Termite sociogenomics: evolution and regulation of caste-specific expressed genes

Differential Expression of Hormone-Related Genes in the Heads of Adult and Nymphal Woodroaches (Cryptocercus)

Termites are eusocial cockroaches, but the crucial distinctions in gene expression during the evolution of eusociality remain unclear. One reason for the lack of this information is that comparative transcriptome analysis of termites with their sister group, the cockroach genus Cryptocercus, has not been conducted. We identified genes associated with three vital hormones (juvenile hormone [JH], 20-hydoroxyecdysone [20E], and insulin) from the genome sequence of Cryptocercus punctulatus and conducted RNA-seq analysis using the heads of female/male adults and nymphs to elucidate their expression levels. The comprehensive gene expression analysis revealed a multitude of genes exhibiting differences in expression between developmental stages rather than between sexes. Subsequently, we compared the differences in expression patterns of each hormone-related gene by combining the results of a previous RNA-seq study conducted on the heads of castes (reproductives, workers, and soldiers) in the termite Reticulitermes speratus. The results indicated that genes with expression differences among castes in R. speratus, particularly those related to JH and 20E, were significantly more abundant compared to genes with expression differences between adults and nymphs in C. punctulatus. While no significant difference was observed in the number of genes within the insulin signaling pathway, a trend of homologs highly expressed in adult woodroaches but not in adult termites was observed, and the expression patterns of positive and negative regulators in the pathway differed significantly between adults and nymphs. The differences in the expression patterns between Cryptocercus and termites are believed to reflect variations in hormone levels and signaling activities between adults and juveniles, the latter encompassing workers and soldiers in the case of termites.

Comparison Between Worker and Soldier Transcriptomes of Termite Neotermes binovatus Reveals Caste Specialization of Host-Flagellate Symbiotic System

Termites are eusocial insects with functionally specialized workers and soldiers, both sharing the same genotype. Additionally, lower termites host flagellates in their hindguts that assist in wood digestion. However, worker-biased and soldier-biased gene expression patterns of the host-flagellate symbiotic system remain underexplored in most taxonomic groups. In this study, we sequenced high-depth transcriptomes from the workers and soldiers of a lower termite, Neotermes binovatus (Kalotermitidae), to investigate the differentially expressed termite transcripts, flagellate transcript abundance, and co-expression patterns of the host-flagellate transcript pairs in both castes. The worker-biased transcripts were enriched in functions related to cuticle development, nervous system regulation, pheromone biosynthesis, and metabolism, whereas the soldier-biased transcripts were predominantly involved in muscle development and kinesis, body morphogenesis, protein modification, and aggression. Flagellate transcripts from the orders Cristamonadida, Trichomonadida, Tritrichomonadida, and Oxymonadida were identified in both workers and soldiers, with the abundance of most flagellate transcripts tending to be higher in workers than in soldiers. Furthermore, we observed a much larger number of strong co-expression correlations between the termite and flagellate transcripts in workers than in soldiers, suggesting the possibility that soldiers depend more on food processed by worker holobionts than on their own symbiotic system. This research provides insights into the functional specialization of the host-flagellate symbiotic system in the worker and soldier castes of termites, supporting the workers' roles in nest maintenance, preliminary food processing, and communication, while emphasizing the defensive role of soldiers. Additionally, it offers new perspectives on the potential termite-flagellate interactions and underscores the need for whole-genome data of termite flagellates in further studies.

