On defining 'key innovations' in an adaptive radiation: Cytochrome P450s and papilionidae

The concept of the key innovation has been central to discussions of specialization and adaptive radiation in herbivorous insects. Rarely, however, have key innovations been defined mechanistically; typically, a key innovation is described broadly as a complex suite of traits (e.g., the ability to feed on angiosperm plants). Identifying the molecular genetic basis for individual traits that collectively characterize such key innovations can be useful in determining whether all species within a lineage share a particular key innovation or whether convergent innovations have evolved independently. Within the genus Papilio, the ability to feed on a furanocoumarin-containing plant has been identified as a key innovation; over 75% of the species in the genus are associated to some degree with plants containing furanocoumarins. In this group, furanocoumarin metabolism is effected by cytochrome P450 monooxygenases, heme-bound proteins known in many organisms to play a role in xenobiotic detoxification. Although furanocoumarin-metabolizing P450s in two species of Papilio do not share high levels of identity overall, conserved elements can be found; in particular, levels of amino acid identity in a putative substrate recognition site are very high. This conservation of amino acid identity contrasts sharply with reportedly high levels of diversification in substrate recognition sites observed in comparisons of proteins within species or between species that do not share a narrow range of host plants. Specialization on a particular group of host plants and subsequent diversification on those hosts may necessitate conservation of function in these regions, which may thus be considered key innovations in a lineage.