Odorant-Binding and Chemosensory Proteins in Fig Wasps: Evolutionary Insights From Comparative Studies.

Fig wasps (Agaonidae; Hymenoptera) are the only pollinating insects of fig trees (Ficus; Moraceae), forming the most closely and highly specific mutualism with the host. We used transcriptome sequences of 25 fig wasps from six genera to explore the evolution of key molecular components of fig wasp chemosensory genes: odorant-binding proteins (OBPs) and chemosensory proteins (CSPs). We identified a total 321 OBPs and 240 CSPs, with each species recording from 6 to 27 OBP genes and 6-19 CSP genes. 318 OBP genes are clustered into 17 orthologous groups and can be divided into two groups: PBP sensitive to pheromone and GOBP sensitive to general odor molecules, such as alcohols, esters, acids, ketones, and terpenoids. 240 CSP genes are clustered into 12 orthologous groups, which can be divided into three major groups and have functions, such as olfactory, tissue formation and/or regeneration, developmental, and some specific and unknown function. The gene sequences of most orthologous groups vary greatly among species and are consistent with the phylogenetic relationships between fig wasps. Strong purifying selection of both OBP and CSP genes was detected, as shown by low ω values. A positive selection was detected in one locus in CSP1. In conclusion, the evolution of chemosensory proteins OBPs and CSPs in fig wasps is relatively conservative, and they play an indispensable role in the life activities of fig wasps. Our results provide a starting point for understanding the molecular basis of the chemosensory systems of fig wasps.

Olfactory and gustatory receptor genes in fig wasps: Evolutionary insights from comparative studies.

The mechanisms of chemoreception in fig wasps (Hymenoptera, Agaonidae) are of primary importance in their co-evolutionary relationship with the fig trees they pollinate. We used transcriptome sequences of 25 fig wasps in six genera that allowed a comparative approach to the evolution of key molecular components of fig wasp chemoreception: their odorant (OR) and gustatory (GR) receptor genes. In total, we identified 311 ORs and 47 GRs, with each species recording from 5 to 30 OR genes and 1-4 GR genes. 304 OR genes clustered into 18 orthologous groups known to be sensitive to cuticular hydrocarbons (CHC), pheromones, acids, alcohols and a variety of floral scents such as cineole, Linalool, and Heptanone. 45 GR genes clustered into 4 orthologous groups that contain sweet, bitter, CO2 and undocumented receptors. Gene sequences in most orthologous groups varied greatly among species, except for ORco (60.0% conserved) and sweet receptors (30.7% conserved). Strong purifying selection of both odorant and gustatory genes was detected, as shown by low ω values. Signatures of positive selection were detected in loci from both OR and GR orthologous groups. Fig wasps have relatively few olfactory and especially gustatory receptors, reflecting the natural history of the system. Amino acid sequences nonetheless vary significantly between species and are consistent with the phylogenetic relationships among fig wasps. The differences in ORs within some orthologous groups from the same species, but different hosts and from closely related species from one host can reach as low as 49.3% and 9.8% respectively, implying the ORs of fig wasps can evolve rapidly to novel ecological environments. Our results provide a starting point for understanding the molecular basis of the chemosensory systems of fig wasps.

A chromosome-level genome assembly of the pollinating fig wasp Valisia javana.

Fig wasp has always been thought the species-specific pollinator for their host fig (Moraceae, Ficus) and constitute a model system with its host to study co-evolution and co-speciation. The availability of a high-quality genome will help to further reveal the mechanisms underlying these characteristics. Here, we present a high-quality chromosome-level genome for Valisa javana developed by a combination of PacBio long-read and Illumina short-read. The assembled genome size is 296.34 Mb from 13 contigs with a contig N50 length of 26.76 kb. Comparative genomic analysis revealed expanded and positively selected genes related to biological features that aid fig wasps living in syconium of its highly specific host. Protein-coding genes associated with chemosensory, detoxification and venom genes were identified. Several differentially expressed genes in transcriptome data of V. javana between odor-stimulated samples and the controls have been identified in some olfactory signal transduction pathways, e.g. olfactory transduction, cAMP, cGMP-PKG, Calcim, Ras and Rap1. This study provides a valuable genomic resource for a fig wasp, and sheds insight into further revealing the mechanisms underlying their adaptive traits to their hosts in different places and co-speciation with their host.