Enzyme patterns in two species of Xenopus and their hybrids

Differences in the isozyme patterns of Xenopus laevis and Xenopus mulleri have been utilized to examine the expression of alleles of both species in hybrid animals. Mitochondrial MDH and tetrazolium oxidase phenotypes were examined during the development of non-hybrid embryos of each species and of reciprocal hybrids. Early stages of the hybrids resemble the enzyme phenotype of the maternal parent. Appearance of paternal enzyme takes place just prior to the active feeding tadpole stage for both mitochondrial MDH and oxidase. The maternal effect disappears shortly thereafter in early feeding tadpoles, at which point reciprocal hybrids have identical isozyme patterns. There is no evidence for a predominance of one species over the other. Examination of feeding tadpoles and adult toads indicates that both laevis and mulleri expression is stable. The appearance of paternal mitochondrial MDH does not correspond to the time when other mitochondrial components begin to increase in Xenopus. Multiple bands of MDH in both species and of oxidase in laevis are probably not due to the aggregation of subunits produced by different alleles at the same locus. There is no evidence for the formation of “hybrid” molecules consisting of subunits of both species.