Sex ratio biases in termites provide evidence for kin selection

Pivotal temperature is not for everyone: Evidence for temperature-dependent sex determination in three gecko species

The prevalence of environmental sex determination (ESD) in squamate reptiles is often overestimated in the literature. This is surprising because we have reliable data demonstrating ESD in only a few species. The documentation of ESD in three species of geckos presented here has significantly increased our knowledge, given that satisfactory evidence for ESD existed in only eight other gecko species. For the first time, we document the occurrence of ESD in the family Sphaerodactylidae. Our finding of unexpected variability in the shapes of reaction norms among geckos highlights that traditional descriptions using parameters such as pivotal temperature, that is, temperature producing a 50:50 sex ratio, are unsatisfactory. For example, the gecko Pachydactylus tigrinus lacks any pivotal temperature and its sex ratios are strongly female-biased across the entire range of viable temperatures. We argue for the effective capture of the relationship between temperature and sex ratio using specific nonlinear models rather than using classical simplistic descriptions and classifications of reaction norms.

Why put all your eggs in one basket? Evolutionary perspectives on the origins of monogenic reproduction

Sexual reproduction is ubiquitous in eukaryotes, but the mechanisms by which sex is determined are diverse and undergo rapid turnovers in short evolutionary timescales. Usually, an embryo's sex is fated at the moment of fertilisation, but in rare instances it is the maternal genotype that determines the offspring's sex. These systems are often characterised by mothers producing single-sex broods, a phenomenon known as monogeny. Monogenic reproduction is well documented in Hymenoptera (ants, bees and wasps), where it is associated with a eusocial lifestyle. However, it is also known to occur in three families in Diptera (true flies): Sciaridae, Cecidomyiidae and Calliphoridae. Here we review current knowledge of monogenic reproduction in these dipteran clades. We discuss how this strange reproductive strategy might evolve, and we consider the potential contributions of inbreeding, sex ratio distorters, and polygenic control of the sex ratio. Finally, we provide suggestions on future work to elucidate the origins of this unusual reproductive strategy. We propose that studying these systems will contribute to our understanding of the evolution and turnover of sex determination systems.

Heritable effects on caste determination and colony-level sex allocation in termites under field conditions

The ecological success of social insects is attributed to the division of labor, where newly hatched offspring differentiate into either fertile progeny or functionally sterile worker castes. There is growing evidence for the heritable (genetic or epigenetic) effects on caste determination based on laboratory experiments. Here, we indirectly demonstrate that heritable factors have the principal role in caste determination and strongly affect colony-level production of both sexes of fertile dispersers (i.e., alates) in field colonies of the termite Reticulitermes speratus. An egg-fostering experiment suggests that the colony-dependent sex-specific caste fates were almost entirely determined before oviposition. Our investigation of field colonies revealed that such colony-dependent sex-specific caste fates result in the intercolonial variation in the numerical sex ratio of differentiated fertile offspring and, eventually, that of alates. This study contributes to better understanding the mechanisms underlying the division of labor and life-history traits in social insects.

King- and queen-specific degradation of uric acid contributes to reproduction in termites

Caste-based reproductive division of labour in social insects is built on asymmetries in resource allocation within colonies. Kings and queens dominantly consume limited resources for reproduction, while non-reproductive castes such as workers and soldiers help reproductive castes. Studying the regulation of such asymmetries in resource allocation is crucial for understanding the maintenance of sociality in insects, although the molecular background is poorly understood. We focused on uric acid, which is reserved and used as a valuable nitrogen source in wood-eating termites. We found that king- and queen-specific degradation of uric acid contributes to reproduction in the subterranean termite Reticulitermes speratus. The urate oxidase gene (RsUAOX), which catalyses the first step of nitrogen recycling from stored uric acid, was highly expressed in mature kings and queens, and upregulated with differentiation into neotenic kings/queens. Suppression of uric acid degradation decreased the number of eggs laid per queen. Uric acid was shown to be provided by workers to reproductive castes. Our results suggest that the capacity to use nitrogen, which is essential for the protein synthesis required for reproduction, maintains colony cohesion expressed as the reproductive monopoly held by kings and queens.

