Spatial distribution of opsin-encoding mRNAs in the tiered larval retinas of the sunburst diving beetle Thermonectus marmoratus (Coleoptera: Dytiscidae)

Abstract

SUMMARYLarvae of the sunburst diving beetle, Thermonectus marmoratus, have a cluster of six stemmata (E1-6) and one eye patch on each side of the head. Each eye has two retinas: a distal retina that is closer to the lens, and a proximal retina that lies directly underneath. The distal retinas of E1 and E2 are made of a dorsal and a ventral stack of at least twelve photoreceptor layers. Could this arrangement be used to compensate for lens chromatic aberration, with shorter wavelengths detected by the distal layers and longer wavelengths by the proximal layers? To answer this question we molecularly identified opsins and their expression patterns in these eyes. We found three opsin-encoding genes. The distal retinas of all six eyes express long-wavelength opsin (TmLW) mRNA, whereas the proximal retinas express ultraviolet opsin (TmUV I) mRNA. In the proximal retinas of E1 and E2, the TmUV I mRNA is expressed only in the dorsal stack. A second ultraviolet opsin mRNA (TmUV II), is expressed in the proximal retinas of E1 and E2 (both stacks). The finding that longer-wavelength opsins are expressed distally to shorter-wavelength opsins makes it unlikely that this retinal arrangement is used to compensate for lens chromatic aberration. In addition, we also described opsin expression patterns in the medial retina of E1 and in the non-tiered retina of the lensless eye patch. To our knowledge, this is also the first report of multiple UV opsins being expressed in the same stemma.

Spectral sensitivity of the principal eyes of sunburst diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae), larvae

“…Interestingly, in contrast to the first instar larvae principal eyes, in which TmUV I mRNA is weakly expressed in the dorsal half of the proximal retina (Maksimovic et al, 2009), we did not find any TmUV I mRNA expression in the principal eyes of third instars. Therefore, TmUV I is either absent or expressed at levels too low to be detected by our methods.…”

“…Our findings demonstrate that in third instar larvae, just as in first instar larvae (Maksimovic et al, 2009) UV and LW opsin mRNA expression is clearly separated, closely following morphological distinctions between different retinas. The in situ hybridization results suggest that the proximal retina of each eye specifically expresses the TmUV II mRNA, whereas the two distal retinas express the TmLW mRNA (Fig.2).…”

“…In situ hybridization was performed using RNA probes against opsin mRNA sequences (TmLW, TmUV I and TmUV II) cloned from T. marmoratus first instar larvae (Maksimovic et al, 2009). The procedure was identical to the one described, except that in this study we used third instar larvae and focused on the opsin expression patterns in the distal and proximal retina of the principal eyes, E1 and E2.…”

“…They are characterized by six eyes (Fig.1A) and an eye spot (not illustrated) on each side of the head. Each of the eyes has at least two distinct, tiered regions of photoreceptor cells that constitute a distal and proximal retina (Maksimovic et al, 2009;Mandapaka et al, 2006). Most unusual are the forward-pointing principal eyes, eye 1 (E1) and eye 2 (E2), which share many anatomical characteristics (Fig.1B).…”

“…We recently found that in each of the principal eyes of first instar larvae, the distal retina expresses one opsin, predicted to be long-wavelength sensitive (TmLW), whereas the proximal retina expresses two opsins, TmUV I and TmUV II. There, TmUV I is expressed weakly in the dorsal region and TmUV II is expressed strongly throughout the entire proximal retina (Maksimovic et al, 2009). Both of these opsins are predicted by sequence homology to be ultraviolet (UV) sensitive.…”