The Divergence of Species-Specific Abalone Sperm Lysins is Promoted by Positive Darwinian Selection

Recognition by sperm lysin of the egg vitelline envelope may be one component in determining the species-specificity of fertilization in abalones. The amino acid sequences of lysin proteins of seven California abalone species were deduced from the cDNA sequences. This is the first extensive comparison of a gamete recognition protein from congeneric species. Each prelysin has a highly conserved signal peptide of 18 amino acids, followed by a mature sequence of 136-138 residues. Of 136 aligned positions, 68 have the same amino acid in all seven sequences. The % identity relative to the red abalone lysin sequence is: white 90%, flat 83%, pinto 82%, pink 78%, black 71%, and green 65%. Hydropathy plots and a distance tree of the seven lysins show that red, white, and flat lysins are more closely related to each other than to the lysins of the other four species. A hypervariable, species-specific, domain exists in all sequences between positions 2-12. Amino acid replacements between any two lysins are mostly nonconservative. Analysis of the cDNA sequences shows the number of nonsynonymous substitutions (amino acid altering) exceeds the number of synonymous substitutions (silent) in 20 of the 21 pairwise comparisons of the seven sequences, indicating that positive Darwinian selection must promote the divergence of lysin sequences.