Transcriptomic analyses of the termite, Cryptotermes secundus, reveal a gene network underlying a long lifespan and high fecundity

Body part-specific development in termite caste differentiation: crosstalk between hormonal actions and developmental toolkit genes

In social insects, interactions among colony members trigger caste differentiation with morphological modifications. During caste differentiation in termites, body parts and caste-specific morphologies are modified during postembryonic development under endocrine controls such as juvenile hormone (JH) and ecdysone. In addition to endocrine factors, developmental toolkit genes such as Hox- and appendage-patterning genes also contribute to the caste-specific body part modifications. These toolkits are thought to provide spatial information for specific morphogenesis. During social evolution, the complex crosstalks between physiological and developmental mechanisms should be established, leading to the sophisticated caste systems. This article reviews recent studies on these mechanisms underlying the termite caste differentiation and addresses implications for the evolution of caste systems in termites.

Termite primary queen - ancestral, but highly specialized eusocial phenotype

Termite eusociality is accompanied by flagrant caste polyphenism manifested by the presence of several sterile (workers and soldiers) and reproductive (imaginal and neotenic kings and queens) caste phenotypes. Imaginal kings and queens are developmentally equivalent to adults of other hemimetabolous insects but display multiple adaptations inherent to their role of eusocial colony founders, such as long lifespan and high fecundity. Herein, we summarize the recent advances in understanding the biology of imaginal (primary) queens as emblematic examples of termite polyphenism acquired during social evolution. We focus on the control of queen development, on dynamics in physiology and fecundity during the queen's life, on new findings about queen fertility signaling, and on proximate mechanisms underlying queen longevity.

A genetic toolkit underlying the queen phenotype in termites with totipotent workers

Social insect castes (e.g., queens, workers) are prime examples of phenotypic plasticity (i.e., different phenotypes arising from the same genotype). Yet, the mechanisms that give rise to highly fertile, long-lived queens versus non-reproducing, short-lived workers are not well understood. Recently, a module of co-expressed genes has been identified that characterizes queens compared to workers of the termite Cryptotermes secundus (Kalotermitidae): the Queen Central Module (QCM). We tested whether the QCM is shared in termite species, in which queens gradually develop via early larval and late larval instars, the latter functioning as totipotent workers (linear development). Similar as in C. secundus, gene expression profiles revealed an enrichment of QCM genes in Zootermopsis angusticollis queens, a species from another termite family (Archotermopsidae). The expression of these QCM genes became gradually enriched during development from early larval instars via workers to queens. Thus, our results support the hypothesis of a conserved genetic toolkit that characterizes termite queens with gradual linear development. Our data also imply a strong caste-specific tissue specificity with the QCM signal being restricted to head-prothorax tissues in termite queens. This tissue-specific expression of key aging-related genes might have facilitated the evolution of a long lifespan in termite queens.

Differential gene expression and microRNA profile in corpora allata-corpora cardiaca of Aedes aegypti mosquitoes with weak juvenile hormone signalling

The corpora allata-corpora cardiaca (CA-CC) is an endocrine gland complex that regulates mosquito development and reproduction through the synthesis of juvenile hormone (JH). Epoxidase (Epox) is a key enzyme in the production of JH. We recently utilized CRISPR/Cas9 to establish an epoxidase-deficient (epox-/-) Aedes aegypti line. The CA from epox-/- mutants do not synthesize epoxidated JH III but methyl farneosate (MF), a weak agonist of the JH receptor, and therefore have reduced JH signalling. Illumina sequencing was used to examine the differences in gene expression between the CA-CC from wild type (WT) and epox-/- adult female mosquitoes. From 18,034 identified genes, 317 were significantly differentially expressed. These genes are involved in many biological processes, including the regulation of cell proliferation and apoptosis, energy metabolism, and nutritional uptake. In addition, the same CA-CC samples were also used to examine the microRNA (miRNA) profiles of epox-/- and WT mosquitoes. A total of 197 miRNAs were detected, 24 of which were differentially regulated in epox-/- mutants. miRNA binding sites for these particular miRNAs were identified using an in silico approach; they target a total of 101 differentially expressed genes. Our results suggest that a lack of epoxidase, besides affecting JH synthesis, results in the diminishing of JH signalling that have significant effects on Ae. aegypti CA-CC transcriptome profiles, as well as its miRNA repertoire.

