, 2007), suggesting that OBPs play an important role in ecological diversification. In our view, elucidating the precise role of this interesting gene family should be a prioritized task for the field. Like the OBPs, the gene family encoding odorant receptors (ORs) in insects is also an insect exclusive radiation. The ORs form a large and highly divergent gene family (Clyne et al., 1999 and Vosshall et al., 1999), selleckchem which shows no homology to the OR families
of vertebrates and nematodes. The insect ORs and the related gustatory receptors (GRs, Clyne et al., 2000 and Scott et al., 2001) together form an arthropod-specific chemoreceptor superfamily, in which the ORs constitute a single highly expanded branch (Robertson et al., 2003). Members of this superfamily essentially share no homology to any known gene family, and encodes for seven transmembrane-domain receptors with an inverted transmembrane topology learn more as compared to the G protein-coupled olfactory receptors of vertebrates (Benton et al., 2006). In contrast to the ORs of vertebrates, the insect ORs form heteromeric
complexes typically composed of a single ligand-binding OR (Störtkuhl and Kettler, 2001 and Dobritsa et al., 2003, but see Goldman et al., 2005) and the OR coreceptor Orco (Vosshall et al., 2000 and Larsson et al., 2004). Orco acts as a chaperone (Larsson et al., 2004) and also takes part in signal transduction (Sato et al., 2008 and Wicher et al., 2008). The rise of the OR family is assumed to date back to the early
Devonian and the first insects as an adaptation to terrestrial life not (Robertson et al., 2003). However, one could also envision that the OR radiation occurred at a later stage (perhaps first with the rise of Neoptera); being driven by the diversification of vascular plants and the increasing abundance of volatile chemicals in the environment. The latter scenario is in our view more likely. Insect ORs form a large and highly divergent gene family, with no close orthologies (apart from Orco) or apparent subfamily structure conserved across insect orders Figure 3). Thus-far-identified OR repertoires range in size from ten in Phthiraptera (i.e., lice, Kirkness et al., 2010) to ∼200–400 in Hymenoptera (i.e., bees, ants, and wasps; Robertson et al., 2010 and Wurm et al., 2011). As with the OBPs, the OR family is characterized by species-specific expansions of single genes or gene subfamilies. Recently duplicated OR loci gain novel functions through positive selection, presumably driven by needs arising from host shifts or host specializations (see below, Gardiner et al., 2008). These processes may also render previous adaptations in the chemosensory repertoire void, resulting in the loss of OR genes that no longer serve a functional purpose. Analysis of the OR repertoires of the five closest relatives of D.