Transitory hemizygosity of paternally derived alleles in hybrid trout embryos

Abstract

IT is generally assumed that in species with yolk-rich eggs early embryonic development is sustained by maternally transmitted storage substances in the egg cytoplasm^1,2, while embryonic genes become activated later in development. As a rule, the maternally and paternally derived alleles of a particular gene locus start to function simultaneously at a defined developmental stage. But preferential activation of the maternally derived allele has been observed^3–7. This prompted the suggestion^2,4 that the information required to switch on embryonic structural gene loci is transcribed by the diploid nucleus of the mother during oogenesis. According to this hypothesis, regulatory substances from the egg cytoplasm bind to receptor sequences adjacent to the structural genes, and thus induce their initial derepression. When the parentally derived receptor sequences are identical, the two alleles at the respective structural gene locus become activated synchronously. But when the base sequence of the paternally derived receptor site is different from the maternally transmitted one, for example when the two haploid genomes stem from different species, the recognition of the paternally derived receptor sequence by the regulatory substance of the egg might become difficult or even impossible. This would lead to either transitory or permanent exclusive expression of the maternally transmitted structural gene. Using the first appearance of embryonically specified α-glycerophosphate dehydrogenase (α-GPDH) isozymes as markers during the ontogeny of interspecific hybrid trout, we have now observed preferential activation of the paternally derived alleles. This seems to contradict the model of gene activation outlined here.