INTROGRESSIVE HYBRIDIZATION BETWEEN TWO SOUTHEASTERN TREEFROGS

Abstract

Natural interspecific hybridization in anuran amphibians is not rare, and a number of examples are recorded. There are, however, few conclusively proven instances of introgressive hybridization in the restricted sense of gene exchange between species. The best known case is that of the toads Bufo woodhousei fowleri and Bufo terrestris americanus (A. P. Blair, 1941; Volpe, 1952; and others). A related example is reported by A. P. Blair (1955) between Bufo w. woodhousei and Bufo microscaphus, and Wasserman (1957) provides evidence of introgression in the spadefoot toads Scaphioputs hurteri and Scaphio pus couchi. Most other presumed examples are based largely on simple identification of hybrids with no evidence for introgression, or belong to the borderline category of closely related contiguously allopatric forms that interbreed freely along a narrow zone of contact. This paper deals with a clear case of interspecific gene exchange between the green treefrog, Hyla cinerea, and the barking treefrog, Hyla gratiosa, near Auburn, Alabama. Both species are widely distributed over the southeastern coastal plain. H. gratiosa ranges from North Carolina along the coastal plain (exclusive of southern Florida) to southeastern Louisiana, extending locally above the fall line, and onto the Cumberland Plateau. H. cinerea ranges from Maryland to south-central Texas, up the Mississippi Basin to southern Illinois, and into southern Florida. The two forms are, therefore, sympatric over the greater