UV-green dichromacy in the basal hymenopteran Sirex noctilio (Hymenoptera: Siricidae).

Abstract

A precondition for colour vision is the presence of at least two spectral types of photoreceptors in the eye. The order Hymenoptera is traditionally divided into the Apocrita (ants, bees, wasps) and the Symphyta (sawflies, woodwasps, horntails). Most apocritan species possess three different photoreceptor types. In contrast, physiological studies in the Symphyta have reported one to four photoreceptor types. To better understand the evolution of photoreceptor diversity in the Hymenoptera, we studied Sirex noctilio, which belongs to the superfamily Siricoidea, a sister group of the Apocrita. Our aim was to i) identify the photoreceptor types of the compound eye by electroretinography (ERG), ii) characterise the visual opsins genes of S. noctilio by genomic comparisons and phylogenetic analyses and iii) analyse opsin mRNA expression. ERG measurements revealed two photoreceptor types in the compound eye, maximally sensitive to 527 and 364 nm. In addition, we identified three opsins in the genome, homologous to the hymenopteran LW1, LW2 and UV opsin genes. The LW1 and UV opsins were found to be expressed in the compound eyes, and LW2 and UV opsins in the ocelli. The lack of a SW-homologous opsin gene and a corresponding receptor suggests that S. noctilio is a UV-green dichromate.