Allozyme variation and population genetic structure during the life history of Bufo woodhousii fowleri (Amphibia: Anura)

Allozyme variation in Bufo woodhousii fowleri was assessed to identify unique patterns of expression during the life history of this toad in eastern Mississippi and to determine the toad's population genetic structure. Of 35 loci screened, only five show differential patterns of expression among tissues; three isozymes were expressed differentially during development, and two isozymes showed tissue-specific patterns of expression. Allozyme variation at nine loci showed spatial genetical heterogeneity among breeding sites. Although previous ecological studies indicate a stepping-stone model of population structure for anuran populations which predicts isolation by distance among populations, isolation by distance did not explain population subdivision on the geographic scale of our study. Loci that show differential patterns of gene expression did not contribute any more to population structure than loci that do not show unique patterns of gene expression. Although we cannot eliminate selection as a plausible explanation for the population subdivision we observed, the most likely causes of the genetic heterogeneity among populations of this toad are genetic drift and historical effects. Temporal changes in allele frequencies between the adult and larval cohorts occurred at all the loci studied while there were fewer changes from one life history stage to the next within the cohort. Deviation from Hardy–Weinberg Equilibrium, which occurred at only 25% of the loci showing temporal variation in allele frequencies, was associated with either heterozygote deficiencies or heterozygote excess at different loci.