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

The macromolecules mediating species-specific events during fertilization a nd early development and their molecular evolution are only beginning to be understood. We screened sea urchin ovary mRNA for species-specific gene pr oducts using representational differential analysis to identify unique tran scripts in Strongylocentrotus franciscanus that are absent or divergent fro m a closely related species, S. purpuratus. One of the transcripts identifi ed by this screening process is SfEGF-II, which contains four EGF repeats. SfEGF-II is orthologous to the previously reported genes S. purpuratus SpEG F-II and Anthocidaris crassispina AcEGF-II, encoding exogastrulation-induci ng peptides (EGIP). EGF peptides derived from EGIP induce exogastrulation, a classical developmental defect, when added to embryos prior to gastrulati on. 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 E GF-II genes display a relatively high nonsynonymous-to-synonymous ratio, a significant excess of radical compared to conservative amino acid substitut ions, and a lack of polymorphism within SfEGF-II, indicating that these gen es have been subjected to positive Darwinian selection. Recombinant EGF3 fr om S. franciscanus induces exogastrulation in both S. franciscanus and S. p urpuratus. In contrast, recombinant EGF4 from both S. franciscanus and S. p urpuratus induces exogastrula in a species-specific manner. In hybrid embry os, both species of EGF4 induce exogastrulation, suggesting that the recept or for this EGF molecule is expressed from both parental genomes during dev elopment. Both EGF3 and EGF4 induce the phosphorylation of membrane protein s of the blastula stage embryos, but EGF4 stimulates phosphorylation of pro teins only in membranes prepared from homologous embryos, suggesting that i t 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 speci es-specific signal transduction pathway. (C) 2000 Academic Press.