Hybridization and introgression of toads of the Bufo americanus group

Toads in the Bufo americanus group are capable of extensive interbreeding. In the laboratory hybridization frequently results in viable sexually mature offspring. The complex of toads found in eastern Texas and western Louisiana is remarkably heterogeneous suggesting either extremely close phylogenetic relationships among the toads, or wide-spread hybridization and introgression. Morphological and biochemical characters of individual toads sampled from this complex were examined. Blood proteins were analysed by acrylamide gel disc electrophoresis and the morphological characters by multiple-group discriminant analysis. Analysis of the electrophoretic banding patterns over a wide range of protein systems by cluster analysis failed to reveal meaningful differences between species or populations. Discriminant analysis showed significant overlap between populations indicating that successful hybridization is occurring. Isolation mechanisms appear to be inoperative. The breakdown of these isolating mechanisms appear to be contributing to the removal of reproductive separation between the populations and the formation of a heterogeneous interbreeding complex.