Same-sex Pairs Retain Their Reproductive Capacity as a Potential Opportunity for Individual Reproductive Success in Termites

In eusocial termites, successful pairing is an essential element of dispersal and distribution after the departure of alates from natal colonies. Two situations could arise during the pairing process: mixed-sex pairs and same-sex pairs. However, most previous studies focused on mixed-sex pairs, overlooking groups formed by same-sex pairings, especially potential fecundity (the total number of oocytes or ovarioles), oogenesis and the development stage of oocytes of females in female-female pairs, and spermatogenesis and testis development of males in male-male pairs. In this study, through experimentation, we investigated the reproductive ability of virgin dealates based on various pairing types as mentioned above. We found that the life spans of virgin dealates can cover 1 yr or even more when they establish a nest with a partner, which is more than 10-fold longer than the life span of individuals establishing a colony alone. After 1 yr of pairing, the potential fecundity of virgin same sex dealates did not degenerate significantly compared with newly emerged dealates, including the number of ovarioles, size of testis, oogenesis, and the development stage of the oocytes. Moreover, when individuals of same-sex pairings experimentally changed into mixed-sex pairs after 1 yr, the eggs produced in the colony hatched into larvae. These findings suggest that dealates which through same-sex pairs retain fecundity after 1 yr have more reproductive potential than dealates that failed to pair with heterosexuals, shedding light on the ecological significance of homosexual behaviors in terms of the successful extension and fecundity of eusocial termites.

Female-biased sex allocation and lack of inbreeding avoidance in Cubitermes termites

Sexually reproducing organisms face a strong selective pressure to find a mate and ensure reproduction. An important criterion during mate-selection is to avoid closely related individuals and subsequent potential fitness costs of resulting inbred offspring. Inbreeding avoidance can be active through kin recognition during mate choice, or passive through differential male and female-biased sex ratios, which effectively prevents sib-mating. In addition, sex allocation, or the resources allotted to male and female offspring, can impact mating and reproductive success. Here, we investigate mate choice, sex ratios, and sex allocation in dispersing reproductives (alates) from colonies of the termite Cubitermes tenuiceps. Termites have a short time to select a mate for life, which should intensify any fitness consequences of inbreeding. However, alates did not actively avoid inbreeding through mate choice via kin recognition based on genetic or environmental cues. Furthermore, the majority of colonies exhibited a female-biased sex ratio, and none exhibited a male-bias, indicating that differential bias does not reduce inbreeding. Sex allocation was generally female-biased, as females also were heavier, but the potential fitness effect of this costly strategy remains unclear. The bacterium Wolbachia, known in other insects to parasitically distort sex allocation toward females, was present within all alates. While Wolbachia is commonly associated with termites, parasitism has yet to be demonstrated, warranting further study of the nature of the symbiosis. Both the apparent lack of inbreeding avoidance and potential maladaptive sex allocation implies possible negative effects on mating and fitness.

Interspecific Hybridization between the Two Sympatric Termite Reticulitermes Species under Laboratory Conditions

Closely related species have similar reproductive behaviors and recognition systems, which contribute to interspecific interactions. However, few studies have explored interspecific reproduction choice and mating in termites. We investigated whether hybridization between two sympatric termites, Reticulitermes flaviceps and R. chinensis, occurs under laboratory conditions. We found that frequencies of acceptance were significantly higher than those of agonism between interspecific partners. There were no significant differences in frequencies of tandem and mating behaviors between intraspecific and interspecific partners. However, the allogrooming frequencies of interspecific partners were significantly higher than intraspecific partners. There were no significant differences in the duration of tandem, allogrooming, or mating behavior at each time between conspecific partners and heterospecfic partners. Genotyping analyses further showed that both intraspecific and interspecific mating were able to produce offspring. We conclude that interspecific hybridization does occur between two termite Reticulitermes species under laboratory conditions.

Widespread occurrence of asexual reproduction in higher termites of the Termes group (Termitidae: Termitinae)

BACKGROUND

A decade ago, the mixed reproductive strategy Asexual Queen Succession (AQS) was first described in termites. In AQS species, the workers, soldiers and dispersing reproductives are produced through sexual reproduction, while non-dispersing (neotenic) queens arise through automictic thelytokous parthenogenesis, replace the founding queen and mate with the founding king. As yet, AQS has been documented in six species from three lineages of lower (Rhinotermitidae) and higher (Termitinae: Termes group and Syntermitinae) termites. Independent evolution of the capacity of thelytoky as a preadaptation to AQS is supported by different mechanisms of automixis in each of the three clades. These pioneering discoveries prompt the question on the extent of thelytoky and AQS in the diversified family of higher termites.

