POSITIVE DARWINIAN SELECTION ON 2 HOMOLOGOUS FERTILIZATION PROTEINS -WHAT IS THE SELECTIVE PRESSURE DRIVING THEIR DIVERGENCE

Most examples of positive selection inferred from nucleotide sequence data involve host-pathogen interactions. However, positive selection a lso promotes the divergence of proteins mediating sperm-egg recognitio n in marine invertebrates. The abalone spermatozoon has a large acroso mal vesicle containing two proteins of 16 kDa and 18 kDa. Lysin, the 1 6-kDa protein, exhibits species-specificity in dissolving a hole in th e egg vitelline envelope through which the sperm swims to reach the eg g plasma membrane. The 18-kDa protein coats the sperm acrosomal proces s and probably mediates fusion of the two gametes, In this review, we compare sequences of both proteins from five species of California aba lones. Both proteins show extensive divergence which has been promoted by positive Darwinian selection. The ratios of nonsynonymous to synon ymous nucleotide substitutions may be the highest yet discovered for f ull-length sequences. Although extensive divergence has occurred, ther e is conservation of the shape and polarity of residues in both protei ns. The two acrosomal proteins arose by a gene duplication followed by their extensive divergence. Five hypotheses are presented which attem pt to explain the nature of the unknown selective force responsible fo r the robust positive selection. The positive selection may, in some u nknown way, be related to the establishment of prezygotic barriers to reproduction. Because positive selection promotes the divergence of un related, species-specific gamete recognition proteins in both abalones and sea urchins, we predict that positive selection may be a general phenomenon in the evolution of gamete recognition systems in marine in vertebrates.