Phenotypic Variation in Phytochemical Defense of Trembling Aspen in Western North America: Genetics, Development, and Geography.

Trembling aspen (Populus tremuloides) is arguably the most important deciduous tree species in the Intermountain West of North America. There, as elsewhere in its range, aspen exhibits remarkable genetic variation in observable traits such as morphology and phenology. In contrast to Great Lakes populations, however, relatively little is known about phytochemical variation in western aspen. This survey of phytochemistry in western aspen was undertaken to assess how chemical expression varies among genotypes, cytotypes (diploid vs. triploid), and populations, and in response to development and mammalian browsing. We measured levels of foliar nitrogen, salicinoid phenolic glycosides (SPGs) and condensed tannins (CTs), as those constituents influence organismal interactions and ecosystem processes. Results revealed striking genotypic variation and considerable population variation, but minimal cytotype variation, in phytochemistry of western aspen. Levels of SPGs and nitrogen declined, whereas levels of CTs increased, with tree age. Browsed ramets had much higher levels of SPGs, and lower levels of CTs, than unbrowsed ramets of the same genotype. We then evaluated how composite chemical profiles of western aspen differ from those of Great Lakes aspen (assessed in earlier research). Interestingly, mature western aspen trees maintain much higher levels of SPGs, and lower levels of CTs, than Great Lakes aspen. Phenotypic variation in chemical composition of aspen - a foundation species - in the Intermountain West likely has important consequences for organismal interactions and forest ecosystem dynamics. Moreover, those consequences likely play out over spatial and temporal scales somewhat differently than have been documented for Great Lakes aspen.

Genotypic variation rather than ploidy level determines functional trait expression in a foundation tree species in the presence and absence of environmental stress.

BACKGROUND AND AIMS: At the population level, genetic diversity is a key determinant of a tree species' capacity to cope with stress. However, little is known about the relative importance of the different components of genetic diversity for tree stress responses. We compared how two sources of genetic diversity, genotype and cytotype (i.e. differences in ploidy levels), influence growth, phytochemical and physiological traits of Populus tremuloides in the presence and absence of environmental stress. METHODS: In a series of field studies, we first assessed variation in traits across diploid and triploid aspen genotypes from Utah and Wisconsin under non-stressed conditions. In two follow-up experiments, we exposed diploid and triploid aspen genotypes from Wisconsin to individual and interactive drought stress and defoliation treatments and quantified trait variations under stress. KEY RESULTS: We found that (1) tree growth and associated traits did not differ significantly between ploidy levels under non-stressed conditions. Instead, variation in tree growth and most other traits was driven by genotypic and population differences. (2) Genotypic differences were critical for explaining variation of most functional traits and their responses to stress. (3) Ploidy level played a subtle role in shaping traits and trait stress responses, as its influence was typically obscured by genotypic differences. (4) As an exception to the third conclusion, we showed that triploid trees expressed 17 % higher foliar defence (tremulacin) levels, 11 % higher photosynthesis levels and 23 % higher rubisco activity under well-watered conditions. Moreover, triploid trees displayed greater drought resilience than diploids as they produced 35 % more new tissue than diploids when recovering from drought stress. CONCLUSION: Although ploidy level can strongly influence the ecology of tree species, those effects may be relatively small in contrast to the effects of genotypic variation in highly diverse species.

Polyploidy and growth-defense tradeoffs in natural populations of western quaking Aspen.

Polyploidy, the expression of more than two sets of chromosomes, is common in plants, and is thought to influence plant trait expression and drive plant species evolution. The degree to which polyploidy influences interactions among physiological processes such as growth and defense in natural populations through its effect on phenotypic variability is poorly understood. We link broad plant genotypic features (including polyploidy) to phenotypic expression of growth and chemical defense in natural populations of quaking aspen (Populus tremuloides) to examine patterns in resource allocation that might drive growth-defense tradeoffs. Quaking aspen are capable of rapid growth, and are also a primary food plant for a large range of herbivores, including insects and ungulates. While often diploid, aspen can exhibit polyploidy as triploid clones. We tested for the effect of genotype, cytotype (ploidy level, divided between diploids and triploids), and ramet age on relationships between growth and leaf chemistry across natural aspen clones in northern Utah. Substantial genotype variability in growth and leaf chemistry occurred across both cytotypes. Phenolic glycosides, but not condensed tannins, were negatively related to growth. Ramet age was also negatively related to growth. Phenolic glycosides were negatively related to condensed tannins, but only for the diploid clones. Triploid clones exhibited ~ 20% higher levels of phenolic glycosides than diploids. Growth in quaking aspen was likely sacrificed for the production of phenolic glycosides. Our study underscores the importance of considering polyploidy, genetic variability, and ramet age in understanding growth-defense tradeoffs in natural populations of clonal organisms, such as quaking aspen.

