Occurrence of a species-specific nuclear antigen in the germ line of Xenopus and its expression from paternal genes in hybrid frogs.

An abundant acidic germinal vesicle protein of 100,000 Da has been previously described in Xenopus laevis and termed N1. It is supposed to bind stored histones in the oocyte. Species-specific monoclonal antibodies (mABs) have been raised against the oocyte nuclear protein of X. borealis B3, that is equivalent to protein N1 of X. laevis. These mABs have been used to monitor paternal gene expression of B3 in hybrids between X. laevis and X. borealis. Protein B3 is accumulated in oocyte nuclei, shed into the cytoplasm of the egg upon germinal vesicle breakdown, and reaccumulated by the nuclei of the embryo. During development it appears to be gradually diluted in all cells of the embryo, its levels falling below the limits of detection after stage 50. In interspecies hybrids, the paternal antigen is not found in somatic cells, as judged by immunohistological criteria. Therefore it has been concluded that protein B3 is not expressed from the genes of the embryo and that the maternal store of B3 is sufficient to endow the nuclei of the embryo with this protein up to the feeding tadpole stage. This deduction is corroborated by radiolabeling experiments. The paternal antigen B3 is, however, specifically expressed in the germ line. In hybrids and in X. borealis it is first detected in the nuclei of oogonia and spermatogonia, but, in both sexes, it is undetectable during early meiotic prophase. In female germ cells, accumulation of B3 is resumed at the beginning of diplotene, concomitant with the onset of oocyte growth. The significance of the observed cell specificity of B3 during germ cell differentiation is discussed in relation to its postulated function as a histone storage factor.