Apparent Selection Intensity for the Cytochrome Oxidase Subunit I Gene Varies with Mode of Reproduction in Echinoderms

When most amino acid substitutions in protein-coding genes are slightly deleterious rather than selectively neutral, life history differences can potentially modify the effective population size or the selective regime, resulting in altered ratios of non-synonymous to synonymous substitutions among taxa. We studied substitution patterns for the mitochondrial cytochrome oxidase subunit I (COI) gene in a sea star genus (Leptasterias spp.) with an obligate brood-protecting mode of reproduction and small-scale population genetic subdivision, and compared the results to available COI sequences in nine other genera of echinoderms with pelagic larvae: three sea stars, five sea urchins and one brittle star. We predicted that this life history difference would be associated with differences in the ratio of non-synonymous (dN) to synonymous (dS) substitution rates. Leptasterias had a significantly greater dN/dS ratio (both between species and within species), a significantly smaller transition/transversion rate ratio, and a significantly lower average nucleotide diversity within species, than did the non-brooding genera. Other explanations for the results, such as altered mutation rates or selective sweeps, were not supported by the data analysis. These findings highlight the potential influence of reproductive traits and other life history factors on patterns of nucleotide substitution within and between species.