Molecular and physiological innovations of butterfly eyes

Abstract

Color vision is a complex trait that can impact the survivorship of short-lived insects like the Lepidoptera. Within this order, the color vision systems are diverse and are best known among butterflies, which are classified into five families. Several recent reviews have focused on the eyes of the basal papilionid (i.e., Papilio xuthus) and pierid butterfly (i.e., Pieris rapae) lineages (Stavenga and Arikawa 2006; Wakakuwa, Stavenga, and Arikawa 2007). Both of these groups have eyes that differ from each other and from the other butterfly families in terms of the copy number of the opsin genes that encoded the visual pigments, their spatial expression pattern, and the distribution of lateral filtering pigments. Only one study to date has examined the visual pigments in a riodinid butterfly, Apodemia mormo (Frentiu et al. 2007). This chapter focuses on recent advances in our understanding of the unique visual system of lycaenid butterflies, with a special emphasis on the sexually dimorphic retina of Lycaena rubidus (Lycaeninae) and the color vision behavior of Polyommatus icarus (Polyommatinae). It is clear from character mapping of opsin genes and their expression patterns on a phylogeny of butterfly families that all butterfly eyes are derived from a much simpler eye that resembles the nymphalid eye (Briscoe 2008). Hence, to put the innovations of the lycaenid butterfly visual system into an evolutionary framework, we begin by describing the much simpler visual system of nymphalid butterflies. We then trace the molecular changes in the opsin genes and their expression patterns, and the physiological changes in the visual receptors they encode. Lastly, we discuss the potential behavioral outcomes of the unique eye design of lycaenids. In the course of the review, we mentioned some fertile areas of interest for future study.