| This is the first study to assess genetic variation in the endangered Houston toad, Bufo houstonensis (Anura: Bufonidae). Samples from across its range were used to determine the number of populations and the levels of diversity within and among populations. D-loop (mtDNA) sequences from 160 individuals were analyzed, and variation at ten microsatellite loci was examined for 439 individuals. Genetic clustering analyses indicate nine populations across the range; five of these populations occurred in Bastrop County. The most divergent population was in Austin County. Gene flow was generally low, but was higher at distances <4 km. Overall, genetic diversity was high across the range and within populations. Recent surveys indicate population sizes are low and are decreasing; accordingly, annual monitoring of all known populations and increasing the number of toads (e.g., through headstarting programs) are proposed for immediate implementation. More general, but crucial, recommendations include preservation of all three habitat types (breeding/nursery, occupied, and dispersal), special attention towards the Austin County population, and involvement of the general public in conservation.;The coastal plain toad, Bufo nebulifer, is sympatric with B. houstonensis through all of the latter species' range. Examination of more common sympatric congeners may be necessary to effectively manage rare or endangered species, especially in cases where widespread or frequent hybridization is known or when human activities increases the rates of hybridization. Bufo houstonensis and B. nebulifer are known to hybridize, and while recent work has been done to investigate the genetic diversity and structure within B. houstonensis, no comparable data yet exist for B. nebulifer. I investigated population genetic structure and diversity, including migration/movement rates, at both the landscape and fine scales. Much of the range was sampled and nine groups were recovered. Their relationships may be explained by a long residence in much of its present-day distribution (at least tens of thousands of years), with a history of range contraction during glaciation and re-expansion following the retreat of glaciers. Bufo houstonensis and B. nebulifer have comparable levels of genetic diversity, but B. nebulifer seems to migrate less frequently or less distance than its endangered congener.;In order to effectively protect endangered species, natural levels of interspecific hybridization, or admixture, must be characterized, especially in cases where anthropogenic changes to the environment may broaden contact between the species. I investigated the baseline levels of admixture in B. houstonensis using mitochondrial sequence data and microsatellite loci. Admixture between B. houstonensis and two sympatric species (B. nebulifer and B. woodhousii) was detected. Phenotype-based assessments of admixture appear to be temporally stable, but they underestimate true levels of admixture. Bufo nebulifer x B. houstonensis F1 hybrids can be fertile and backcross to B. nebulifer; B. nebulifer x B. houstonensis matings may result in fertile offspring more frequently than previously thought. Admixed individuals with B. houstonensis or B. woodhousii maternal lineages can backcross to B. houstonensis. Phenotypically aberrant individuals were not always F1 hybrids, and F1 hybrids were not always phenotypically aberrant. With continued habitat alteration and rising temperatures, both habitat isolation and offset breeding season have already partially broken down and may deteriorate further, consequently, opportunities for hybridization events will increase. Selection against hybrids at the tadpole stage did not occur among B. houstonensis and B. nebulifer individuals. All these factors may also lead to higher levels of gamete wastage in B. houstonensis, an already critically endangered species.;Thirty-five published microsatellite loci were screened in several Bufo species, chiefly B. houstonensis and its two common, sympatric relatives, B. nebulifer and B. woodhousii. Twelve loci were polymorphic in the three focal species. For some loci, amplification was observed in distantly related species. Natural hybridization occurs within the genus Bufo and laboratory crosses often result in viable or fertile offspring. These microsatellite loci may be used to address questions of interspecific admixture as well as baseline intraspecific genetic variation. |
