Proteins associated with male reproductive function often diverge rapidly between species. In free-spawning marine invertebrates, such male proteins, along with female complements borne on the surface of the egg, determine whether fertilization can occur. The species-specificity of these gamete recognition proteins has been demonstrated in vitro, and cDNA comparisons reveal the signature of positive selection in a significant excess of amino acid altering DNA substitutions relative to silent substitutions. Focusing on trochid snails of the genus Tegula, I have found:
Sister species of Tegula co-occur on the same coastline. Phylogenetic trees (based on over 1100 bp of mitochondrial sequence from each of 23 species) strongly support the monophyly of several coexisting species of Tegula, with single radiations in East Asia and Chile and two radiations (one intertidal, one subtidal) along the Pacific coast of North America. These results suggest that major barriers (e.g. great expanses of ocean, land barriers) are not required for species formation in these free-spawning snails. (Hellberg 1998. Evolution 52: 1311-1324.). Adding an additional seven Neotropical Tegula species to these analyses further strengthens this conclusion.
Tegula lysin dissolves egg coats species-specifically and diverges interspecifically via positive selection. Both the fossil record and a molecular clock (calibrated by geminate species split by the Isthmus of Panama) suggest lineages giving rise to T. funebralis and T. brunnea separated about 10 Mya, however their lysins sequences share just 35% amino acid identity, yielding a nonsynonymous substitution rate of 2.4% per million years. Despite extensive interspecific divergence,Tegula lysins maintain the same a-helical structure as those from abalone (Haliotis), even though these lineages split over 250 million years ago. (Hellberg & Vacquier. 1999. Mol Biol Evol 16: 839-848.)
Like lysin, the major acrosomal protein in Tegula sperm (TMAP) diverges rapidly between species and this divergence is promoted by positive selection. For one sympatric species pair (T. brunnea and T. montereyi) that diverged in the Pleistocene, nonsynonymous substitutions per nonsynonymous site were three times as numerous as synonymous substitutions per synonymous site. TMAP is not homologous to lysin, and thus represents an independent rapidly evolving male reproductive protein. In addition, N-terminal amino acid sequencing shows that a portion of the pre-pro region normally excised from the mature TMAP protein has been retained in the mature protein of one species. Such "pre-pro capture" may be a novel molecular mechanism that speeds divergence in proteins under positive selection. (Hellberg, Moy & Vacquier 2000. Mol Biol Evol. 17: 458-466.)
Recent developments in this project include identifying a portion of the egg-borne receptor for lysin (VERL) from Tegula funebralis, sequencing two paralogs for VERL, finding that lysin and TMAP have been duplicated in some Tegula species, and developing microsatellite and AFLP markers for T. funebralis.