Bindin genes of the Pacific oyster Crassostrea gigas.

When Crassostrea gigas oyster sperm acrosome react a ring of bindin protein is exposed that bonds the sperm to the egg vitelline envelope. The putative functional unit of bindin is a fucose lectin (F-lectin) domain that is structurally conserved among phyla. There is only one bindin gene in C. gigas, which can possess 1-5 tandem F-lectin repeats. Alternative splicing can alter the number of repeats per bindin mRNA. Recombination occurs in a highly variable intron in the middle of each F-lectin repeat to create many different lectin domain sequences [Moy, G.W., Springer, S.A., Adams, S.L., Swanson, W.J., Vacquier, V.D., 2008. Extraordinary intraspecific diversity in oyster sperm bindin. Proc. Natl. Acad. Sci. U.S.A. 105, 1993-1998]. Two bindin genes were sequenced to learn more about bindin introns. The first gene (6914 bp) contained one F-lectin repeat. The second gene (25,932 bp) contained three tandem F-lectin repeats. Four of the introns in this larger gene are conserved in size among individuals. However, the one intron in each F-lectin repeat is highly variable in size and sequence, indicating that it has been a hot spot for recombination. A retroposon with high reverse transcriptase homology is present in the three repeat gene immediately upstream of the first F-lectin repeat, suggesting that retroposition is one mechanism by which F-lectin repeats are duplicated. The retroposon is not present in the one F-lectin repeat bindin gene. Three GA microsatellites, one in each intron immediately upstream of the start of each F-lectin repeat exon, and one downstream CT microsatellite, suggest that loopout strand hybridization can occur, and lectin repeats replicate and transpose within the gene. The CT microsatellite is not found in the one F-lectin repeat containing gene. Oysters appear to use every possible mechanism to create variation in the F-lectin domains of sperm bindin. This is presumably in response to sexual conflict that operates in the prevention of polyspermy.