Endosymbiont Tremblaya phenacola influences the reproduction of cotton mealybugs by regulating the mechanistic target of rapamycin pathway

The intricate evolutionary dynamics of endosymbiotic relationships result in unique characteristics among the genomes of symbionts, which profoundly influence host insect phenotypes. Here, we investigated an endosymbiotic system in Phenacoccus solenopsis, a notorious pest of the subfamily Phenacoccinae. The endosymbiont, "Candidatus Tremblaya phenacola" (T. phenacola PSOL), persisted throughout the complete life cycle of female hosts and was more active during oviposition, whereas there was a significant decline in abundance after pupation in males. Genome sequencing yielded an endosymbiont genome of 221.1 kb in size, comprising seven contigs and originating from a chimeric arrangement between betaproteobacteria and gammaproteobacteria. A comprehensive analysis of amino acid metabolic pathways demonstrated complementarity between the host and endosymbiont metabolism. Elimination of T. phenacola PSOL through antibiotic treatment significantly decreased P. solenopsis fecundity. Weighted gene coexpression network analysis demonstrated a correlation between genes associated with essential amino acid synthesis and those associated with host meiosis and oocyte maturation. Moreover, altering endosymbiont abundance activated the host mechanistic target of rapamycin pathway, suggesting that changes in the amino acid abundance affected the host reproductive capabilities via this signal pathway. Taken together, these findings demonstrate a mechanism by which the endosymbiont T. phenacola PSOL contributed to high fecundity in P. solenopsis and provide new insights into nutritional compensation and coevolution of the endosymbiotic system.

Nutrition- and hormone-controlled developmental plasticity in Blattodea

Blattodea, which includes cockroaches and termites, possesses high developmental plasticity that is mainly controlled by nutritional conditions and insect hormones. Insulin/insulin-like growth factor signaling (IIS), target of rapamycin complex 1 (TORC1), and adenosine monophosphate-activated protein complex are the three primary nutrition-responsive signals. Juvenile hormone (JH) and 20-hydroxyecdysone (20E) constitute the two most vital insect hormones that might interact with each other through the Met, Kr-h1, E93 (MEKRE93) pathway. Nutritional and hormonal signals interconnect to create a complex regulatory network. Here we summarize recent progress in our understanding of how nutritional and hormonal signals coordinately control the developmental plasticity of metamorphosis, reproduction, and appendage regeneration in cockroaches as well as caste differentiation in termites. We also highlight several perspectives that should be further emphasized in the studies of developmental plasticity in Blattodea. This review provides a general landscape in the field of nutrition- and hormone-controlled developmental plasticity in insects.

Costs of reproduction are present but latent in eusocial bumblebee queens

BACKGROUND

The standard evolutionary theory of ageing proposes that ageing occurs because of a trade-off between reproduction and longevity. Eusocial insect queens exhibit positive fecundity-longevity associations and so have been suggested to be counter-examples through not expressing costs of reproduction and through remodelling conserved genetic and endocrine networks regulating ageing and reproduction. If so, eusocial evolution from solitary ancestors with negative fecundity-longevity associations must have involved a stage at which costs of reproduction were suppressed and fecundity and longevity became positively associated. Using the bumblebee (Bombus terrestris), we experimentally tested whether queens in annual eusocial insects at an intermediate level of eusocial complexity experience costs of reproduction, and, using mRNA-seq, the extent to which they exhibit a remodelling of relevant genetic and endocrine networks. Specifically, we tested whether costs of reproduction are present but latent, or whether a remodelling of relevant genetic and endocrine networks has already occurred allowing queens to reproduce without costs.