Remote sensing of cytotype and its consequences for canopy damage in quaking aspen.

Mapping geographic mosaics of genetic variation and their consequences via genotype x environment interactions at large extents and high resolution has been limited by the scalability of DNA sequencing. Here, we address this challenge for cytotype (chromosome copy number) variation in quaking aspen, a drought-impacted foundation tree species. We integrate airborne imaging spectroscopy data with ground-based DNA sequencing data and canopy damage data in 391 km2 of southwestern Colorado. We show that (1) aspen cover and cytotype can be remotely sensed at 1 m spatial resolution, (2) the geographic mosaic of cytotypes is heterogeneous and interdigitated, (3) triploids have higher leaf nitrogen, canopy water content, and carbon isotope shifts (δ13 C) than diploids, and (4) canopy damage varies among cytotypes and depends on interactions with topography, canopy height, and trait variables. Triploids are at higher risk in hotter and drier conditions.

Cytotype and genotype predict mortality and recruitment in Colorado quaking aspen (Populus tremuloides).

Species responses to climate change depend on environment, genetics, and interactions among these factors. Intraspecific cytotype (ploidy level) variation is a common type of genetic variation in many species. However, the importance of intraspecific cytotype variation in determining demography across environments is poorly known. We studied quaking aspen (Populus tremuloides), which occurs in diploid and triploid cytotypes. This widespread tree species is experiencing contractions in its western range, which could potentially be linked to cytotype-dependent drought tolerance. We found that interactions between cytotype and environment drive mortality and recruitment across 503 plots in Colorado. Triploids were more vulnerable to mortality relative to diploids and had reduced recruitment on more drought-prone and disturbed plots relative to diploids. Furthermore, there was substantial genotype-dependent variation in demography. Thus, cytotype and genotype variation are associated with decline in this foundation species. Future assessment of demographic responses to climate change will benefit from knowledge of how genetic and environmental mosaics interact to determine species' ecophysiology and demography.

Genotyping-by-sequencing and ecological niche modeling illuminate phylogeography, admixture, and Pleistocene range dynamics in quaking aspen (Populus tremuloides).

Populus tremuloides is the widest-ranging tree species in North America and an ecologically important component of mesic forest ecosystems displaced by the Pleistocene glaciations. Using phylogeographic analyses of genome-wide SNPs (34,796 SNPs, 183 individuals) and ecological niche modeling, we inferred population structure, ploidy levels, admixture, and Pleistocene range dynamics of P. tremuloides, and tested several historical biogeographical hypotheses. We found three genetic lineages located mainly in coastal-Cascades (cluster 1), east-slope Cascades-Sierra Nevadas-Northern Rockies (cluster 2), and U.S. Rocky Mountains through southern Canadian (cluster 3) regions of the P. tremuloides range, with tree graph relationships of the form ((cluster 1, cluster 2), cluster 3). Populations consisted mainly of diploids (86%) but also small numbers of triploids (12%) and tetraploids (1%), and ploidy did not adversely affect our genetic inferences. The main vector of admixture was from cluster 3 into cluster 2, with the admixture zone trending northwest through the Rocky Mountains along a recognized phenotypic cline (Utah to Idaho). Clusters 1 and 2 provided strong support for the "stable-edge hypothesis" that unglaciated southwestern populations persisted in situ since the last glaciation. By contrast, despite a lack of clinal genetic variation, cluster 3 exhibited "trailing-edge" dynamics from niche suitability predictions signifying complete northward postglacial expansion. Results were also consistent with the "inland dispersal hypothesis" predicting postglacial assembly of Pacific Northwestern forest ecosystems, but rejected the hypothesis that Pacific-coastal populations were colonized during outburst flooding from glacial Lake Missoula. Overall, congruent patterns between our phylogeographic and ecological niche modeling results and fossil pollen data demonstrate complex mixtures of stable-edge, refugial locations, and postglacial expansion within P. tremuloides. These findings confirm and refine previous genetic studies, while strongly supporting a distinct Pacific-coastal genetic lineage of quaking aspen.

Low offspring survival in mountain pine beetle infesting the resistant Great Basin bristlecone pine supports the preference-performance hypothesis.