RESULTS

Here, we investigated the capacity of thelytoky and occurrence of AQS in three species from the phylogenetic proximity of the neotropical AQS species Cavitermes tuberosus (Termitinae: Termes group): Palmitermes impostor, Spinitermes trispinosus, and Inquilinitermes inquilinus. We show that queens of all three species are able to lay unfertilized eggs, which undergo thelytokous parthenogenesis (via gamete duplication as in C. tuberosus) and develop through the transitional stage of aspirants into replacement neotenic queens.

CONCLUSIONS

The breeding system in P. impostor is very reminiscent of that described in C. tuberosus and can be characterized as AQS. In the remaining two species, our limited data do not allow classifying the breeding system as AQS; yet, also in these species the thelytokous production of neotenic females appears to be a systematic element of reproductive strategies. It appears likely that the capacity of thelytokous parthenogenesis evolved once in the Termes group, and may ultimately be found more widely, well beyond these Neotropical species.

Hypoxia adaptation in termites: hypoxic conditions enhance survival and reproductive activity in royals

Termite royals (queen and king) exhibit extraordinary longevity without sacrificing reproductive performance, unlike most animals, in whom lifespan is generally negatively associated with reproduction. Therefore, the regulatory mechanisms underlying longevity have attracted much attention. Although the ageing process is influenced by environmental factors in many insects during their life cycle, it remains unclear whether any factors have an effect on the extended survival and high reproductive capacity of termite royals. Here, we show that hypoxia, possibly an important environmental factor in the nests, enhances survival and reproductive activity in incipient royals of the subterranean termite Reticulitermes speratus compared with those in control conditions. Quantitative real-time PCR analysis revealed that the expression levels of the vitellogenin gene in queens are maintained to a greater extent under hypoxic conditions than under control conditions. The expression levels of the antioxidant enzyme genes RsCAT1 and RsPHGPX are also significantly promoted by hypoxia in queens and kings respectively. These results suggest that hypoxic exposure can contribute in part to achieving high reproductive output by altering gene expression after founding of colonies in the royals. Our study provides novel insights into the effect of a nest environment on the reproductive characteristics in termite royals.

High expression of the breast cancer susceptibility gene BRCA1 in long-lived termite kings

Aging is associated with the accumulation of DNA damage. High expression of DNA repair genes has been suggested to contribute to prolonged lifespan in several organisms. However, the crucial DNA repair genes contributing to longevity remain unknown. Termite kings have an extraordinary long lifespan compared with that of non-reproductive individuals such as workers despite being derived from the same genome, thus providing a singular model for identifying longevity-related genes. In this study, we demonstrated that termite kings express higher levels of the breast cancer susceptibility gene BRCA1 than other castes. Using RNA sequencing, we identified 21 king-specific genes among 127 newly annotated DNA repair genes in the termite Reticulitermes speratus. Using quantitative PCR, we revealed that some of the highly expressed king-specific genes were significantly upregulated in reproductive tissue (testis) compared to their expression in somatic tissue (fat body). Notably, BRCA1 gene expression in the fat body was more than 4-fold higher in kings than in workers. These results suggest that BRCA1 partly contributes to DNA repair in somatic and reproductive tissues in termite kings. These findings provide important insights into the linkage between BRCA1 gene expression and the extraordinary lifespan of termite kings.

Scaling the phase-planes of social dilemma strengths shows game-class changes in the five rules governing the evolution of cooperation

Game theory has been extensively applied to help explain how cooperative behaviour is promoted in human and animal societies. How do humans and animals establish reciprocity when confronting a social dilemma? In 2006, Nowak theoretically proved that numerous mechanisms and models for evolving cooperative behaviour reported over the last few decades can be distilled into five reciprocity mechanisms (rules). Additionally, universal scaling parameters were proposed to measure two different types of dilemmas, namely, the gamble-intending dilemma (GID) and risk-averting dilemma (RAD). In this work, by drawing a RAD-GID phase-plane diagram for pair-wise games, we prove that these five rules are indeed quite different for the resolution (relaxation) of the two dilemmas. These diagrams also demonstrate whether and when game-class change (resolution of a dilemma) occurs, thus implying how defectors can be eliminated.

