Chemical Characterization of Young Virgin Queens and Mated Egg-Laying Queens in the Ant Cataglyphis cursor: Random Forest Classification Analysis for Multivariate Datasets

Integrative characterization of genetic and phenotypic differentiation in an ant species complex with strong hierarchical population structure and low dispersal abilities

Low dispersal, occurrence of asexual reproduction and geographic discontinuity increase genetic differentiation between populations, which ultimately can lead to speciation. In this work, we used a multidisciplinary framework to characterize the genetic and phenotypic differentiation between and within two cryptic ant species with restricted dispersal, Cataglyphis cursor and C. piliscapa and used behavioral experiments to test for reproductive isolation. Their distribution is segregated by the Rhône River and they have been traditionally distinguished only by hair numbers, although a statistical assessment is still lacking. We found strong genetic (microsatellites, nuclear and mitochondrial sequences), morphological (number of hairs, tibia length, male genitalia) and chemical (cuticular hydrocarbons) differentiation not only between species but also among localities within species. However, inter-specific differentiation was slightly higher than intra-specific differentiation for most markers. Overall, this pattern could either reflect reproductive isolation or could result from a longer period of geographic isolation between species than among localities within species without necessarily involving reproductive isolation. Interestingly, our behavioral experiments showed an absence of mating between species associated to a higher aggressiveness of workers towards heterospecific males. This suggests that sexual selection may, at least partially, fuel reproductive isolation. We also showed that cuticular hydrocarbons, mtDNA sequences and number of hairs provide reliable criteria allowing species discrimination. Overall, this species complex offers a case study to further investigate varying stages of a speciation continuum by estimating reproductive isolation between pairs of localities varying by their level of genetic differentiation.

Experimental disruption of social structure reveals totipotency in the orchid bee, Euglossa dilemma

Eusociality has evolved multiple times across the insect phylogeny. Social insects with greater levels of social complexity tend to exhibit specialized castes with low levels of individual phenotypic plasticity. In contrast, species with simple social groups may consist of totipotent individuals that transition among behavioral and reproductive states. However, recent work has shown that in simple social groups, there can still be constraint on individual plasticity, caused by differences in maternal nourishment or social interaction. It is not well understood how these constraints arise, ultimately leading to the evolution of nonreproductive workers. Some species of orchid bees form social groups of a dominant and-one to two subordinate helpers where all individuals are reproductive. Females can also disperse to start their own nest as a solitary foundress, which includes a nonreproductive phase characterized by ovary inactivation, not typically expressed by subordinates. Little is known about individual flexibility across these trajectories. Here, using the orchid bee Euglossa dilemma, we assess the plasticity of subordinate helpers, finding that they are capable of the same behavioral, physiological, transcriptomic, and chemical changes seen in foundresses. Our results suggest that the lack of nonreproductive workers in E. dilemma is not due to a lack of subordinate plasticity.

Queen Control or Queen Signal in Ants: What Remains of the Controversy 25 Years After Keller and Nonacs' Seminal Paper?

Ant queen pheromones (QPs) have long been known to affect colony functioning. In many species, QPs affect important reproductive functions such as diploid larvae sexualization and egg-laying by workers, unmated queens (gynes), or other queens. Until the 1990s, these effects were generally viewed to be the result of queen manipulation through the use of coercive or dishonest signals. However, in their seminal 1993 paper, Keller and Nonacs challenged this idea, suggesting that QPs had evolved as honest signals that informed workers and other colony members of the queen's presence and reproductive state. This paper has greatly influenced the study of ant QPs and inspired numerous attempts to identify fertility-related compounds and test their physiological and behavioral effects. In the present article, we review the literature on ant QPs in various contexts and pay special attention to the role of cuticular hydrocarbons (CHCs). Although the controversy generated by Keller and Nonacs' (Anim Behav 45:787-794, 1993) paper is currently less intensively debated, there is still no clear evidence which allows the rejection of the queen control hypothesis in favor of the queen signal hypothesis. We argue that important questions remain regarding the mode of action of QPs, and their targets which may help understanding their evolution.