Evidence for UV-green dichromacy in the basal hymenopteran Sirex noctilio (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 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 identified 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.

Chemical and visual ecology of the Symphyta

“…A single electroretinogram study (Peitsch et al, 1992) of four species demonstrated that some Symphyta possess an extra photoreceptor sensitive to red light. This photoreceptor seems to be lost in the rest of the Hymenoptera (Guignard et al, 2021;Peitsch et al, 1992).…”

“…Narrow-band LED and paint could also be used to determine the most attractive colour, if any. In addition, the loss of blue photoreceptor and associated gene in S. noctilio demonstrates that Symphyta possess from two to four photoreceptors in a few closely related families (Guignard et al, 2021). As a result, the Symphyta offer a unique opportunity to study the ecological mechanisms (e.g., feeding habits) underlying the rapid gain and loss of visual opsins and associated photoreceptors.…”

“…The ultraviolet and blue photoreceptors are assumed to be present in all species, but were not found during intracellular recordings of T. scrophulariae , X. cameus and U. gigas (Briscoe & Chittka, 2001). A more recent study showed that S. noctilio has lost the blue photoreceptor and its associated opsin gene (Guignard et al, 2021). Males and females of this woodwasp express both the ultraviolet (λ max = 364 nm) and green photoreceptors (λ max = 527 nm), but the red photoreceptor found in other Symphyta seems to have been lost in the Siricidae and in the Apocrita (Guignard et al, 2022).…”

“…A single electroretinogram study (Peitsch et al, 1992) of four species demonstrated that some Symphyta possess an extra photoreceptor sensitive to red light. This photoreceptor seems to be lost in the rest of the Hymenoptera (Guignard et al, 2021; Peitsch et al, 1992). The role of the extra red photoreceptor is unknown, but it could allow Symphyta to respond to light not visible to other Hymenoptera.…”

Colour vision in ants (Formicidae, Hymenoptera)

“…The absence of a blue light-sensitive PR is common to many ants and has been noted for several other insect taxa. For example, the loss of the blue-light-sensitive PR has been shown in the basal hymenopteran Sirex noctilio [44] and in most beetle species [21]. The functional significance of this absence raises several questions, which should be addressed in future studies.…”