Molecular Evolution and Phylogeny of Actin Genes in Haliotis Species (Mollusca: Gastropoda)

Gene conversion and positive selection are often proposed as mechanisms for the evolution of biological molecules. Changes in sequence similarity have been used as evidence for gene conversion (Crain et al., 1987). Evidence for the evolution of the actin gene family by gene conversion in the echinoderms, Strongylocentrotus purpuratus and Pisaster ochraceus, came from amino acid and DNA sequence analyses (Crain et al. 1987, Kowbel and Smith 1989, White and Crother 2000). Comparison of the cytoplasmic and muscle actin genes of the sea star, P. ochraceus, revealed that the N-terminal domains of the genes were apparently conserved, but the C-terminal domains showed higher levels of variation (White and Crother 2000). Muscle-specific amino acids were only found in the C-terminal domain of the P. ochraceus muscle actin gene (Kowbel and Smith, 1989). In the sea urchin, S. purpuratus, Crain et al. (1987) found that sequence similarity was high between codons 61 and 120 in muscle and cytoplasmic actin genes with 7.9% synonymous substitutions, compared to 43.3% in the rest of the gene. Further evidence for gene conversion in the actin genes of P. ochraceus and S. purpuratus was based on sequence analyses using the GENECONV program (Drouin 2002). The role of gene conversion in the actin genes of haliotids has not been reported. Positive selection is inferred when the numMolecular Evolution and Phylogeny of Actin Genes in Haliotis Species (Mollusca: Gastropoda)