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 (sawfies, woodwasps, horntails). Most apocritan species possess three diferent
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 the Symphyta Sirex noctilio, which belongs to the superfamily Siricoidea, a
closely related group of the Apocrita suborder. Our aim was to (i) identify the photoreceptor types of
the compound eye by electroretinography (ERG), (ii) characterise the visual opsin 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 identifed three opsins in the genome, homologous to the hymenopteran
green or long-wavelength sensitive (LW) LW1, LW2 and ultra-violet sensitive (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 blue or short-wavelength sensitive (SW) homologous opsin gene and a
corresponding receptor suggests that S. noctilio is a UV-green dichromate.