The preference-performance hypothesis states that ovipositing phytophagous insects will select host plants that are well-suited for their offspring and avoid host plants that do not support offspring performance (survival, development and fitness). The mountain pine beetle (Dendroctonus ponderosae), a native insect herbivore in western North America, can successfully attack and reproduce in most species of Pinus throughout its native range. However, mountain pine beetles avoid attacking Great Basin bristlecone pine (Pinus longaeva), despite recent climate-driven increases in mountain pine beetle populations at the high elevations where Great Basin bristlecone pine grows. Low preference for a potential host plant species may not persist if the plant supports favorable insect offspring performance, and Great Basin bristlecone pine suitability for mountain pine beetle offspring performance is unclear. We infested cut bolts of Great Basin bristlecone pine and two susceptible host tree species, limber (P. flexilis) and lodgepole (P. contorta) pines with adult mountain pine beetles and compared offspring performance. To investigate the potential for variation in offspring performance among mountain pine beetles from different areas, we tested beetles from geographically-separated populations within and outside the current range of Great Basin bristlecone pine. Although mountain pine beetles constructed galleries and laid viable eggs in all three tree species, extremely few offspring emerged from Great Basin bristlecone pine, regardless of the beetle population. Our observed low offspring performance in Great Basin bristlecone pine corresponds with previously documented low mountain pine beetle attack preference. A low preference-low performance relationship suggests that Great Basin bristlecone pine resistance to mountain pine beetle is likely to be retained through climate-driven high-elevation mountain pine beetle outbreaks.

Repurposing environmental DNA samples-detecting the western pearlshell (Margaritifera falcata) as a proof of concept.

Information on the distribution of multiple species in a common landscape is fundamental to effective conservation and management. However, distribution data are expensive to obtain and often limited to high-profile species in a system. A recently developed technique, environmental DNA (eDNA) sampling, has been shown to be more sensitive than traditional detection methods for many aquatic species. A second and perhaps underappreciated benefit of eDNA sampling is that a sample originally collected to determine the presence of one species can be re-analyzed to detect additional taxa without additional field effort. We developed an eDNA assay for the western pearlshell mussel (Margaritifera falcata) and evaluated its effectiveness by analyzing previously collected eDNA samples that were annotated with information including sample location and deposited in a central repository. The eDNA samples were initially collected to determine habitat occupancy by nonbenthic fish species at sites that were in the vicinity of locations recently occupied by western pearlshell. These repurposed eDNA samples produced results congruent with historical western pearlshell surveys and permitted a more precise delineation of the extent of local populations. That a sampling protocol designed to detect fish was also successful for detecting a freshwater mussel suggests that rapidly accumulating collections of eDNA samples can be repurposed to enhance the efficiency and cost-effectiveness of aquatic biodiversity monitoring.

Reproductive isolation and environmental adaptation shape the phylogeography of mountain pine beetle (Dendroctonus ponderosae).

Chromosomal rearrangement can be an important mechanism driving population differentiation and incipient speciation. In the mountain pine beetle (MPB, Dendroctonus ponderosae), deletions on the Y chromosome that are polymorphic among populations are associated with reproductive incompatibility. Here, we used RAD sequencing across the entire MPB range in western North America to reveal the extent of the phylogeographic differences between Y haplotypes compared to autosomal and X-linked loci. Clustering and geneflow analyses revealed three distinct Y haplogroups geographically positioned within and on either side of the Great Basin Desert. Despite close geographic proximity between populations on the boundaries of each Y haplogroup, there was extremely low Y haplogroup mixing among populations, and gene flow on the autosomes was reduced across Y haplogroup boundaries. These results are consistent with a previous study suggesting that independent degradation of a recently evolved neo-Y chromosome in previously isolated populations causes male sterility or inviability among Y haplotype lineages. Phylogeographic results supported historic contraction of MPB into three separate Pleistocene glacial refugia followed by postglacial range expansion and secondary contact. Distinct sets of SNPs were statistically associated with environmental data among the most genetically distinct sets of geographic populations. This finding suggests that the process of adaptation to local climatic conditions is influenced by population genetic structure, with evidence for largely independent evolution in the most genetically isolated Y haplogroup.

Detection of individual ploidy levels with genotyping-by-sequencing (GBS) analysis.

