A rapidly diverging EGF protein regulates species-specific signal transduction in early sea urchin development

Abstract

The macromolecules mediating species-specific events during fertilization and early development and their molecular evolution are only beginning to be understood. We screened sea urchin ovary mRNA for species-specific gene products using representational differential analysis to identify unique transcripts in Strongylocentrotus franciscanus that are absent or divergent from a closely related species, S. purpuratus. One of the transcripts identified by this screening process is SfEGF-II, which contains four EGF repeats. SfEGF-II is orthologous to the previously reported genes S. purpuratus SpEGF-II and Anthocidaris crassispina AcEGF-II, encoding exogastrulation-inducing peptides (EGIP). EGF peptides derived from EGIP induce exogastrulation, a classical developmental defect, when added to embryos prior to gastrulation. The first three EGF repeats (EGF1-3) share 50 to 60% identity among the three species, but the fourth repeat (EGF4) is more divergent, displaying only 30% identity. Analysis of the sequence divergence indicates that the EGF-II genes display a relatively high nonsynonymous-to-synonymous ratio, a significant excess of radical compared to conservative amino acid substitutions, and a lack of polymorphism within SfEGF-II, indicating that these genes have been subjected to positive Darwinian selection. Recombinant EGF3 from S. franciscanus induces exogastrulation in both S. franciscanus and S. purpuratus. In contrast, recombinant EGF4 from both S. franciscanus and S. purpuratus induces exogastrula in a species-specific manner. In hybrid embryos, both species of EGF4 induce exogastrulation, suggesting that the receptor for this EGF molecule is expressed from both parental genomes during development. Both EGF3 and EGF4 induce the phosphorylation of membrane proteins of the blastula stage embryos, but EGF4 stimulates phosphorylation of proteins only in membranes prepared from homologous embryos, suggesting that it utilizes a unique pathway involving a species-specific receptor for EGF4. Thus, species-specific events of gastrulation and early development may be controlled by these rapidly diverging EGF molecules, through a novel species-specific signal transduction pathway.