RESULTS

We experimentally increased queens' costs of reproduction by removing their eggs, which caused queens to increase their egg-laying rate. Treatment queens had significantly reduced longevity relative to control queens whose egg-laying rate was not increased. Reduced longevity in treatment queens was not caused by increased worker-to-queen aggression or by increased overall activity in queens. In addition, treatment and control queens differed in age-related gene expression based on mRNA-seq in both their overall expression profiles and the expression of ageing-related genes. Remarkably, these differences appeared to occur principally with respect to relative age, not chronological age.

CONCLUSIONS

This study represents the first simultaneously phenotypic and transcriptomic experimental test for a longevity cost of reproduction in eusocial insect queens. The results support the occurrence of costs of reproduction in annual eusocial insects of intermediate social complexity and suggest that reproductive costs are present but latent in queens of such species, i.e. that these queens exhibit condition-dependent positive fecundity-longevity associations. They also raise the possibility that a partial remodelling of genetic and endocrine networks underpinning ageing may have occurred in intermediately eusocial species such that, in unmanipulated conditions, age-related gene expression depends more on chronological than relative age.

The remoulding of dietary effects on the fecundity / longevity trade-off in a social insect

BACKGROUND

In many organisms increased reproductive effort is associated with a shortened life span. This trade-off is reflected in conserved molecular pathways that link nutrient-sensing with fecundity and longevity. Social insect queens apparently defy the fecundity / longevity trade-off as they are both, extremely long-lived and highly fecund. Here, we have examined the effects of a protein-enriched diet on these life-history traits and on tissue-specific gene expression in a termite species of low social complexity.

RESULTS

On a colony level, we did not observe reduced lifespan and increased fecundity, effects typically seen in solitary model organisms, after protein enrichment. Instead, on the individual level mortality was reduced in queens that consumed more of the protein-enriched diet - and partially also in workers - while fecundity seemed unaffected. Our transcriptome analyses supported our life-history results. Consistent with life span extension, the expression of IIS (insulin/insulin-like growth factor 1 signalling) components was reduced in fat bodies after protein enrichment. Interestingly, however, genes involved in reproductive physiology (e.g., vitellogenin) were largely unaffected in fat body and head transcriptomes.

CONCLUSION

These results suggest that IIS is decoupled from downstream fecundity-associated pathways, which can contribute to the remoulding of the fecundity/longevity trade-off in termites as compared to solitary insects.

The vitellogenin receptor gene is involved in lifespan regulation of Zeugodacus cucurbitae (Coquillett) after short-term high-temperature treatment

Zeugodacus cucurbitae (Coquillett) is a highly damaging agricultural pest in many tropical and subtropical countries around the world and high temperatures usually affect its survival. To clarify the effect of short-term high temperatures on the survival and lifespan of Z. cucurbitae, newly emerged adults of three consecutive generations (F₁, F₂, and F₃) were exposed to 25 °C, 33 °C, 37 °C, 41 °C, or 45 °C treatments for 1 h. The effect of these temperatures on survival and lifespan was evaluated using biological indicators such as lifespan and pupation rate. Then, to study the molecular regulatory mechanism of the lifespan of Z. cucurbitae after short-term high-temperature treatment, we exposed the newly emerged adults to 25 °C or 45 °C treatments for 1 h and used siRNA to interfere with the expression of the vitellogenin receptor (VgR) gene in the female to study the effect of the VgR gene on the lifespan of Z. cucurbitae. The results showed that the survival rate, lifespan, pupae weight, pupation rate, and emergence rate of Z. cucurbitae decreased with increased temperature, while the female sex ratio of offspring increased. The heat resistance of females was higher than that of males. Interference with the expression of the VgR gene resulted in shortening of the female's lifespan by approximately 60% after exposure to 25 °C or 45 °C treatments for 1 h, which indicated involvement of the VgR gene in the regulation of Z. cucurbitae lifespan. This study provides a reference to guide integrated control of Z. cucurbitae in high-temperature seasons.