Ploidy levels sometimes vary among individuals or populations, particularly in plants. When such variation exists, accurate determination of cytotype can inform studies of ecology or trait variation and is required for population genetic analyses. Here, we propose and evaluate a statistical approach for distinguishing low-level ploidy variants (e.g. diploids, triploids and tetraploids) based on genotyping-by-sequencing (GBS) data. The method infers cytotypes based on observed heterozygosity and the ratio of DNA sequences containing different alleles at thousands of heterozygous SNPs (i.e. allelic ratios). Whereas the method does not require prior information on ploidy, a reference set of samples with known ploidy can be included in the analysis if it is available. We explore the power and limitations of this method using simulated data sets and GBS data from natural populations of aspen (Populus tremuloides) known to include both diploid and triploid individuals. The proposed method was able to reliably discriminate among diploids, triploids and tetraploids in simulated data sets, and this was true for different levels of genetic diversity, inbreeding and population structure. Power and accuracy were minimally affected by low coverage (i.e. 2×), but did sometimes suffer when simulated mixtures of diploids, autotetraploids and allotetraploids were analysed. Cytotype assignments based on the proposed method closely matched those from previous microsatellite and flow cytometry data when applied to GBS data from aspen. An R package (gbs2ploidy) implementing the proposed method is available from CRAN.

Phylogeny of the most species-rich freshwater bivalve family (Bivalvia: Unionida: Unionidae): Defining modern subfamilies and tribes.

Freshwater mussels of the order Unionida are key elements of freshwater habitats and are responsible for important ecological functions and services. Unfortunately, these bivalves are among the most threatened freshwater taxa in the world. However, conservation planning and management are hindered by taxonomic problems and a lack of detailed ecological data. This highlights the urgent need for advances in the areas of systematics and evolutionary relationships within the Unionida. This study presents the most comprehensive phylogeny to date of the larger Unionida family, i.e., the Unionidae. The phylogeny is based on a combined dataset of 1032bp (COI+28S) of 70 species in 46 genera, with 7 of this genera being sequenced for the first time. The resulting phylogeny divided the Unionidae into 6 supported subfamilies and 18 tribes, three of which are here named for the first time (i.e., Chamberlainiini nomen novum, Cristariini nomen novum and Lanceolariini nomen novum). Molecular analyses were complemented by investigations of selected morphological, anatomical and behavioral characters used in traditional phylogenetic studies. No single morphological, anatomical or behavioral character was diagnostic at the subfamily level and few were useful at the tribe level. However, within subfamilies, many tribes can be recognized based on a subset of these characters. The geographical distribution of each of the subfamilies and tribes is also presented. The present study provides important advances in the systematics of these extraordinary taxa with implications for future ecological and conservation studies.

Defense traits in the long-lived Great Basin bristlecone pine and resistance to the native herbivore mountain pine beetle.

Mountain pine beetle (MPB, Dendroctonus ponderosae) is a significant mortality agent of Pinus, and climate-driven range expansion is occurring. Pinus defenses in recently invaded areas, including high elevations, are predicted to be lower than in areas with longer term MPB presence. MPB was recently observed in high-elevation forests of the Great Basin (GB) region, North America. Defense and susceptibility in two long-lived species, GB bristlecone pine (Pinus longaeva) and foxtail pine (P. balfouriana), are unclear, although they are sympatric with a common MPB host, limber pine (P. flexilis). We surveyed stands with sympatric GB bristlecone-limber pine and foxtail-limber pine to determine relative MPB attack susceptibility and constitutive defenses. MPB-caused mortality was extensive in limber, low in foxtail and absent in GB bristlecone pine. Defense traits, including constitutive monoterpenes, resin ducts and wood density, were higher in GB bristlecone and foxtail than in limber pine. GB bristlecone and foxtail pines have relatively high levels of constitutive defenses which make them less vulnerable to climate-driven MPB range expansion relative to other high-elevation pines. Long-term selective herbivore pressure and exaptation of traits for tree longevity are potential explanations, highlighting the complexity of predicting plant-insect interactions under climate change.

Imaging spectroscopy links aspen genotype with below-ground processes at landscape scales.

