Nuclear and cytoplasmic contributions to dehydrogenase phenotypes in hybrid frog embryos

Abstract

Frogs in the Rana pipiens group including R. palustris and geographic races of R. pipiens form viable diploid hybrids which develop through metamorphosis. There are differences in electrophoretic mobilities of several enzymes among the frogs used for hybridization. After electrophoresis of extracts of hybrid frog embryos on starch gel the maternal, paternal, and possible hybrid molecules are detected with appropriate specific enzyme staining. In early embryos only the maternal type enzymes are present. In viable diploid hybrids the paternal form and a ‛hybrid” molecule of 6-phosphogluconate dehydrogenase (6PGD) are detected by stage 17 ½ (late tail bud). Similarly, evidence of gene expression for the B subunit of lactate dehydrogenase (LDH) is seen at stage 18 ½ (muscular movement). Expression of the gene for soluble isocitrate dehydrogenase (IDH-s) is seen by stage 19 (heart beat). A nuclear gene controls mitochondrial malate dehydrogenase (MDH-m), and its expression is also evident by stage 19 heart beat). A maternal effect on each of the enzyme phenotypes persists even after these events; in the case of LDH and MDH, until after stage 25 (feeding tadpole). Androgenetic haploid hybrids show a range of development. The homologous combinations show the typical haploid syndrome but develop to the feeding stage. Heterologous combinations show more severe symptoms; some arrest at neurula stages, but others develop through hatching. In androgenetic haploid hybrids the paternal (nuclear) forms of the dehydrogenases studied appear at the same time as the diploid controls. This occurs even though the haploids are grossly abnormal and retarded in morphogenesis. Since the maternal genome was not present in such haploid embryos, the maternal effect on enzyme phenotypes clearly must be cytoplasmic in origin. No hybrid enzymes (composed of both maternal and paternal type subunits) are detected in androgenetic haploid hybrids. This and other evidence is consistent with the idea that the maternal effects are due to enzymes present in the egg prior to fertilization, not to continued synthesis directed by stable messenger RNA. It also indicates that the degradation of maternal (cytoplasmic) enzymes in vivo does not yield subunits capable of reaggregation with newly synthesized subunits to form active enzymes.