Protein localization and potential function of lipocalin in Reticulitermes speratus queens

To understand the mechanisms underlying social evolution and caste development in social insects, caste-specific organs and genes should be investigated. In the rhinotermitid termite, Reticulitermes speratus, the lipocalin gene RS008881, which encodes a protein transporter, is expressed in the ovarian accessory glands of primary queens. To obtain additional data on its expression and product localization, we conducted real-time quantitative polymerase chain reaction and protein assays using a peptide antibody. Gene expression analysis of the castes revealed that RS008881 was highly expressed in female primary and secondary reproductives. Further analysis of its expression during reproductive caste differentiation showed that its expression levels increased prior to molting into reproductive individuals, even during the winged imago (alates) stage. Western blotting and fluorescent immunohistochemical staining revealed that the RS008881 product was localized in the ovary as well as the eggshells produced by female reproductives. RS008881 may play a significant role in the reproductive biology of R. speratus; protein localization in both the ovary and eggshell suggests multiple functions related to embryo protection and potential pheromone interactions.

Pervasive relaxed selection in termite genomes

Genetic changes that enabled the evolution of eusociality have long captivated biologists. More recently, attention has focussed on the consequences of eusociality on genome evolution. Studies have reported higher molecular evolutionary rates in eusocial hymenopteran insects compared with their solitary relatives. To investigate the genomic consequences of eusociality in termites, we analysed nine genomes, including newly sequenced genomes from three non-eusocial cockroaches. Using a phylogenomic approach, we found that termite genomes have experienced lower rates of synonymous substitutions than those of cockroaches, possibly as a result of longer generation times. We identified higher rates of non-synonymous substitutions in termite genomes than in cockroach genomes, and identified pervasive relaxed selection in the former (24-31% of the genes analysed) compared with the latter (2-4%). We infer that this is due to reductions in effective population size, rather than gene-specific effects (e.g. indirect selection of caste-biased genes). We found no obvious signature of increased genetic load in termites, and postulate efficient purging of deleterious alleles at the colony level. Additionally, we identified genomic adaptations that may underpin caste differentiation, such as genes involved in post-translational modifications. Our results provide insights into the evolution of termites and the genomic consequences of eusociality more broadly.

Gene regulation supporting sociality shared across lineages and variation in complexity

Across evolutionary lineages, insects vary in social complexity, from those that exhibit extended parental care to those with elaborate divisions of labor. Here, we synthesize the sociogenomic resources from hundreds of species to describe common gene regulatory mechanisms in insects that regulate social organization across phylogeny and levels of social complexity. Different social phenotypes expressed by insects can be linked to the organization of co-expressing gene networks and features of the epigenetic landscape. Insect sociality also stems from processes like the emergence of parental care and the decoupling of ancestral genetic programs. One underexplored avenue is how variation in a group's social environment affects the gene expression of individuals. Additionally, an experimental reduction of gene expression would demonstrate how the activity of specific genes contributes to insect social phenotypes. While tissue specificity provides greater localization of the gene expression underlying social complexity, emerging transcriptomic analysis of insect brains at the cellular level provides even greater resolution to understand the molecular basis of social insect evolution.

Genomic signatures of eusocial evolution in insects

The genomes of eusocial insects allow the production and regulation of highly distinct phenotypes, largely independent of genotype. Although rare, eusociality has evolved convergently in at least three insect orders (Hymenoptera, Blattodea and Coleoptera). Despite such disparate origins, eusocial phenotypes show remarkable similarity, exhibiting long-lived reproductives and short-lived sterile workers and soldiers. In this article, we review current knowledge on genomic signatures of eusocial evolution. We confirm that especially an increased regulatory complexity and the adaptive evolution of chemical communication are common to several origins of eusociality. Furthermore, colony life itself can shape genomes of divergent taxa in a similar manner. Future research should be geared towards generating more high-quality genomic resources, especially in hitherto understudied clades, such as ambrosia beetles and termites. The application of more sophisticated tools such as machine learning techniques may allow the detection of more subtle convergent genomic footprints of eusociality.

