RAPID RATE OF CONTROL-REGION EVOLUTION IN PACIFIC BUTTERFLYFISHES (CHAETODONTIDAE)

Sequence differences in the tRNA-proline (tRNA(pro)) end of the mitoch ondrial control-region of three species of Pacific butterflyfishes acc umulated 33-43 times more rapidly than did changes within the mitochon drial cytochrome b gene (cytb). Rapid evolution in this region was acc ompanied by strong transition/transversion bias and large variation in the probability of a DNA substitution among sites. These substitution constraints placed an absolute ceiling on the magnitude of sequence d ivergence that could be detected between individuals. This divergence ''ceiling'' was reached rapidly and led to a decay in the relative rat e of control-region/cyt b b evolution. A high rate of evolution in thi s section of the control-region of butterflyfishes stands in marked co ntrast to the patterns reported in some other fish lineages. Although the mechanism underlying rate variation remains unclear, all taxa with rapid evolution in the 5'-end of the control-region showed extreme tr ansition biases. By contrast, in taxa with slower control-region evolu tion, transitions accumulated at nearly the same rate as transversions . More information is needed to understand the relationship between nu cleotide bias and the rate of evolution in the 5'-end of the control-r egion. Despite strong constraints on sequence change, phylogenetic inf ormation was preserved in the group of recently differentiated species and supported the clustering of sequences into three major mtDNA grou pings. Within these groups, very similar control-region sequences were widely distributed across the Pacific Ocean and were shared between r ecognized species, indicating a lack of mitochondrial sequence monophy ly among species.