CYP6B1 and CYP6B3 of the black swallowtail (Papilio polyxenes): adaptive evolution through subfunctionalization

Gene duplication provides essential material for functional divergence of proteins and hence allows organisms to adapt to changing environments. Following duplication events, redundant paralogs may undergo different evolutionary paths via processes known as nonfunctionalization, neofunctionalization, or subfunctionalization. Studies of adaptive evolution at the molecular level have progressed rapidly by computationally analyzing nucleotide substitution patterns but such studies are limited by the absence of information relating to alterations of function of the encoded enzymes. In this respect, evolution of the Papilio polyxenes cytochrome P450 monooxygenases (P450s) responsible for the adaptation of this insect to furanocoumarin-containing host plants provides an excellent model for elucidating the evolutionary fate of duplicated genes. Evidence from sequence and functional analysis in combination with molecular modeling indicates that the paralogous CYP6B1 and CYP6B3 genes in P. polyxenes have probably evolved via subfunctionalization after the duplication event by which they arose. Both enzymes have been under independent purifying selection as evidenced by the low dN/dS ratio in both the coding region and substrate recognition sites. Both enzymes have maintained their ability to metabolize linear and angular furanocoumarins albeit at different efficiencies. Comparisons of molecular models developed for the CYP6B3 and CYP6B1 proteins highlight differences in their binding modes that account for their different activities toward linear and angular furanocoumarins. That P. polyxenes maintains these 2 furanocoumarin-metabolizing loci with somewhat different activities and expression patterns provides this species with the potential to acquire P450s with novel functions while maintaining those most critical to its exclusive feeding on its current range of host plants.