Historic hybridization and persistence of a novel mito-nuclear combination in red-backed voles (genus Myodes)

<p>Abstract</p> <p>Background</p> <p>The role of hybridization in generating diversity in animals is an active area of discovery and debate. We assess hybridization across a contact zone of northern (<it>Myodes rutilus</it>) and southern (<it>M. gapperi</it>) red-backed voles using variation in skeletal features and both mitochondrial and nuclear loci. This transect extends approximately 550 km along the North Pacific Coast of North America and encompasses 26 populations (n = 485). We establish the history, geographic extent and directionality of hybridization, determine whether hybridization is ongoing, and assess the evolutionary stability of novel genomic combinations.</p> <p>Results</p> <p>Identification of <it>M. rutilus </it>and <it>M. gapperi </it>based on the degree of closure of the post-palatal bridge was concordant with the distribution of diagnostic nuclear MYH6 alleles; however, an 80 km zone of introgressed populations was identified. The introgressant form is characterized by having mitochondrial haplotypes closely related to the northern <it>M. rutilus </it>on a nuclear background and morphological characteristics of southern <it>M. gapperi</it>.</p> <p>Conclusion</p> <p>Introgression appears to have been historic as pure populations of <it>M. rutilus </it>are now isolated to the north from introgressants or pure <it>M. gapperi </it>by the LeConte Glacier. As we do not find pure <it>M. rutilus </it>or <it>M. gapperi </it>individuals throughout the distribution of the introgressant form, it appears that the introgressants are a self-sustaining entity not requiring continued hybridization between pure parental forms to generate this novel combination of characters.</p

Low genetic diversity in an endangered species: recent or historic pattern?

Examining patterns of genetic diversity has become an integral component of many management plans concerning endangered species, yet interpreting the processes underlying such patterns remains challenging. We demonstrate low genetic diversity in a critically endangered small mammal population. A common interpretation of this pattern would be that it is the result of a known, recent decline in this population. We test this interpretation and find it to be incorrect. Instead, by using museum voucher specimens, we show that the pattern of low genetic diversity is historical. This study demonstrates the importance of choosing appropriate reference groups by which to interpret modern levels of genetic diversity in endangered species. We conclude that analysis of archival specimens may be essential in cases where genetic diversity is driving conservation management decisions because it may allow us to distinguish the effects of low genetic diversity from the process of losing diversity. We recognize that this approach can be limited due to several sampling issues: archival material may not be available, statistical power needs to be evaluated, sample sizes and sequence lengths may be suboptimal due to intrinsic difficulties associated with amplification of degraded DNA. These issues are discussed and possible solutions identified

Speciation along a shared evolutionary trajectory

Groups of organisms-whether multiple species or populations of a single species-can differ in several non-exclusive ways. For example, groups may have diverged phenotypically, genetically, or in the evolutionary responses available to them. We tested for the latter of these-response divergence-between 2 species of woodrats: Neotoma fuscipes and Neotoma macrotis. Based on random skewers analyses we found that, despite being well differentiated both phenotypically and genetically, N. fuscipes and N. macrotis appear to be diverging along a shared evolutionary trajectory (r degrees = 0.895, P = 0.114). Because these species are currently in secondary contact, their phenotypic evolution being along a shared evolutionary axis has important implications. In particular, that their response to selection arising from interspecific interactions will be constrained along the same evolutionary trajectory may reduce the potential for reinforcing selection to maintain species boundaries

Philopatry, kin clusters, and genetic relatedness in a population of woodrats (Neotoma macrotis)

Studies of highly kin-structured mammal societies have revealed the importance of natal philopatry in determining the distribution of genetic variation within populations. In comparison, the relationship between philopatry and genetic diversity within populations of moderately kin-structured societies has received relatively little attention. Previous studies of Neotoma macrotis have suggested that females form distinct kin clusters. Each kin cluster overlaps spatially with the home range(s) of one or more males that are not related to each other or to the females with which they are spatially associated. To examine interactions between philopatry and genetic structure in this apparently moderately kin-structured species, we characterized spatial and genetic relationships among individually marked females in a population of N. macrotis from central coastal California. Our field studies revealed that, contrary to expectation, females in this population were not strongly philopatric and spatially clustered females were not characterized by high levels of genetic relatedness. Nevertheless, genetic structure was evident within the study population; spatial and genetic distances among females were significantly correlated, suggesting that dispersal patterns influenced genetic structure even in the absence of marked female philopatry. Because females with overlapping spatial distributions were not typically closely related to one another, opportunities for the evolution of kin-selected social behavior (e.g., cooperative care of young) appear to be limited in this population. Copyright 2004.Neotoma macrotis; Neotoma fuscipes; relatedness; female kin clusters; natal philopatry

