Geographic variation of blood and liver proteins in cricket frogs

Population and species structure of the cricket frogs, Acris crepitans and A. gryllus, were examined in terms of protein variation throughout their geographic ranges. Eight hundred and fifty animals, collected from 32 populations in 20 states, were processed to obtain electrophoretic evidence on 20 blood and liver proteins. Molecular data confirm the reproductive isolation of the two species. They had different albumins, transferrins, and hemoglobins; frogs with hybrid protein patterns were not found in regions where the two species are sympatric. Within A. crepitans eight proteins were invariant across the species range; twelve exhibited one or more variant phenotypes. The genetic integrity of the species was illustrated by the uniformity of the albumin, plasma esterases, liver LDH, GOT, GDH, MDH-NADP, NADP-reductase, and hemoglobin polypeptide 4. Its polytypic nature, with geographic divergence into Plains, Delta, and Appalachian groups, was suggested by the presence of specific hemoglobin, transferrin, and liver esterase variants in animals of certain regions. Within-population polymorphism was exhibited by the transferrins, MDH-NAD, and three complexes of liver esterases; heterozygosity was greater in central than in peripheral populations. Biochemical evidence supports the hypothesis that the two species diverged during a Pleistocene disjunction of proto-Acris populations in Florida and Mexican refuges. With the recession of the glaciers, A crepitans presumably moved from its Mexican refuge northward and northeastward, to become adapted both to grassland and deciduous forest biomes and to attain contact and overlap with A. gryllus in southeastern United States.