Animals live in diverse habitats that vary in their light spectral quality. In terrestrial habitats the most extreme differences in spectral light quality occur between low and high altitudes, and between open habitats like grassland and dense forest (Endler 1993). However, most studies so far fail to show correlations between the spectral sensitivity of photoreceptors and the photic environment of the specific habitat, in spite of a distinct variation of spectral light quality among habitats of the species under comparison. Therefore, the possible mechanisms responsible for adaptation of terrestrial animals to their photic environment are still under debate.Bumblebees (Bombus) provide an ideal system to address these questions since they inhabit almost every terrestrial habitat and for many taxa the spectral sensitivity of their visual system is well characterized (Briscoe & Chittka 2001). Here we investigate whether bumblebees adapt to different photic environments by varying the relative level of visual pigments (opsins) in order to change the sensitivity of the respective photoreceptor types. The complex eye of bumblebees is composed of three different receptor types sensitive in the UV, blue and green part of the light spectrum (Briscoe & Chittka 2001). It has already been shown that the mRNA expression level of the long-wavelength (LWRh) opsin of Apis mellifera varies significantly over a 24 h cycle (Sasagawa et al 2003). We hypothesize that opsin expression levels correlate with the photic environments, e.g. bumblebees in alpine habitats, where UV radiation is high, express lower levels of UV opsin mRNA than bumblebees in lowland habitats or forests, where light spectrum is strongly shifted to longer wavelengths. Using real-time PCR, we first characterised the expression level of three opsin mRNAs (UV, blue and green) of Bombus terrestris individuals kept under controlled 12:12 hours L:D light regime over a 24 h period. We then entrained two experimental groups to short- and longwavelength shifted illumination, respectively, using color filters, and measured possible changes in the mRNA levels of the three opsins. Finally, we compared opsin expression levels of individuals of different Bombus species caught in the field from alpine habitats at altitudes > 2500 meters asl and from low land habitats.

Hummeln leben in unterschiedlichen Habitaten, ein Merkmal, das diese Habitate unterscheidet ist die spektrale Zusammensetzung des Habitats. Die größten Unterschiede in der spektralen Umgebung von Hummeln finden sich zwischen alpinen und Tiefland- Habitaten (Endler 1993). Anpassungen der spektralen Empfindlichkeit der Photorezeptoren an die photische Umwelt wurden bisher nicht gefunden (Peitsch et al 1992). In unserer Arbeit gingen wir der Frage nach, ob Anpassungen an unterschiedliche spektrale Bedingungen auf der Ebene der Genexpression zu finden sind. Zu diesem Zweck wurden Hummeln in unterschiedlichen spektralen Umgebungen gehalten und ihre Opsinexpression mittel Real-Time PCR gemessen. Weiters wurden Hummeln aus unterschiedlichen Höhenlagen gesammelt und die Opsinexpression wurde ebenfalls bestimmt. Wir konnten zeigen, daß das Sehsystem von Hummeln auf unterschiedliche spektrale Umgebungen auf der Ebene der Genexpression reagiert.