Identification of a queen pheromone mediating the rearing of adult sexuals in the pharaoh ant Monomorium pharaonis

Chemical Resemblance of Egg Surface Compounds and Dufour's Gland in Two Neotropical Polistinae Wasps Polistes versicolor (Olivier) and Mischocyttarus metathoracicus (de Saussure, 1854)

Chemical communication plays a major role in regulating social dynamics in social insect colonies. The most studied class of chemical compounds are the cuticular hydrocarbons (CHCs), compounds with high molecular weight that cover the insect body. CHCs are used in nestmate recognition and to signal reproductive status. Brood, in the form of larvae and eggs, is known to participate in chemical communication and social dynamics by performing hunger behaviour and inducing interaction with adults and conferring nest and maternity identity. CHCs of adults and egg surface compounds are similar in composition in social insect species. The main source of egg compounds is proposed to be Dufour's gland, an accessory reproductive gland found in several Hymenoptera females. There is still a lack of information about the level of similarity among CHCs, compounds of egg surface and Dufour's gland for several wasp species, which could provide correlational evidence about the origins of egg-marking compounds. Thus, we investigated whether egg surface compounds were more similar to CHCs or Dufour's gland secretions in two Neotropical primitively eusocial wasp species, Polistes versicolor (Olivier) and Mischocyttarus metathoracicus (de Saussure, 1854). As expected, there was a higher chemical similarity between eggs and Dufour's gland secretions in both studied species, supporting the hypothesis that this gland is the source of chemical compounds found over the eggs in these two primitively eusocial species.

Conditional indirect genetic effects of caregivers on brood in the clonal raider ant

Caregivers shape the rearing environment of their young. Consequently, offspring traits are influenced by the genes of their caregivers via indirect genetic effects (IGEs). However, the extent to which IGEs are modulated by environmental factors, other than the genotype of social partners (i.e., intergenomic epistasis), remains an open question. Here we investigate how brood are influenced by the genotype of their caregivers in the clonal raider ant, Ooceraea biroi, a species in which the genotype, age and number of both caregivers and brood can be experimentally controlled. First, we used four clonal lines to establish colonies that differed only in the genotype of caregivers and measured effects on foraging activity, as well as IGEs on brood phenotypes. In a second experiment, we tested whether these IGEs are conditional on the age and number of caregivers. We found that caregiver genotype affected the feeding and foraging activity of colonies, and influenced the rate of development, survival, body size, and caste fate of brood. Caregiver genotype interacted with other factors to influence the rate of development and survival of brood, demonstrating that IGEs can be conditional. Thus, we provide an empirical example of phenotypes being influenced by IGE-by-environment interactions beyond intergenomic epistasis, highlighting that IGEs of caregivers/parents are alterable by factors other than their brood's/offspring's genotype.

Evidence of queen-rearing suppression by mature queens in the little fire ant, Wasmannia auropunctata

The little fire ant (LFA), Wasmannia auropunctata, is a serious invasive pest first reported on Hawaii Island in 1999, and has since spread and established itself across the island. LFA is considered one of the worst 100 invasive species and has significant ecological, agricultural, and public health impacts in invaded areas, which include much of the tropical New World. Although localized eradication efforts have proven successful, they are intensive and difficult to implement. Furthermore, LFA's high invasive-ability resists these control efforts in areas where the species is established and can re-infest treated areas. This research set out to determine whether LFA queens have a suppressant effect on new queen production in nests, as a first step in identifying a potential queen pheromone for LFA. A queen pheromone could offer a means to shutdown LFA reproductive capability, potentially by suppressing the production of new queens or inducing the execution of queens or queen-destined larvae. When queenless experimental nests and polygyne experimental nests were compared, six out of eight queenless nests successfully reared both new alate queens (2.25 queens/nest) and drones (3.63 drones/nest) to adulthood, whereas only three of eight polygyne nests reared sexual larvae that failed to develop to adulthood or even the pupal stage. These results suggest that dealate mature LFA queens suppress the production of new alate queens in LFA nests, and is the first evidence that LFA may utilize a queen pheromone.

