Restricted expression of paternal genes in sea urchin interspecies hybrids

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We have used two-dimensional electrophoresis to analyze the synthesis of paternal proteins in embryos of three interspecies hybrids which form healthy pluteus larvae: the reciprocal crosses of Strongylocentrotus purpuratus and Strongylocentrotus droebachiensis as well as S. purpuratus eggs fertilized with Lytechinus pictus sperm. No proteins specific to the paternal species were detectably synthesized at the two- to four-cell stage. By hatching blastula stage the synthesis of a few proteins specified by the paternal genome was detected, but these did not increase significantly during later development, and the synthesis of most distinctly paternal proteins was never detected. Radioactive cDNA probes were prepared by reversed transcription of polysome-enriched polyadenylated RNA of S. purpuratus or L. pictus gastrulae. The rate and extent of annealing of these probes to homologous sperm DNA or hybrid embryo DNA indicated that DNA coding for mRNA normally translated in embryos of the paternal species is fully retained in S. purpuratus × L. pictus hybrid gastrulae. Hybridization of these probes to excess cytoplasmic polyadenylated RNA of hybrid embryos indicated substantial underrepresentation of paternal transcripts, particularly those which are normally prevalent. These observations may be explained if much of the mRNA translated into proteins detected on two-dimensional gels is persistent maternal RNA. Alternatively, the synthesis or processing of many paternal mRNA sequences may be impaired in the hybrid embryos.