Caste-specific expressions and diverse roles of takeout genes in the termite Reticulitermes speratus

Acquisition of novel functions caused by gene duplication may be important for termite social evolution. To clarify this possibility, additional evidence is needed. An important example is takeout, encoding juvenile hormone binding protein. We identified 25 takeouts in the termite Reticulitermes speratus genome. RNA-seq revealed that many genes were highly expressed in specific castes. Two novel paralogs (RsTO1, RsTO2) were tandemly aligned in the same scaffold. Real-time qPCR indicated that RsTO1 and RsTO2 were highly expressed in queens and soldiers, respectively. Moreover, the highest RsTO1 expression was observed in alates during queen formation. These patterns were different from vitellogenins, encoding egg-yolk precursors, which were highly expressed in queens than alates. In situ hybridization showed that RsTO1 mRNA was localized in the alate-frontal gland, indicating that RsTO1 binds with secretions probably used for the defence during swarming flight. In contrast, increased RsTO2 expression was observed approximately 1 week after soldier differentiation. Expression patterns of geranylgeranyl diphosphate synthase, whose product functions in the terpenoid synthesis, were similar to RsTO2 expression. In situ hybridization indicated RsTO2-specific mRNA signals in the soldier-frontal gland. RsTO2 may interact with terpenoids, with a soldier-specific defensive function. It may provide additional evidence for functionalization after gene duplication in termites.

Efficient RNA interference method during caste differentiation with hormone treatment in the termite Reticulitermes speratus (Isoptera: Rhinotermitidae)

Unveiling the proximate mechanism of caste differentiation is crucial for understanding insect social evolution, and gene function analysis is an important tool in this endeavor. The RNA interference (RNAi) technique is useful in termites, but its knockdown effects may differ among species. One of the most important model species in the field of termite sociogenomics is Reticulitermes speratus Kolbe (Isoptera: Rhinotermitidae). Presoldier and worker differentiation of this species can be artificially induced by juvenile hormone and 20-hydroxyecdysone application, respectively. However, appropriate RNAi technique of genes expressed during caste differentiation has never been considered. To clarify this issue, first, we injected nine different volumes of nuclease-free water (NFW, 0-404.8 nL) into workers and found that survival and caste differentiation rates were strongly reduced by the application of the top three largest volumes. Second, we injected double-stranded (ds) RNA of ecdysone receptor homolog (RsEcR) (2.0 µg/151.8 nL NFW) into workers with hormone treatments. The expression levels of RsEcR were significantly reduced at 9 days after dsRNA injection. RsEcR RNAi strongly affected both molting events during presoldier and worker differentiation induced by hormone treatments. The present results highlight the need for caution regarding injection volumes for RNAi experiments using hormone treatments. We suggest that the injection of dsRNA solution (2 µg; approximately 100-200 nL) is suitable for RNAi experiments during caste differentiation induced by hormone application in R. speratus.

Transcriptomics on Social Interactions in Termites: Effects of Soldier Presence

The organization of social insect colonies requires sophisticated mechanisms to regulate caste composition according to colony demands. In termites, the soldier caste is responsible for the inhibition of soldier differentiation, but the mechanism underlying the regulation of soldier differentiation is still unclear. In this study, we performed transcriptome analyses to identify genes expressed in workers that fluctuated in the presence of soldiers in the subterranean termite Reticulitermes speratus. First, soldier differentiation was artificially induced via juvenile hormone (JH) application, and the inhibitory effects of soldier differentiation on soldier presence were evaluated. Second, transcriptomes were prepared from workers with or without soldiers under JH treatment, and expression analyses were performed to identify differentially expressed genes (DEGs) for each treatment. The expression levels of several DEGs were verified by quantitative real-time PCR. The results indicated that only a small number of DEGs were upregulated by the presence of soldiers. A homology search of DEGs and gene ontology (GO) analysis of the DEGs showed that some genes were responsible for the regulation of hormone levels, social interaction, and response to xenobiotic substances, suggesting that they could be involved in developmental arrest and pheromonal regulation in workers. Moreover, GO analysis indicated that the expression of many genes, including those involved in hormone metabolic processes, fluctuated with JH application. Suppression of soldier differentiation in the presence of soldiers could be accomplished by the expression of a large number of genes required for soldier differentiation.