Queens control caste allocation in the ant Cardiocondyla obscurior

Social insect queens and workers can engage in conflict over reproductive allocation when they have different fitness optima. Here, we show that queens have control over queen-worker caste allocation in the ant Cardiocondyla obscurior, a species in which workers lack reproductive organs. We describe crystalline deposits that distinguish castes from the egg stage onwards, providing the first report of a discrete trait that can be used to identify ant caste throughout pre-imaginal development. The comparison of queen and worker-destined eggs and larvae revealed size and weight differences in late development, but no discernible differences in traits that may be used in social interactions, including hair morphology and cuticular odours. In line with a lack of caste-specific traits, adult workers treated developing queens and workers indiscriminately. Together with previous studies demonstrating queen control over sex allocation, these results show that queens control reproductive allocation in C. obscurior and suggest that the fitness interests of colony members are aligned to optimize resource allocation in this ant.

Comprehensive analysis of male-free reproduction in Monomorium triviale (Formicidae: Myrmicinae)

We report comprehensive evidence for obligatory thelytokous parthenogenesis in an ant Monomorium triviale. This species is characterized by distinct queen–worker dimorphism with strict reproductive division of labor: queens produce both workers and new queens without mating, whereas workers are completely sterile. We collected 333 nests of this species from 14 localities and three laboratory-reared populations in Japan. All wild queens dissected had no sperm in their spermathecae. Laboratory observation confirmed that virgin queens produced workers without mating. Furthermore, microsatellite genotyping showed identical heterozygous genotypes between mothers and their respective daughters, suggesting an extremely low probability of sexual reproduction. Microbial analysis detected no bacterial genera that are known to induce thelytokous parthenogenesis in Hymenoptera. Finally, the lack of variation in partial sequences of mitochondrial DNA among individuals sampled from across Japan suggests recent rapid spread or selective sweep. M. triviale would be a promising model system of superorganism-like adaptation through comparative analysis with well-studied sexual congeners, including the pharaoh ant M. pharaonis.

Genetic basis of chemical communication in eusocial insects

Social behavior is one of the most fascinating and complex behaviors in humans and animals. A fundamental process of social behavior is communication among individuals. It relies on the capability of the nervous system to sense, process, and interpret various signals (e.g., pheromones) and respond with appropriate decisions and actions. Eusocial insects, including ants, some bees, some wasps, and termites, display intriguing cooperative social behavior. Recent advances in genetic and genomic studies have revealed key genes that are involved in pheromone synthesis, chemosensory perception, and physiological and behavioral responses to varied pheromones. In this review, we highlight the genes and pathways that regulate queen pheromone-mediated social communication, discuss the evolutionary changes in genetic systems, and outline prospects of functional studies in sociobiology.

Identification of a queen pheromone mediating the rearing of adult sexuals in the pharaoh ant Monomorium pharaonis

The division of labour between reproductive queens and mostly sterile workers is among the defining characteristics of social insects. Queen-produced chemical signals advertising her presence and fertility status, i.e. queen pheromones, are normally used to assert the queen's reproductive dominance in the colony. Most queen pheromones identified to date are chemicals that stop the daughter workers from reproducing. Nevertheless, it has long been suggested that queen pheromones could also regulate reproduction in different ways. In some multiple-queen ants with obligately sterile workers, for example-such as fire ants and pharaoh ants-queen pheromones are thought to regulate reproduction by inhibiting the rearing of new sexuals. Here, we identify the first such queen pheromone in the pharaoh ant Monomorium pharaonis and demonstrate its mode of action via bioassays with the pure biosynthesized compound. In particular, we show that the monocyclic diterpene neocembrene, which in different Monomorium species is produced solely by fertile, egg-laying queens, strongly inhibits the rearing of new sexuals (queens and males) and also exerts a weakly attractive 'queen retinue' effect on the workers. This is the first time that a queen pheromone with such a dual function has been identified in a social insect species with obligately sterile workers.