Variation in constraint versus positive selection as an explanation for evolutionary rate variation among anthocyanin genes.

It has been argued that downstream enzymes in metabolic pathways are expected to be subject to reduced selective constraint, while upstream enzymes, particularly those at pathway branch points, are expected to exhibit more frequent adaptive substitution than downstream enzymes. We examined whether these expectations are met for enzymes in the anthocyanin biosynthetic pathway in Ipomoea. Previous investigations have demonstrated that downstream enzymes in this pathway have substantially higher rates of nonsynonymous substitution than upstream enzymes. We demonstrate here that the difference in rates between the most upstream enzyme (CHS) and the two most downstream enzymes (ANS and UFGT) is explained almost entirely by differences in levels of selective constraint. Adaptive substitutions were not detected in any of these genes. Our results are consistent with suggestions that constraint is greater on enzymes with greater connectivity.