The disposable male- the ultimate emancipation of females?

Sexual reproduction is costly compared to asexual reproduction, in particular because males generally contribute little to offspring. Research published today in BMC Biology shows that some populations of a termite species have disposed of males altogether. However, this need not necessarily be seen as a victory for the females, since males in most termite societies are active colony members that contribute their fair share to colony tasks.

A Genomic Imprinting Model of Termite Caste Determination: Not Genetic but Epigenetic Inheritance Influences Offspring Caste Fate

Eusocial insects exhibit the most striking example of phenotypic plasticity. There has been a long controversy over the factors determining caste development of individuals in social insects. Here we demonstrate that parental phenotypes influence the social status of offspring not through genetic inheritance but through genomic imprinting in termites. Our extensive field survey and genetic analysis of the termite Reticulitermes speratus show that its breeding system is inconsistent with a genetic caste determination model. We therefore developed a genomic imprinting model, in which queen- and king-specific epigenetic marks antagonistically influence sexual development of offspring. The model accounts for all known empirical data on caste differentiation of R. speratus and other related species. By conducting colony-founding experiments and additively incorporating relevant socio-environmental factors into our genomic imprinting model, we show the relative importance of genomic imprinting and environmental factors in caste determination. The idea of epigenetic inheritance of sexual phenotypes solves the puzzle of why parthenogenetically produced daughters carrying only maternal chromosomes exclusively develop into queens and why parental phenotypes (nymph- or worker-derived reproductives) strongly influence caste differentiation of offspring. According to our model, the worker caste is seen as a "neuter" caste whose sexual development is suppressed due to counterbalanced maternal and paternal imprinting and opens new avenues for understanding the evolution of caste systems in social insects.

Evolution of the asexual queen succession system and its underlying mechanisms in termites

One major advantage of sexual reproduction over asexual reproduction is its promotion of genetic variation, although it reduces the genetic contribution to offspring. Queens of social insects double their contribution to the gene pool, while overuse of asexual reproduction may reduce the ability of the colony to adapt to environmental stress because of the loss of genetic diversity. Recent studies have revealed that queens of some termite species can solve this tradeoff by using parthenogenesis to produce the next generation of queens and sexual reproduction to produce other colony members. This reproductive system, known as asexual queen succession (AQS), has been identified in the subterranean termites Reticulitermes speratus, Reticulitermes virginicus and Reticulitermes lucifugus and in the Neotropical higher termites Embiratermes neotenicus and Cavitermes tuberosus The studies presented here have uncovered the unusual modes of reproduction in termites and have aimed to identify their underlying mechanisms. The study of AQS, the mixed use of sexual and asexual reproduction, is of fundamental importance as it may provide a key to solve the evolutionary paradox of sex.

Caste-, sex-, and age-dependent expression of immune-related genes in a Japanese subterranean termite, Reticulitermes speratus

Insects protect themselves from microbial infections through innate immune responses, including pathogen recognition, phagocytosis, the activation of proteolytic cascades, and the synthesis of antimicrobial peptides. Termites, eusocial insects inhabiting microbe-rich wood, live in closely-related family groups that are susceptible to shared pathogen infections. To resist pathogenic infection, termite families have evolved diverse immune adaptations at both individual and societal levels, and a strategy of trade-offs between reproduction and immunity has been suggested. Although termite immune-inducible genes have been identified, few studies have investigated the differential expression of these genes between reproductive and neuter castes, and between sexes in each caste. In this study, we compared the expression levels of immune-related genes among castes, sexes, and ages in a Japanese subterranean termite, Reticulitermes speratus. Using RNA-seq, we found 197 immune-related genes, including 40 pattern recognition proteins, 97 signalling proteins, 60 effectors. Among these genes, 174 showed differential expression among castes. Comparing expression levels between males and females in each caste, we found sexually dimorphic expression of immune-related genes not only in reproductive castes, but also in neuter castes. Moreover, we identified age-related differential expression of 162 genes in male and/or female reproductives. In addition, although R. speratus is known to use the antibacterial peptide C-type lysozyme as an egg recognition pheromone, we determined that R. speratus has not only C-type, but also P-type and I-type lysozymes, as well as other termite species. Our transcriptomic analyses revealed immune response plasticity among all castes, and sex-biased expression of immune genes even in neuter castes, suggesting a sexual division of labor in the immune system of R. speratus. This study heightens the understanding of the evolution of antimicrobial strategies in eusocial insects, and of sexual roles in insect societies as a whole.