Fine-scale biodiversity is increasingly recognized as important to ecosystem-level processes. Remote sensing technologies have great potential to estimate both biodiversity and ecosystem function over large spatial scales. Here, we demonstrate the capacity of imaging spectroscopy to discriminate among genotypes of Populus tremuloides (trembling aspen), one of the most genetically diverse and widespread forest species in North America. We combine imaging spectroscopy (AVIRIS) data with genetic, phytochemical, microbial and biogeochemical data to determine how intraspecific plant genetic variation influences below-ground processes at landscape scales. We demonstrate that both canopy chemistry and below-ground processes vary over large spatial scales (continental) according to aspen genotype. Imaging spectrometer data distinguish aspen genotypes through variation in canopy spectral signature. In addition, foliar spectral variation correlates well with variation in canopy chemistry, especially condensed tannins. Variation in aspen canopy chemistry, in turn, is correlated with variation in below-ground processes. Variation in spectra also correlates well with variation in soil traits. These findings indicate that forest tree species can create spatial mosaics of ecosystem functioning across large spatial scales and that these patterns can be quantified via remote sensing techniques. Moreover, they demonstrate the utility of using optical properties as proxies for fine-scale measurements of biodiversity over large spatial scales.

Genotyping-by-sequencing for Populus population genomics: an assessment of genome sampling patterns and filtering approaches.

Continuing advances in nucleotide sequencing technology are inspiring a suite of genomic approaches in studies of natural populations. Researchers are faced with data management and analytical scales that are increasing by orders of magnitude. With such dramatic advances comes a need to understand biases and error rates, which can be propagated and magnified in large-scale data acquisition and processing. Here we assess genomic sampling biases and the effects of various population-level data filtering strategies in a genotyping-by-sequencing (GBS) protocol. We focus on data from two species of Populus, because this genus has a relatively small genome and is emerging as a target for population genomic studies. We estimate the proportions and patterns of genomic sampling by examining the Populus trichocarpa genome (Nisqually-1), and demonstrate a pronounced bias towards coding regions when using the methylation-sensitive ApeKI restriction enzyme in this species. Using population-level data from a closely related species (P. tremuloides), we also investigate various approaches for filtering GBS data to retain high-depth, informative SNPs that can be used for population genetic analyses. We find a data filter that includes the designation of ambiguous alleles resulted in metrics of population structure and Hardy-Weinberg equilibrium that were most consistent with previous studies of the same populations based on other genetic markers. Analyses of the filtered data (27,910 SNPs) also resulted in patterns of heterozygosity and population structure similar to a previous study using microsatellites. Our application demonstrates that technically and analytically simple approaches can readily be developed for population genomics of natural populations.

Widespread triploidy in Western North American aspen (Populus tremuloides).

We document high rates of triploidy in aspen (Populus tremuloides) across the western USA (up to 69% of genets), and ask whether the incidence of triploidy across the species range corresponds with latitude, glacial history (as has been documented in other species), climate, or regional variance in clone size. Using a combination of microsatellite genotyping, flow cytometry, and cytology, we demonstrate that triploidy is highest in unglaciated, drought-prone regions of North America, where the largest clone sizes have been reported for this species. While we cannot completely rule out a low incidence of undetected aneuploidy, tetraploidy or duplicated loci, our evidence suggests that these phenomena are unlikely to be significant contributors to our observed patterns. We suggest that the distribution of triploid aspen is due to a positive synergy between triploidy and ecological factors driving clonality. Although triploids are expected to have low fertility, they are hypothesized to be an evolutionary link to sexual tetraploidy. Thus, interactions between clonality and polyploidy may be a broadly important component of geographic speciation patterns in perennial plants. Further, cytotypes are expected to show physiological and structural differences which may influence susceptibility to ecological factors such as drought, and we suggest that cytotype may be a significant and previously overlooked factor in recent patterns of high aspen mortality in the southwestern portion of the species range. Finally, triploidy should be carefully considered as a source of variance in genomic and ecological studies of aspen, particularly in western U.S. landscapes.

Spatial genetic structure of the mountain pine beetle (Dendroctonus ponderosae) outbreak in western Canada: historical patterns and contemporary dispersal.

Environmental change has a wide range of ecological consequences, including species extinction and range expansion. Many studies have shown that insect species respond rapidly to climatic change. A mountain pine beetle epidemic of record size in North America has led to unprecedented mortality of lodgepole pine, and a significant range expansion to the northeast of its historic range. Our goal was to determine the spatial genetic variation found among outbreak population from which genetic structure, and dispersal patterns may be inferred. Beetles from 49 sampling locations throughout the outbreak area in western Canada were analysed at 13 microsatellite loci. We found significant north-south population structure as evidenced by: (i) Bayesian-based analyses, (ii) north-south genetic relationships and diversity gradients; and (iii) a lack of isolation-by-distance in the northernmost cluster. The north-south structure is proposed to have arisen from the processes of postglacial colonization as well as recent climate-driven changes in population dynamics. Our data support the hypothesis of multiple sources of origin for the outbreak and point to the need for population specific information to improve our understanding and management of outbreaks. The recent range expansion across the Rocky Mountains into the jack/lodgepole hybrid and pure jack pine zones of northern Alberta is consistent with a northern British Columbia origin. We detected no loss of genetic variability in these populations, indicating that the evolutionary potential of mountain pine beetle to adapt has not been reduced by founder events. This study illustrates a rapid range-wide response to the removal of climatic constraints, and the potential for range expansion of a regional population.

