In eusocial insects, worker polymorphism is shaped by several factors, including colony size, queen mating frequency, and the timing of queen-worker differentiation during larval development. In a comparative study of 18 species of Cataglyphis desert ants representing a wide range of worker sizes, we used phylogenetically controlled analyses to examine correlations between worker head width variation (i.e., worker polymorphism) and multiple social traits, namely, mature colony size, mean worker head width, queen head width, queen-worker head width dimorphism, and within-colony genetic relatedness, resulting from multiple mating by queens. We found that worker polymorphism was positively correlated with mature colony size, mean worker head width, and queen head width. In contrast, worker polymorphism was not correlated with queen-worker dimorphism and withincolony genetic relatedness. These results underscore that evolution of worker polymorphism and social traits are correlated. They also illustrate that additional research using multivariate approaches is needed to further clarify the evolution of insect societies. Significance statementIn eusocial insects, worker morphological variation (i.e., worker polymorphism) is tightly linked to division of labor. Multiple factors are supposed to shape the evolution of worker polymorphism. Using phylogenetically controlled analyses of worker head width variation from 18 species of Cataglyphis desert ants, we show that worker polymorphism positively correlates with mature colony size, mean worker head width, and queen size. These results highlight that the evolution of worker polymorphism and social traits are correlated. Identifying the mechanisms underlying these relationships could provide major insights into the development and evolution of insect societies. KeywordsWorker polymorphism • Head width • Colony size • Polyandry • Queen-worker dimorphism • Evolution Communicated by L. Keller.

Kin selection and inclusive fitness are thought to be key factors explaining the reproductive altruism displayed by workers in eusocial insect species. However, when a colony’s queen has mated with <2 males, workers may increase their fitness by producing their own male offspring. Conversely, when the queen has mated with ≥2 males, workers are expected to increase their inclusive fitness by eschewing the production of their sons and preventing other workers from reproducing as well. Here, we investigated sociogenetic structure and worker reproduction in the red honey ant, Melophorus bagoti. Morphometric analyses revealed that workers belong to one of two distinct subcastes: they are either majors or minors. Using DNA microsatellite markers, we showed that all the colonies had a single, multiple-mated queen and that there was no relationship between worker patriline and worker subcaste. Furthermore, we found that workers were producing males in the presence of the queen, which contrasts with the predictions of inclusive fitness theory. Although our results are based on a small sample, they can serve as the foundation for future research examining worker reproduction in M. bagoti.

In ants, reproductive division of labour is typically associated with queen-worker dimorphism. In some species with ergatoid queens (wingless worker-like queens), this polymorphism is drastically reduced and virgin queens may integrate the worker force. While ergatoid queens have been described in several species, their colony and population genetic structure remain largely unstudied. Here, we investigated the population structure and sociogenetic organisation of the desert ant Ocymyrmex robustior. All Ocymyrmex species have only ergatoid queens that are worker-sized. Workers, queens, and males from a large population were genotyped at ten polymorphic microsatellite loci. Our results show that the study population is genetically structured, consistent with dependent colony foundation. Genetic analyses revealed that 17.6% of the males were diploid; diploid males are fertile, siring triploid females. Nests were typically headed by a single queen, and queens were strictly monandrous. However, several nests in the population shared matrilines, indicating polygyny, polydomy, dependent colony foundation, serial polygyny, or a combination of these processes. Dissections reveal that workers lay eggs in both queenright and queenless nests, while virgin ergatoid queens lay eggs in queenright nests only. However, our genetic analyses show that male offspring in queenright nests are all queen-produced, suggesting worker policing and/or trophic egg laying.

Ancestral eusocial insect societies were probably headed by a single and singly mated reproductive queen. However, eusocial Hymenoptera have since secondarily evolved highly diverse mating systems and social structures, which include multiple mating by queens, the coexistence of several reproductive queens within a given colony, and queen replacement over time. Here, we report pronounced plasticity in sociogenetic structure and reproductive strategies in the erratic ant, Tapinoma erraticum. Notably, colonies can be headed by one or several queens. Nestmate queens display a varying degree of genetic relatedness; they range from being unrelated to full sisters to mothers and daughters. Queens mate with one to three males. There is again variability in the degree of relatedness. The queens and their male mate(s) are sometimes related and sometimes unrelated; the same is true for the males that had mated with nestmate queens. Finally, genotypic comparisons between age-based cohorts (i.e. adult workers vs. worker larvae) indicate that new queens, both related and unrelated, appear in established colonies. We suggest that the highly plastic reproductive systems found in T. erraticum might have opened the door evolutionarily to supercoloniality and invasiveness in the genus Tapinoma.