TGF β signaling related genes are involved in hormonal mediation during termite soldier differentiation

Abstract

A working knowledge of the proximate factors intrinsic to sterile caste differentiation is necessary to understand the evolution of eusocial insects. Genomic and transcriptomic analyses in social hymenopteran insects have resulted in the hypothesis that sterile castes are generated by the novel function of co-opted or recruited universal gene networks found in solitary ancestors. However, transcriptome analysis during caste differentiation has not been tested in termites, and evolutionary processes associated with acquiring the caste are still unknown. Termites possess the soldier caste, which is regarded as the first acquired permanently sterile caste in the taxon. In this study, we performed a comparative transcriptome analysis in termite heads during 3 molting processes, i.e., worker, presoldier and soldier molts, under natural conditions in an incipient colony of the damp-wood termite Zootermopsis nevadensis. Although similar expression patterns were observed during each molting process, more than 50 genes were shown to be highly expressed before the presoldier (intermediate stage of soldier) molt. We then performed RNA interference (RNAi) of the candidate 13 genes, including transcription factors and uncharacterized protein genes, during presoldier differentiation induced by juvenile hormone (JH) analog treatment. Presoldiers induced after RNAi of two genes related to TGF β (Transforming growth factor beta) signaling were extremely unusual and possessed soldier-like phenotypes. These individuals also displayed aggressive behaviors similar to natural soldiers when confronted with Formica ants as hypothetical enemies. These presoldiers never molted into the next instar, presumably due to the decreased expression levels of the molting hormone (20-hydroxyecdysone; 20E) signaling genes. These results suggest that TGF β signaling was acquired for the novel function of regulating between JH and 20E signaling during soldier differentiation in termites.

Author summary

The acquisition of a sterile caste is a key step in animal eusocial evolution. The soldier is the first acquired permanently sterile caste in termites, which are distantly related to hymenopteran eusocial insects (bees, ants and wasps). However, the evolutionary background to acquiring the soldier caste is still largely unclear. Here we performed transcriptome analysis of heads during worker and soldier caste differentiation under natural conditions in Zootermopsis nevadensis, for which the whole-genome sequence is available. Soldiers differentiate from workers via a double molting processes, and the intermediate stage is called a presoldier. Based on the comparison among molting stages, some presoldier-specific highly expressed genes, including transcription factors and several uncharacterized protein genes, were identified. RNAi of two genes presumably involved in the TGF β signaling resulted in the formation of presoldiers possessing soldier-like phenotypes (e.g. well-tunned cuticle) without further molting into soldiers. Expression levels of 20E signaling genes were negatively affected by the RNAi treatment of these genes. This study provides a novel insight into the hormonal control for termite caste differentiation.