Rapid rate of control-region evolution in PaciWc butterXyWshes (Chaetodontidae

Abstract. Sequence differences in the tRNA-proline (tRNApro) end of the mitochondrial control-region of three species of Pacific butterflyfishes accumulated 33– 43 times more rapidly than did changes within the mi-tochondrial cytochrome b gene (cytb). Rapid evolution in this region was accompanied by strong transition/ transversion bias and large variation in the probability of a DNA substitution among sites. These substitution con-straints placed an absolute ceiling on the magnitude of sequence divergence that could be detected between in-dividuals. This divergence ‘‘ceiling’ ’ was reached rap-idly and led to a decay in the relative rate of control-region/cytb b evolution. A high rate of evolution in this section of the control-region of butterflyfishes stands in marked contrast to the patterns reported in some other fish lineages. Although the mechanism underlying rate variation remains unclear, all taxa with rapid evolution in the 58-end of the control-region showed extreme transi-tion biases. By contrast, in taxa with slower control-region evolution, transitions accumulated at nearly the same rate as transversions. More information is needed to understand the relationship between nucleotide bias and the rate of evolution in the 58-end of the control-region. Despite strong constraints on sequence change, phy-logenetic information was preserved in the group of re-cently differentiated species and supported the clustering of sequences into three major mtDNA groupings. Within these groups, very similar control-region sequences were widely distributed across the Pacific Ocean and were shared between recognized species, indicating a lack of mitochondrial sequence monophyly among species