Asexual queen succession mediates an accelerated colony life cycle in the termite Silvestritermes minutus

Mixed modes of reproduction, combining sexual processes with thelytokous parthenogenesis, occur in all major clades of social insects. In several species of termites, queens maximize their genetic input into nondispersing replacement queens through parthenogenesis, while maintaining genetically diverse sterile offspring and dispersing reproductives via sexual reproduction. This so-called asexual queen succession (AQS) has multiple independent origins and its presumed advantages are diverse as well, ranging from multiplication of colony reproductive potential to extension of its lifespan beyond that of the foundress. However, how AQS shapes colony life cycles under natural conditions remains poorly known. The neotropical termite Silvestritermes minutus inhabits small but conspicuous nests, offering a unique opportunity to investigate the impact of AQS on life history. We report on its breeding system, life cycle and sex allocation using social structure census in 137 nests and genotyping of 12 colonies at 12 microsatellite loci. We show that colonies are established by an outbred pair of primary reproductives. In less than 2 years, the foundress is replaced by multiple neotenic queens, arising mostly through automixis with central fusion. Sterile castes, male and most (93%) female dispersers are produced sexually. Colony reproduction is usually restricted to a single dispersal of alates with unbiased sex ratio, taking place after 3 years. We conclude that S. minutus benefits from AQS to maximize colony growth rate and alate production within a very short life cycle rather than to extend colony lifespan. This highlights the versatile role of AQS in different cases of its polyphyletic origin.

Termite queens close the sperm gates of eggs to switch from sexual to asexual reproduction

Males and females are in conflict over genetic transmission in the evolution of parthenogenesis, because it enhances female reproductive output but deprives the males' genetic contribution. For males, any trait that coerces females into sexual reproduction should increase their fitness. However, in the termite Reticulitermes speratus, queens produce their replacements (neotenic queens) parthenogenetically while using normal sexual reproduction to produce other colony members. Here, we show that termite queens produce parthenogenetic offspring in the presence of kings by closing the micropyles (sperm gates; i.e., openings for sperm entry) of their eggs. Our field survey showed that termite eggs show large variation in numbers of micropyles, with some having none. Microsatellite analysis showed that embryos of micropyleless eggs develop parthenogenetically, whereas those of eggs with micropyles are fertilized and develop sexually. Surveys of eggs among queens of different age groups showed that queens begin to lay micropyleless eggs when they are older and thus, need to produce their replacements parthenogenetically. In addition, we found clear seasonality in new neotenic queen differentiation and micropyleless egg production. This micropyle-dependent parthenogenesis is the first identification, to our knowledge, of the mechanism through which females control egg fertilization over time in diploid animals, implying a novel route of the evolution of parthenogenesis in favor of female interests without interference from males.

The soldiers in societies: defense, regulation, and evolution

The presence of reproductively altruistic castes is one of the primary traits of the eusocial societies. Adaptation and regulation of the sterile caste, to a certain extent, drives the evolution of eusociality. Depending on adaptive functions of the first evolved sterile caste, eusocial societies can be categorized into the worker-first and soldier-first lineages, respectively. The former is marked by a worker caste as the first evolved altruistic caste, whose primary function is housekeeping, and the latter is highlighted by a sterile soldier caste as the first evolved altruistic caste, whose task is predominantly colony defense. The apparent functional differences between these two fundamentally important castes suggest worker-first and soldier-first eusociality are potentially driven by a suite of distinctively different factors. Current studies of eusocial evolution have been focused largely on the worker-first Hymenoptera, whereas understanding of soldier-first lineages including termites, eusocial aphids, gall-dwelling thrips, and snapping shrimp, is greatly lacking. In this review, we summarize the current state of knowledge on biology, morphology, adaptive functions, and caste regulation of the soldier caste. In addition, we discuss the biological, ecological and genetic factors that might contribute to the evolution of distinct caste systems within eusocial lineages.