Molecular characterization and expression of the UV opsin in bumblebees:three ommatidial subtypes in the retina and a new photoreceptor organ in the lamina

Abstract

SUMMARY Ultraviolet-sensitive photoreceptors have been shown to be important for a variety of visual tasks performed by bees, such as orientation, color and polarization vision, yet little is known about their spatial distribution in the compound eye or optic lobe. We cloned and sequenced a UV opsin mRNA transcript from Bombus impatiens head-specific cDNA and, using western blot analysis, detected an eye protein band of ∼41 kDa,corresponding to the predicted molecular mass of the encoded opsin. We then characterized UV opsin expression in the retina, ocelli and brain using immunocytochemistry. In the main retina, we found three different ommatidial types with respect to the number of UV opsin-expressing photoreceptor cells,namely ommatidia containing two, one or no UV opsin-immunoreactive cells. We also observed UV opsin expression in the ocelli. These results indicate that the cloned opsin probably encodes the P350 nm pigment, which was previously characterized by physiological recordings. Surprisingly, in addition to expression in the retina and ocelli, we found opsin expression in different parts of the brain. UV opsin immunoreactivity was detected in the proximal rim of the lamina adjacent to the first optic chiasm, which is where studies in other insects have found expression of proteins involved in the circadian clock, period and cryptochrome. We also found UV opsin immunoreactivity in the core region of the antennal lobe glomeruli and different clusters of perikarya within the protocerebrum, indicating a putative function of these brain regions, together with the lamina organ, in the entrainment of circadian rhythms. In order to test for a possible overlap of clock protein and UV opsin spatial expression, we also examined the expression of the period protein in these regions.

Opsins inLimuluseyes: Characterization of three visible light-sensitive opsins unique to and co-expressed in median eye photoreceptors and a peropsin/RGR that is expressed in all eyes

“…Despite the puzzling PCR results, our present study provides strong evidence that the visible light-sensitive opsins expressed in Limulus MEs are different from those expressed in LE and larval eyes. Ocellar-specific expression of visible light-sensitive opsins has been described in insects (Pollock and Benzer, 1988;Spaethe and Briscoe, 2005;Velarde et al, 2005;Henze et al, 2012) and crustaceans (Oakley and Huber, 2004). The current study suggests that this opsin expression pattern is consistent among the visual systems of three of the four major groups of arthropods: insects, crustaceans and chelicerates.…”

“…By contrast, in Limulus and some insects, e.g. honeybees (Velarde et al, 2005), bumblebees (Spaethe and Briscoe, 2005) and fruit flies (Pollock and Benzer, 1988), the same UV opsin is expressed in ocelli and compound eyes, whereas crickets express an ocellar-specific UV opsin (Henze et al, 2012).…”

Absence of eye shine and tapetum in the heterogeneous eye ofAnthocharisbutterflies (Pieridae)

“…Ommatidial heterogeneity is a widespread characteristic of insect compound eyes. In the honeybee and bumblebee retina, three randomly distributed ommatidial types were distinguished on the basis of the ultraviolet-and blue-sensitive distal photoreceptors (Spaethe and Briscoe, 2005;Wakakuwa et al, 2005). The retina of some nymphaline butterflies is probably organized in a very similar way (Briscoe and Bernard, 2005).…”

“…The three types of ommatidia are randomly distributed in the regularly arranged hexagonal lattice of the compound eye. The ommatidial heterogeneity and random distribution of the different ommatidial types are shared by other insects (Ribi, 1978;White et al, 2003;Spaethe and Briscoe, 2005;Wakakuwa et al, 2005) and are probably related to color vision (Frisch, 1914;Kelber and Henique, 1999;Kelber and Pfaff, 1999;Kinoshita et al, 1999;Kelber et al, 2002;Zaccardi et al, 2006).…”

Daily Changes in Ultraviolet Light Levels Can Synchronize the Circadian Clock of Bumblebees (Bombus terrestris)

“…This results in significant daily changes of available UV levels, which are more pronounced than the corresponding daily changes in absolute light intensity (Krüll, 1976) and might therefore be able to trigger rhythmic wavelength-dependent behavior (Hut et al, 2000;Krüll, 1976;Nuboer et al, 1983;Pohl, 1999;Stelzer & Chittka, 2010). A new photoreceptor organ that expresses UV-sensitive opsins has recently been discovered in bumblebees in the lamina, a brain area that has been associated with circadian clock activity in other insects (Spaethe & Briscoe, 2005). It is therefore possible that the molecular clock of bumblebees can be entrained by daily cycles in UV radiation levels, which could explain the robust diurnal rhythms in their foraging patterns observed during the arctic summer (Stelzer & Chittka, 2010).…”