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

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 approximately 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.