Molecular signatures of selection associated with host-plant differences in Pieris butterflies

Abstract

Adaptive traits that enable organisms to conquer novel niches and experience subsequent diversification are ecologically and evolutionarily important. The larvae of Pieris butterflies express nitrile-specifier proteins (NSPs), a key innovation for overcoming the glucosinolate (GLS)-myrosinase-based defense system of their Brassicales host-plants. NSPs are a member of the NSP-like gene family, which includes the major allergen (MA) protein, a paralog of NSP with a GLS-disarming function, and a single domain major allergen (SDMA) protein, whose function is unknown. The arms-race between a highly variable host-plant defense system and members of the NSP-like gene family is suggested to mediate diversification in both Pierid butterflies and Brassicales plants. Here, we combined feeding experiments using 25 Brassicaceae plants and five Pieris species with larval transcriptome data to investigate the evolutionary forces acting on NSP-like gene family members associated with patterns of host-plant usage. Although we observed significantly elevated nonsynonymous to synonymous substitution ratios in NSPs, no such pattern was observed in MAs or SDMAs. Furthermore, we found a signature of positive selection of NSP at a phylogenetic branch which reflects different host-plant preferences. Our data indicate that NSPs have evolved in response to shifting preferences for host plants among five Pieris butterflies, whereas MAs and SDMAs appear to have more conserved functions. Our results show that the evolution and functional differentiation of key genes used in host-plant adaptation play a crucial role in the chemical arms-race between Pieris butterflies and their Brassicales host-plants.