Gamete Recognition Proteins and Speciation

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 snails of the genus Tegula, I have found:

Sister species co-occur on the same coastline. Phylogenetic trees 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 are not required for species formation in these free-spawning snails. (Hellberg 1998). Additional Neotropical Tegula species have strengthened this conclusion, as have studies on conchs (Latiolais et. al 2006) and Caribbean reef fish (Taylor & Hellberg 2005).

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, even though these lineages split over 250 million years ago. (Hellberg & Vacquier 1999).

The lysin-binding region of the Vitelline Envelope Receptor for Lysin (VERL) also diverges rapidly by positive selection, and these interacting gamete recognition proteins coevolve (Hellberg et. al 2012).  Intriguingly, gene trees between these two proteins and mtDNA are sometimes discordant.

Along with exploring the latter, I would love to help build a more solid species tree for the genus using NGS, study the evolution of the ZP family of proteins to which VERL belongs, and look more deeply into the gene duplications evident in some Tegula GaRPs.  In collaboration with Carlos Prada, we are also trying to identify GaRPs in Caribbean sea fans.