Ionotropic Receptor Genes in Fig Wasps: Evolutionary Insights from Comparative Studies

TL;DR

This study explores how fig wasps detect chemical signals from fig trees by analyzing the evolution of ionotropic receptor genes in 25 fig wasp species.

Contribution

The study identifies and classifies ionotropic receptor genes in fig wasps, revealing patterns of conservation and divergence linked to chemosensation.

Findings

205 ionotropic receptor genes were identified across 25 fig wasp species, grouped into three types: IRco, antennal IRs, and divergent IRs.

Strong purifying selection and some signatures of positive selection were detected in IR genes, suggesting functional importance.

Gene sequences of IRs are consistent with phylogenetic relationships among fig wasp species, similar to odorant and gustatory receptors.

Abstract

Fig wasps mainly depend on the specific chemical cues released by receptive syconia to find their fig hosts. However, we know little about the molecular mechanisms of chemosensation in fig wasps. This study compares the evolutionary characters of ionotropic receptors (IRs) among 25 fig wasp taxa with three types: IRco, antennal IRs, and divergent IRs. There are considerable differences in IR gene sequences between species, which are consistent with the phylogenetic relationships among fig wasps. In addition, strong purifying selection of IRs was found. The results give us a better understanding of the molecular basis of the peripheral chemosensory system in fig wasps.

The mechanisms of chemoreception in fig wasps (Hymenoptera, Agaonidae) are of primary importance in their co-evolutionary relationship with the fig trees they pollinate. As the supplementary receptors to odorant receptors (ORs) and gustatory receptors (GRs) in insects, we compare the evolutionary characters of ionotropic receptors (IRs) among 25 fig wasp taxa in six genera. In total, we identified 205 IRs in 25 fig wasps, with each taxon recording from 5 to 12 IR genes. We found 189 IR genes clustered into 18 orthologous groups that can be divided into three types: IRco, antennal IRs, and divergent IRs. More IRs belong to antennal IRs in fig wasps, which can be sensitive to acids, aldehydes, polyamines, salt, amino acids, and temperature/humidity according to homology comparison. Additionally, some IR genes in fig wasps do not cluster with those of outgroup species (e.g., Drosophila melanogaster, Apis mellifera), suggesting they may represent a unique group and may have special functions in fig wasps. Divergent IRs are very few, with large sequence variation between species. Compared to ORs and GRs in fig wasps, gene sequences in most IR orthologous groups are more conserved between genera, with the lowest sequence similarity in 10 orthologous groups (including three IRco) exhibiting above 58.5%. Gene sequences are consistent with the phylogenetic relationships among fig wasps, which is the same as ORs and GRs. Strong purifying selection of IR genes was detected, as shown by the low ω values. Signatures of positive selection were detected in loci from three orthologous groups. Our results provide important molecular information for further studies on chemosensory mechanisms in fig wasps.

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