Eight Polymorphic Microsatellite Loci Developed and Characterized From Townsend’s Big-Eared Bat, \u3ci\u3eCorynorhinus Townsendii\u3c/i\u3e

Two of the five subspecies of the western big-eared bat, Corynorhinus townsendii, are listed as federally endangered with the remaining three being of conservation concern. Knowing the degree of connectivity among populations would aid in the establishment of sound conservation and management plans for this taxon. For this purpose, we have developed and characterized eight polymorphic microsatellite markers

Data from: Ecological segregation in a small mammal hybrid zone: habitat-specific mating opportunities and selection against hybrids restrict gene flow on a fine spatial scale

The degree to which closely related species interbreed is determined by a complex interaction of ecological, behavioral, and genetic factors. We examine the degree of interbreeding between two woodrat species, Neotoma bryanti and N. lepida, at a sharp ecological transition. We identify the ecological association of each genotypic class, assess the opportunity for mating between these groups, and test whether they have similar patterns of year-to-year persistence on our study site. We find that 13% of individuals have a hybrid signature but that the two parental populations and backcrosses are highly segregated by habitat type and use. Also, we find that adult hybrids are comparable to parental types in terms of year-to-year persistence on our site but that, among juveniles, significantly fewer hybrids reach adulthood on site compared to their purebred counterparts. Our analyses show that this hybrid zone is maintained by occasional nonassortative mating coupled with hybrid fertility, but that these factors are balanced by lower apparent survival of juvenile hybrids and habitat-based preference or selection that limits heterospecific mating while promoting backcrossing to habitat-specific genotypes. This system presents a novel example of the role that sharp resource gradients play in reproductive isolation and the potential for genetic introgression

Population genetic structure of two ecologically distinct Amazonian spiny rats: Separating history and current ecology

Population history and current demographic and ecological factors determine the amount of genetic variation within and the degree of differentiation among populations. Differences in the life history and ecology of codistributed species may lead to differences in hierarchical population genetic structure. Here, we compare patterns of genetic diversity and structure of two species of spiny rats in the genus Proechimys from the Rio Jurua of western Amazonian Brazil. Based on the ecological and life-history differences between the two species, we make predictions as to how they might differ in patterns of genetic diversity and structure. We use mitochondrial sequence data from the cytochrome b gene to test these predictions. Although both species maintain nearly the same number of mitochondrial haplotypes across the sampled range, they differ in levels of genetic diversity and geographic structure. Patterns of gene flow are also different between the two species with average M-values of nearly three in P. steerei and less than one in P. simonsi. Our initial predictions are largely upheld by the genetic data and where conflicting hypotheses arise, we suggest further studies that may allow us to distinguish among evolutionary scenarios. Separating the effects of history and ongoing demography on patterns of genetic diversity is challenging. Combining genetic analyses with field studies remains essential to disentangling these complex processes

Molecular evidence for historical and recent population size reductions of tiger salamanders (Ambystoma tigrinum) in Yellowstone National Park

Abstract Population declines caused by natural and anthropogenic factors can quickly erode genetic diversity in natural populations. In this study, we examined genetic variation within 10 tiger salamander populations across northern Yellowstone National Park in Wyoming and Montana, USA using eight microsatellite loci. We tested for the genetic signature of population decline using heterozygosity excess, shifts in allele frequencies, and low ratios of allelic number to allelic size range (M-ratios). We found different results among the three tests. All 10 populations had low M-ratios, five had shifts in allele frequencies and only two had significant heterozygosity excesses. These results support theoretical expectations of different temporal signatures among bottleneck tests and suggest that both historical fish stocking, recent, sustained drought, and possibly an emerging amphibian disease have contributed to declines in effective population size