction, determination of population structure, evolution and spread, and evaluation of the effectiveness and impact of control measures. Novel, polymorphic simple sequence repeat (SSR) markers were developed after screening 149 candidate loci. Eighty isolates of P. ramorum, broadly representing the Belgian population, were analyzed using four previously described and three newly identified polymorphic microsatellite loci as well as amplified fragment length polymorphisms. SSR analysis was most informative and was used to screen the entire Belgian population. Thirty multilocus genotypes were identified, but 68% of the isolates belonged to the main genotype EU1MG1. Although accumulated mutation events were detected, the overall level of genetic diversity within the Belgian isolates of P. ramorum appears to be limited, indicating a relatively recent clonal expansion. Based on our SSR analysis there is no evidence of sexual recombination in the Belgian population of P. ramorum. Metalaxyl use decreased the genetic diversity of P. ramorum until 2005, when the majority of the isolates had become resistant. Most genotypes were site-specific and despite systematic removal of symptomatic and neighbouring plants, some genotypes were detected over a period of several years at a single site, sometimes discontinuously, indicating (latent) survival of the pathogen at those sites.

Population Genetic Structure of Tetranychus urticae and Its Sibling Species Tetranychus cinnabaribus (Acari: Tetranychidae) in China as Inferred From Microsatellite Data

Abstract The population structure of the twospotted spider mite, Tetranychus urticae Koch, and its sibling species the carmine spider mite, Tetranychus cinnabarinus (Boisduval) (Acari: Tetranychidae), from China were investigated using microsatellite markers. The two species exhibited significant polymorphism but generally low genetic diversity. The genetic differentiation between populations of T. urticae and T. cinnabarinus was extreme and was significantly related to geographical distance between the populations (Mantel test). The overall level of genetic diversity of T. cinnabarinus was significantly higher than that of T. urticae. However, the various geographical levels of T. cinnabarinus were lower than those of T. urticae. The average pairwise FST between geographical populations of T. cinnabarinus was far higher than that of T. urticae, suggesting that the genetic differentiation of T. cinnabarinus was much higher than that of T. urticae. A comparison of T. urticae and T. cinnabarinus collected in adjacent and sympatric areas provided no evidence that genetic differentiation was related to geographical distances, suggesting a lack of gene flow between populations of these two species. In a phylogenetic analysis, seven geographical populations of T. urticae clustered in one branch and six geographical populations of T. cinnabarinus were distributed in other branches.

CONSERVATION GENETICS OF THE SONOMA TREE VOLE (ARBORIMUS POMO) BASED ON MITOCHONDRIAL AND AMPLIFIED FRAGMENT LENGTH POLYMORPHISM MARKERS

We used a comparative, multimarker approach to investigate the conservation genetics of an arboreal vole (the Sonoma tree vole [Arborimus pomo]) in the Pacific Northwest of North America. We compared geographic patterns and overall levels of genetic diversity based on 55 amplified fragment length polymorphism (AFLP) loci with those based on a single, commonly used mitochondrial locus, the control region. Although examination of the control region data revealed the presence of 2 distinct mitochondrial DNA (mtDNA) clades within A. pomo (1 in the north and 1 in the south of the species' range), the nuclear perspective provided by AFLP did not reveal a similar geographic division within the species, supporting instead that A. pomo consists of a single panmictic population. Genetic diversity estimates based on the mtDNA data (gene diversity = 0.79 and 0.80 for the 2 clades) were much greater than those based on AFLP (gene diversity = 0.31 and 0.19 for the 2 clades). These contrasting results reflect inherent differences between mitochondrial and nuclear loci in mutation rate, effective population size, expected time to monophyly, and mode of inheritance, and highlight the utility of using the combination of AFLP and mtDNA when assessing the genetic status of wild populations and species of mammals, especially those of conservation concern. In the case of A. pomo, our combined AFLP and mtDNA data support the recognition of the southern Sonoma tree voles as a distinct management unit within the species.