Cryptic postzygotic isolation in an eruptive species of bark beetle (Dendroctonus ponderosae).

Studies of postzygotic isolation often involve well-differentiated taxa that show a consistent level of incompatibility, thereby limiting our understanding of the initial stages and development of reproductive barriers. Dendroctonus ponderosae provides an informative system because recent evidence suggests that distant populations produce hybrids with reproductive incompatibilities. Dendroctonus ponderosae shows an isolation-by-distance gene flow pattern allowing us to characterize the evolution of postzygotic isolation (e.g., hybrid inviability, hybrid sterility) by crossing populations along a continuum of geographic/genetic divergence. We found little evidence of hybrid inviability among these crosses. However, crosses between geographically distant populations produced sterile males (consistent with Haldane's rule). This effect was not consistent with the fixation of mutations in an isolation-by-distance pattern, but instead is spatially localized. These reproductive barriers are uncorrelated with a reduction in gene flow suggesting their recent development. Crosses between geographically proximal populations bounding the transition from compatibility to hybrid male sterility showed evidence of unidirectional reduction in hybrid male fecundity. Our study describes significant postzygotic isolation occurring across a narrow and molecularly cryptic geographic zone between the states of Oregon and Idaho. This study provides a view of the early stages of postzygotic isolation in a geographically widespread species.

Genetic structuring in the freshwater mussel Anodonta corresponds with major hydrologic basins in the western United States.

Freshwater mussels (unionids) are increasingly recognized as important providers of ecosystem services, yet are among the most endangered fauna in the world. Because unionids are generally sessile and require specific fish hosts for development and dispersal, they are particularly vulnerable to habitat degradation. Surprisingly, little is known about the distribution of genetic diversity in freshwater mussels and this gap has a negative impact on taxonomy, monitoring, conservation and ecological research in these species. Here, we focus on western North American Anodonta, one of only three genera known to exist in this broad landscape and which contains three highly divergent lineages. We describe phylogeographical subdivision in the most widespread and diverse of these lineages, which includes Anodonta californiensis and Anodonta nuttalliana and occurs from Canada to Mexico. Using mitochondrial and nuclear data, we found that genetic structuring within this clade is inconsistent with morphologically based species designations, but instead follows patterns of vicariance among major hydrogeologic basins. Furthermore, there was a strong tendency for population diversity within drainage systems to increase downstream, implying greater habitat or host fish availability in this direction. Microsatellite results indicated that sampling locations were all genetically distinct, even at short distances. Many of our sample populations showed evidence of a recent demographic bottleneck, although this effect seemed to be very local and not drainage or basin-specific. This study provides a foundation for the establishment of appropriate management units and future research on adaptive differentiation and host fish relationships.

Isolation and characterization of microsatellite loci in western North American Anodonta species.

We have developed and characterized 13 microsatellite loci from a group of Anodonta species in western North America, and demonstrated their utility in populations representing two major clades in this genus. Allelic diversity and polymorphic information content were high for all loci, although these characteristics varied across populations. Deviations from Hardy-Weinberg genotypic ratios were not detected, although the estimated frequency of null alleles was high in one population for one locus. This is the first set of microsatellite loci to be developed for freshwater mussels in western North America, and will be useful for describing gene flow patterns among populations.

Isolation and characterization of microsatellite loci in the western pearlshell mussel, Margaritifera falcata (Gould).

Ten microsatellite loci were isolated from the western pearlshell, Margaritifera falcata (Gould, 1850) and characterized in populations from Washington and Montana, USA. We also assessed eight microsatellite loci developed in M. margaritifera, two of which showed utility. Both of our test populations showed significant heterozygote deficiencies at most loci, consistent with a hermaphroditic life history. Populations differed markedly with respect to allelic richness, allele frequencies and numbers of identical multilocus genotypes. This panel of loci should prove useful in describing gene flow and genetic diversity patterns among M. falcata populations, information that should aid future conservation efforts.