Complex regulatory role of DNA methylation in caste- and age-specific expression of a termite

The reproductive castes of eusocial insects are often characterized by extreme lifespans and reproductive output, indicating an absence of the fecundity/longevity trade-off. The role of DNA methylation in the regulation of caste- and age-specific gene expression in eusocial insects is controversial. While some studies find a clear link to caste formation in honeybees and ants, others find no correlation when replication is increased across independent colonies. Although recent studies have identified transcription patterns involved in the maintenance of high reproduction throughout the long lives of queens, the role of DNA methylation in the regulation of these genes is unknown. We carried out a comparative analysis of DNA methylation in the regulation of caste-specific transcription and its importance for the regulation of fertility and longevity in queens of the higher termite Macrotermes natalensis. We found evidence for significant, well-regulated changes in DNA methylation in mature compared to young queens, especially in several genes related to ageing and fecundity in mature queens. We also found a strong link between methylation and caste-specific alternative splicing. This study reveals a complex regulatory role of fat body DNA methylation both in the division of labour in termites, and during the reproductive maturation of queens.

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

Termite genomes have been sequenced in at least five species from four different families. Genome-based transcriptome analyses have identified large numbers of protein-coding genes with caste-specific expression patterns. These genes include those involved in caste-specific morphologies and roles, for example high fecundity and longevity in reproductives. Some caste-specific expressed genes belong to multi-gene families, and their genetic architecture and expression profiles indicate they have evolved via tandem gene duplication. Candidate regulatory mechanisms of caste-specific expression include epigenetic regulation (e.g. histone modification and non-coding RNA) and diversification of transcription factors and cis-regulatory elements. We review current knowledge in the area of termite sociogenomics, focussing on the evolution and regulation of caste-specific expressed genes, and discuss future research directions.

Genomic and transcriptomic analyses of the subterranean termite Reticulitermes speratus: Gene duplication facilitates social evolution

Gene duplication is a major source of evolutionary innovation and is associated with the increases in biological complexity and adaptive radiation. Termites are model social organisms characterized by a sophisticated caste system. We analyzed the genome of the Japanese subterranean termite, an ecologically and economically important insect acting as a destructive pest. The analyses revealed the significance of gene duplication in social evolution. Gene duplication associated with caste-biased gene expression was prevalent in the termite genome. Many of the duplicated genes were related to social functions, such as chemical communication, social immunity, and defense, and they were often expressed in caste-specific organs. We propose that gene duplication facilitates social evolution through regulatory diversification leading to caste-biased expression and functional specialization.

Termites are model social organisms characterized by a polyphenic caste system. Subterranean termites (Rhinotermitidae) are ecologically and economically important species, including acting as destructive pests. Rhinotermitidae occupies an important evolutionary position within the clade representing a transitional taxon between the higher (Termitidae) and lower (other families) termites. Here, we report the genome, transcriptome, and methylome of the Japanese subterranean termite Reticulitermes speratus. Our analyses highlight the significance of gene duplication in social evolution in this termite. Gene duplication associated with caste-biased gene expression was prevalent in the R. speratus genome. The duplicated genes comprised diverse categories related to social functions, including lipocalins (chemical communication), cellulases (wood digestion and social interaction), lysozymes (social immunity), geranylgeranyl diphosphate synthase (social defense), and a novel class of termite lineage–specific genes with unknown functions. Paralogous genes were often observed in tandem in the genome, but their expression patterns were highly variable, exhibiting caste biases. Some of the assayed duplicated genes were expressed in caste-specific organs, such as the accessory glands of the queen ovary and the frontal glands of soldier heads. We propose that gene duplication facilitates social evolution through regulatory diversification, leading to caste-biased expression and subfunctionalization and/or neofunctionalization conferring caste-specialized functions.

Histone acetylation regulates the expression of genes involved in worker reproduction in the ant Temnothorax rugatulus

Background

In insect societies, queens monopolize reproduction while workers perform tasks such as brood care or foraging. Queen loss leads to ovary development and lifespan extension in workers of many ant species. However, the underlying molecular mechanisms of this phenotypic plasticity remain unclear. Recent studies highlight the importance of epigenetics in regulating plastic traits in social insects. Thus, we investigated the role of histone acetylation in regulating worker reproduction in the ant Temnothorax rugatulus. We removed queens from their colonies to induce worker fecundity, and either fed workers with chemical inhibitors of histone acetylation (C646), deacetylation (TSA), or the solvent (DMSO) as control. We monitored worker number for six weeks after which we assessed ovary development and sequenced fat body mRNA.