A space‐for‐time substitution reveals the long‐term decline in genotypic diversity of a widespread salt marsh plant, Spartina alterniflora, over a span of 1500 years

Summary Clonal populations face a trade‐off between sexual recruitment and vegetative growth and, once established, may undergo continuous declines in genotypic diversity if their sexual recruits make poor competitors. The geological history of delta formation in the Lower Mississippi River Valley was used to age eight S. alterniflora marshes for use in a space‐for‐time substitution ranging over 1500 years, in order to determine the long‐term effects of clonal growth on genotypic diversity in natural populations. We also predicted that highly heterozygous clones are competitively superior, leading to an increase in the overall level of genetic diversity as a marsh ages and/or to an increasingly positive relationship between clone size and individual heterozygosity, and that the clumping of ramets within clones will occur over increasingly large distances as populations age, while the clumping of genetically related clones will become less pronounced as intraclonal competition begins to obscure the initial effects of localized seedling recruitment. Using molecular markers to differentiate clones, we documented a decline in clonal richness at the rate of approximately 1% 100 years−1 that was accompanied for the first 300–500 years by an increase in the distance over which clumping of ramets within genets occurred. Older populations, in the 500–1500‐year range, showed evidence of clone fragmentation. The spatial clustering of kin was observed for only two marshes, and exhibited no clear relationship with marsh age. Whereas the overall level of genetic diversity was consistent among marshes and showed no clear relationship with marsh age, the relationship between heterozygosity and individual clone size became increasingly pronounced within older marshes. Our results suggest that under natural conditions S. alterniflora marshes will rarely reach ages sufficient for the loss of all clonal diversity, or for the effects of inbreeding and drift to pose a significant threat to population viability.

Genetic diversity analysis of captive populations: The Vietnamese sika deer (Cervus nippon pseudaxis) in zoological parks

AbstractThe Vietnamese sika deer (Cervus nippon pseudaxis) is an endangered subspecies; it has disappeared in the wild, but is being bred in zoological parks. We studied the neutral genetic diversity and population structure of herds kept in different European zoos, using nine microsatellite loci. The goal was to evaluate the consequences of founding effects and breeding practices on the level and structure of genetic variability. The level of genetic diversity within the European zoos is not lower than that of the populations kept in Vietnamese farms. Strong differences among zoological parks and between the European group and the Vietnamese population were detected. This is probably due to founding effects, genetic drift, and possibly hybridization in both Europe and Vietnam. We expected to find a much lower level of genetic diversity in Europe. The current overall level of genetic diversity is probably due to the recent introduction of Cuc Phuong individuals, and to important differences among the populations of different zoological parks, which increase the total genetic variability. Although the current level of genetic variability is not particularly low, future levels are probably threatened by the current herd sizes and structure. Based on these results, management guidelines are proposed. Zoo Biol 22:465–475, 2003. © 2003 Wiley‐Liss, Inc.

Genetic Variation among Populations of Valeriana ciliata T.&G. (Prairie valerian) in Midwestern Prairie Fens

lations of V. ciliata in Ohio. These southern, edge-of-range populations were compared with genetic variation in four Michigan populations at the middle of this species' range. Allozyme electrophoresis of individuals in populations of this subdioecious species revealed overall levels of genetic diversity (Hes = 0.245) that were higher than expected for an endemic. Though small in size, the two Ohio populations of V. ciliata had intermediate levels of genetic diversity (He = 0.224, 0.254) in comparison to the Michigan populations (He = 0.0540.305). Differences between census population size and effective population size, uneven sex ratios, location within the species' range, and variation in habitat quality all contribute to the level of genetic variation recorded among these V. ciliata populations.

Gene flow and genetic diversity: a comparison of freshwater bryozoan populations in Europe and North America.

We have used microsatellite and mitochondrial sequence data to gain insight into patterns of gene flow and genetic diversity among North American and European populations of the freshwater bryozoan Cristatella mucedo. Mitochondrial sequence data reveal numerous, widely distributed, divergent genetic lineages in North America that can be broadly categorized into two groups, one of which is genetically homogeneous and relatively similar to the European haplotypes, the other of which is more diverse. The maximum North American sequence differentiation translates into a divergence time of approximately 1.5 Myr BP. In contrast, European populations contained only three haplotypes that are all closely related. Microsatellite data reveal higher overall levels of genetic diversity in North America than Europe, although levels of within-population genetic variation are similar on the two continents. In North America, two of the three microsatellite loci show bimodal distributions of allele sizes which are significantly associated between the two loci. As a result, two microsatellite lineages are evident, and these are assortatively distributed between the mitochondrial haplotype groupings. The combined mitochondrial and microsatellite data suggest two distinct genetic lineages in North America that may represent cryptic species. Hybridization between the two presumptive species or subspecies may have contributed to the high levels of genetic diversity in North America. The overall lower levels of genetic diversity in Europe can be attributed to postglacial derivation of extant populations from a single mitochondrial lineage, and conformation to a metapopulation structure.