Aberrant Gene Expression during the Development of Hybrid Sunfishes (Perciformes, Teleostei)

Reciprocal intergeneric hybrid crosses were made between two species of centrarchids (Perciformes, Teleostei), the largemouth bass (Micropterus salmoides) (LMB) and the green sunfish (Lepomis cyanellus) (GSF). The LMB ♀ × GSF ♂ hybrid embryos underwent apparently normal morphological development. However, the hybrid embryos derived from the reciprocal GSF ♀ × LMB ♂ cross were morphologically abnormal, and all died near the time of hatching. Starch gel electrophoresis and specific histochemical staining techniques were utilized to investigate the ontogeny of enzymes encoded at each of 13 loci during the development of embryos of both parental species and both types of hybrids. Enzymes encoded in eight loci were present throughout the embryogenesis of one or both parental species. Enzymes encoded in these eight loci were also expressed throughout the development of both types of hybrid embryos. The electrophoretic mobilities of isozymes encoded in five of these eight loci were different for the two parental species. Although the maternal alleles at each of these five loci were expressed continuously during the development of both types of hybrid embryos, the parental alleles were not expressed until later in development. The five remaining loci were not expressed in the unfertilized eggs of either parental species, but were first expressed after fertilization. The onset of the expression of homologous loci was similar in embryos of the two parental species. During the development of the LMB ♀ × GSF ♂ hybrid embryos, the onset of gene expression for these five loci was delayed compared to that in the embryos of the two parental species. However, during the development of the GSF ♀ × LMB ♂ hybrid embryos, the onset of gene expression for most of these loci was earlier than that in the embryos of the two parental species. This precocious gene expression may reflect an aberrant pattern of gene regulation responsible for the abnormal development and lethality at hatching of the GSF ♀ × LMB ♂ hybrid embryos. A model is proposed to explain the various patterns of gene expression observed during embryogenesis of these reciprocal hybrids.