Results

Workers survived better in queenless colonies. They also developed their ovaries after queen removal in control colonies as expected, but not in colonies treated with the chemical inhibitors. Both inhibitors affected gene expression, although the inhibition of histone acetylation using C646 altered the expression of more genes with immunity, fecundity, and longevity functionalities. Interestingly, these C646-treated workers shared many upregulated genes with infertile workers from queenright colonies. We also identified one gene with antioxidant properties commonly downregulated in infertile workers from queenright colonies and both C646 and TSA-treated workers from queenless colonies.

Conclusion

Our results suggest that histone acetylation is involved in the molecular regulation of worker reproduction, and thus point to an important role of histone modifications in modulating phenotypic plasticity of life history traits in social insects.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08196-8.

Molecular underpinnings of division of labour among workers in a socially complex termite

Division of labour characterizes all major evolutionary transitions, such as the evolution of eukaryotic cells or multicellular organisms. Social insects are characterized by reproductive division of labour, with one or a few reproducing individuals (queens) and many non-reproducing nestmates (workers) forming a colony. Among the workers, further division of labour can occur with different individuals performing different tasks such as foraging, brood care or building. While mechanisms underlying task division are intensively studied in social Hymenoptera, less is known for termites, which independently evolved eusociality. We investigated molecular mechanisms underlying task division in termite workers to test for communality with social Hymenoptera. We compared similar-aged foraging workers with builders of the fungus-growing termite Macrotermes bellicosus using transcriptomes, endocrine measures and estimators of physiological condition. Based on results for social Hymenoptera and theory, we tested the hypotheses that (i) foragers are in worse physiological conditions than builders, (ii) builders are more similar in their gene expression profile to queens than foragers are, and (iii) builders invest more in anti-ageing mechanism than foragers. Our results support all three hypotheses. We found storage proteins to underlie task division of these similar-aged termite workers and these genes also characterize reproductive division of labour between queens and workers. This implies a co-option of nutrient-based pathways to regulate division of labour across lineages of termites and social Hymenoptera, which are separated by more than 133 million years.

Disentangling the aging gene expression network of termite queens

Background

Most insects are relatively short-lived, with a maximum lifespan of a few weeks, like the aging model organism, the fruit-fly Drosophila melanogaster. By contrast, the queens of many social insects (termites, ants and some bees) can live from a few years to decades. This makes social insects promising models in aging research providing insights into how a long reproductive life can be achieved. Yet, aging studies on social insect reproductives are hampered by a lack of quantitative data on age-dependent survival and time series analyses that cover the whole lifespan of such long-lived individuals. We studied aging in queens of the drywood termite Cryptotermes secundus by determining survival probabilities over a period of 15 years and performed transcriptome analyses for queens of known age that covered their whole lifespan.

Results

The maximum lifespan of C. secundus queens was 13 years, with a median maximum longevity of 11.0 years. Time course and co-expression network analyses of gene expression patterns over time indicated a non-gradual aging pattern. It was characterized by networks of genes that became differentially expressed only late in life, namely after ten years, which associates well with the median maximum lifespan for queens. These old-age gene networks reflect processes of physiological upheaval. We detected strong signs of stress, decline, defense and repair at the transcriptional level of epigenetic control as well as at the post-transcriptional level with changes in transposable element activity and the proteostasis network. The latter depicts an upregulation of protein degradation, together with protein synthesis and protein folding, processes which are often down-regulated in old animals. The simultaneous upregulation of protein synthesis and autophagy is indicative of a stress-response mediated by the transcription factor cnc, a homolog of human nrf genes.

Conclusions

Our results show non-linear senescence with a rather sudden physiological upheaval at old-age. Most importantly, they point to a re-wiring in the proteostasis network and stress as part of the aging process of social insect queens, shortly before queens die.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07649-4.