Characterisation of a functional allatotropin receptor in the bumblebee, Bombus terrestris (Hymenoptera, Apidae).

Allatotropins (ATs) are multifunctional neuropeptides initially isolated from the tobacco hornworm, Manduca sexta, where they were found to stimulate juvenile hormone synthesis and release from the corpora allata. ATs have been found in a wide range of insects, but appear to be absent in Drosophila. The first AT receptor (ATR) was characterised in 2008 in the lepidopteran Bombyx mori. Since then ATRs have been characterised in Coleoptera and Diptera and in 2012, an AT precursor gene was identified in hymenopteran species. ATRs show large sequence and structural similarity to vertebrate orexin receptors (OXR). Also, AT in insects and orexin in vertebrates show some overlap in functions, including modulation of feeding behaviour and reproduction. The goal of this study was to identify a functional ATR in a hymenopteran species. We used ATRs (insect sequences) and OXRs (vertebrate sequences) to search the genome of the bumblebee, Bombus terrestris. Two receptors (XP_003402490 and XP_003394933) with resemblance to ATRs and OXRs were found. Phylogenetic analysis provided the first indication that XP_003402490 was more closely related to ATRs than XP_003394933. We investigated the transcript level distribution of both receptors and the AT precursor gene by means of quantitative real-time reverse transcriptase PCR. XP_003402490 displayed a tissue distribution comparable with ATRs in other species, with high transcript levels in the male accessory glands. After pharmacological characterisation, it appeared that XP_003402490 is indeed a functional ATR. Activation of the receptor causes an increase in intracellular calcium and cyclic AMP levels with an EC50 value in the low nanomolar to picomolar range. XP_003394933 remains an orphan receptor.