Recent relaxation of purifying selection on the tandem-repetitive early-stage histone H3 gene in brooding sea stars

Patterns of nucleotide substitution differ between marine species that have a pelagic feeding (planktotrophic) larval stage and related species that lack such a stage, for both adaptive and non-adaptive reasons. Here, patterns of nucleotide and inferred amino acid substitution are analyzed for the tandem-repetitive early-stage histone H3 gene in 36 sea star species of the order Forcipulatida with documented larval habitat. The relative rate of nonsynonymous substitution (expressed as ω = d_N/d_S) was significantly higher in lineages with a brooded non-feeding (lecithotrophic) larval form than in lineages with a planktotrophic larval form. There was also a significant excess of conservative over radical substitutions. The increase in ω for brooders as compared to non-brooders was much greater than for previously analyzed mitochondrial sequences in echinoderms. These data are consistent with the hypothesis that purifying selection on this gene has been relaxed in brooding lineages compared to non-brooding lineages. The hypotheses of adaptive or neutral evolution are less plausible, although recent pseudogenization following a period of relaxed purifying selection could also explain the results.