Expression and evolutionary divergence of the non-conventional olfactory receptor in four species of fig wasp associated with one species of fig

A newly identified Or83b orthologous gene was named Or2. Expressions of Or2 were restricted to the heads of all wingless male fig wasps, which usually live in the dark cavity of a fig throughout their life cycle. However, expressions were widely detected in the antennae, legs and abdomens of all female fig wasps that fly from one fig to another for oviposition, and secondarily pollination. Weak expression was also observed in the thorax of PFWs. Compared with NPFWs, the Or2 gene in C. solmsi had an elevated rate of substitutions and lower codon usage. Analyses using Tajima's D, Fu and Li's D* and F* tests indicated a non-neutral pattern of nucleotide variation in all fig wasps. Unlike in NPFWs, this non-neutral pattern was also observed for synonymous sites of Or2 within PFWs.The sex- and species-specific expression patterns of Or2 genes detected beyond the known primary olfactory tissues indicates the location of cryptic olfactory inputs. The specialized ecological niche of these wasps explains the unique habits and adaptive evolution of Or2 genes. The Or2 gene in C. solmsi is evolving very rapidly. Negative deviation from the neutral model of evolution reflects possible selection pressures acting on Or2 sequences of fig wasp, particularly on PFWs who are more host-specific to figs.The interactions of flowers and insect pollinators are the classic examples of co-evolution. The intimate relationships and specialization between figs and fig wasps are among the best studied cases [1-3]. Pollinating fig wasps (PFWs) usually exclusively pollinate species of figs [2,4-7], although recent discoveries indicate that the species-specific association is much less specific than has been thought previously [8-13]. The pollinators completely depend on figs for their life cycles [14-17]. In addition to PFWs, several non-pollinating species of fig wasps (NPFWs) also exploit figs [18]. Evidently, non-pollinators are less specific to a given host species than pollinators [13,18-23].