POSITIVE SELECTION IS A GENERAL PHENOMENON IN THE EVOLUTION OF ABALONE SPERM LYSIN

Lysin is a 16kDa acrosomal protein used by abalone sperm to create a h ole in the egg vitelline envelope (VE). The interaction of lysin with the VE is species-selective and is one step in the multistep fertiliza tion process that restricts heterospecific (cross-species) fertilizati on. For this reason, the evolution of lysin could play a role in estab lishing prezygotic reproductive isolation between species. Previously, we sequenced sperm lysin cDNAs from seven California abalone species and showed that positive Darwinian selection promotes their divergence . In this paper an additional 13 lysin sequences are presented represe nting species from Japan, Taiwan, Australia, New Zealand, South Africa , and Europe. The total of 20 sequences represents the most extensive analysis of a fertilization protein to date. The phylogenetic analysis divides the sequences into two major clades, one composed of species from the northern Pacific (California and Japan) and the other compose d of species from other parts of the world. Analysis of nucleotide sub stitution demonstrates that positive selection is a general process in the evolution of this fertilization protein. Analysis of nucleotide a nd codon usage bias shows that neither parameter can account for the r obust data supporting positive selection. The selection pressure respo nsible for the positive selection on lysin remains unknown.