Hybridization between the estuarine fishes yellowfin bream, Acanthopagrus australis (Gunther), and black bream, A. butcheri (Munro) (Pisces : Sparidae)

Abstract

The status of bream, from estuaries near Narooma on the south coast of New South Wales, which display morphological characteristics intermediate to those distinguishing the species A. australis and A. butcheri was investigated. The two species were separated electrophoretically at a malate dehydrogenase locus (Mdh-1) where they are fixed for alternative alleles, and by a significant difference in allele frequency at the lactate dehydrogenase locus Ldh-A. Because four individual bream, which displayed intermediate morphometric characteristics, have inherited both species-specific Mdh-1 alleles and are heterozygous at the Ldh-A locus, their status as hybrids is determined. The presence of three bream with allelic characteristics of A. australis at the Mdh-1 locus but allelic characteristics of A. butcheri at the Ldh-A locus is evidence for the presence of later generation or backcross hybrids. No evidence of introgression was found. The reproduction of A. australis and A. butcheri is temporally and spatially isolated under normal circumstances but in some landlocked, coastal lakes there is a breakdown of these isolating mechanisms. Because A. australis and A. butcheri differ morphometrically and ecologically over their sampled ranges, and only appear to hybridize in the unusual environmental conditions of closed lakes, it is suggested that their specific status be retained. Using electrophoretic data, a high level of genic similarity (I = 0.91) was found to exist between the two species. This, together with the close relationship indicated by the production of fertile F₁ hybrids, suggests a recent speciation.