Meeting Abstract
11.10 Sunday, Jan. 4 Results from hybrid genotypes question the role of M7 lysin in blue mussel gamete recognition MCCARTNEY, Michael A.*; LIMA, Thiago G.; YUND, Philip O.; Univ of North Carolina, Wilmington; Univ of North Carolina, Wilmington; University of New England mccartneym@uncw.edu
M7 lysin is a sperm protein that dissolves the egg vitelline coat, prior to fertilization, in blue mussels in the Mytilus species complex. As in abalone lysin, DNA sequences of M7 lysin show an excess of nonsynonymous substitutions indicative of positive selection. These results have been used to conclude that M7 lysin is a gamete recognition protein; that is, that it controls species-specific fertilization. We tested this hypothesis in the Gulf of Maine hybrid zone between M. edulis and M. trossulus. We predicted that introgression at a gamete recognition locus should be reduced compared to marker loci not involved in species recognition. Results were to the contrary: we found a much higher rate of introgression of M7 lysin alleles into M. trossulus genetic backgrounds compared to alleles at marker loci. Introgression at M7 lysin is biased, and does not occur into M. edulis backgrounds. Sequencing of cloned alleles showed multiple amino acid substitutions within the fifth exon, confirming that hybrid lysin heterozygotes carry one M. edulis and one M. trossulus allele. We have found no consistent differences between the compatibility of sperm from male lysin heterozygotes with eggs from pure M. edulis females, compared to homozygous males. Both the pattern of introgression and these fertilization results question the role of M7 lysin in species-specific gamete recognition. However, evidence for positive selection indicates that the gene is under selection, and the high rate of retention of introgressed alleles in highly backcrossed hybrids suggests that alleles contributed through hybridization provide some advantage. Selection within species (e.g. under sperm competition) may favor these divergent lysin alleles, and may help drive adaptive evolution of Mytilus lysin.
