Patterns of evolutionary rate variation among genes of the anthocyanin biosynthetic pathway

The anthocyanin biosynthetic pathway is responsible for the production of a nthocyanin pigments in plant tissues and shares a number of enzymes with ot her biochemical pathways. The sir core structural genes of this pathway hav e been cloned and characterized in two taxonomically diverse plant species (maize and snapdragon). We have recently cloned these genes for a third spe cies, the common morning glory, Ipomoea purpurea. This additional informati on provides an opportunity to examine patterns of evolution among genes wit hin a single biochemical pathway. We report here that upstream genes in the anthocyanin pathway have evolved substantially more slowly than downstream genes and suggest that this difference in evolutionary rates may be explai ned by upstream genes being more constrained because they participate in se veral different biochemical pathways. In addition, regulatory genes associa ted with the anthocyanin pathway tend to evolve mon rapidly than the struct ural genes they regulate, suggesting that adaptive evolution of flower colo r may be mediated more by regulatory than by structural genes. Finally, for individual anthocyanin genes, we found an absence of rate heterogeneity am ong three major angiosperm lineages. This rate constancy contrasts with an accelerated rate of evolution of three CHS-like genes in the Ipomoea lineag e, indicating that these three genes have diverged without coordinated adju stment by other pathway genes.