Polygynous supercolonies of the acacia-ant Pseudomyrmex peperi, an inferior colony founder

Ant-plant sociometry in the Azteca-Cecropia mutualism

A holistic understanding of superorganism biology requires study of colony sociometry, or the quantitative relationships among growth, nest architecture, morphology, and behavior. For ant colonies that obligately nest within plant hosts, their sociometry is likely intertwined with the plant, which has implications for the evolution, strength, and stability of the mutualism. In the Azteca-Cecropia mutualism, plants provide ants with food rewards and hollow stems for nesting in return for protection from herbivores. Several interesting questions arise when considering ant-plant sociometry: are colony growth and plant growth synchronized? How do colonies distribute themselves within the stem of their host plant? How do plant traits influence worker morphology? How is collective personality related to tree structure, nest organization, and worker morphology? To address these questions, we investigated patterns within and relationships among five major sociometric categories of colonies in the field - plant traits, colony size, nest organization, worker morphology, and collective personality. We found that colony sociometry was intimately intertwined with host plant traits. Colony and plant growth rates were synchronized, suggesting that positive feedback between plant and colony growth stabilizes the mutualism. The colony's distribution inside the host tree tended to follow leaf growth, with most workers, brood, and the queen in the top half of the tree. Worker morphology correlated with plant size instead of colony size or age, which suggests that plant traits influence worker development. Colony personality was independent of colony distribution and tree structure but may correlate with worker size such that colonies with smaller, less variable workers had more aggressive personalities. This study provides insights into how ant-plant structural relationships may contribute to plant protection and the strength of mutualisms.

Polygyny does not explain the superior competitive ability of dominant ant associates in the African ant-plant, Acacia (Vachellia) drepanolobium

The Acacia drepanolobium (also known as Vachellia drepanolobium) ant-plant symbiosis is considered a classic case of species coexistence, in which four species of tree-defending ants compete for nesting space in a single host tree species. Coexistence in this system has been explained by trade-offs in the ability of the ant associates to compete with each other for occupied trees versus the ability to colonize unoccupied trees. We seek to understand the proximal reasons for how and why the ant species vary in competitive or colonizing abilities, which are largely unknown. In this study, we use RADseq-derived SNPs to identify relatedness of workers in colonies to test the hypothesis that competitively dominant ants reach large colony sizes due to polygyny, that is, the presence of multiple egg-laying queens in a single colony. We find that variation in polygyny is not associated with competitive ability; in fact, the most dominant species, unexpectedly, showed little evidence of polygyny. We also use these markers to investigate variation in mating behavior among the ant species and find that different species vary in the number of males fathering the offspring of each colony. Finally, we show that the nature of polygyny varies between the two commonly polygynous species, Crematogaster mimosae and Tetraponera penzigi: in C. mimosae, queens in the same colony are often related, while this is not the case for T. penzigi. These results shed light on factors influencing the evolution of species coexistence in an ant-plant mutualism, as well as demonstrating the effectiveness of RADseq-derived SNPs for parentage analysis.

Dispersal strategies in the highly polygynous ant Crematogaster (Orthocrema) pygmaea Forel (Formicidae: Myrmicinae)

In ants, dispersal strategies and morphology of female sexuals are generally linked to the mode of colony founding. In species using long-range dispersal tactics, queen/worker dimorphism is generally high and young queens are able to initiate new colonies by themselves, using their metabolic reserves. By contrast, in species using short-range dispersal strategies, queen/worker dimorphism is generally low and, due to their limited metabolic reserves, queens have lost the capacity to raise their brood alone and to found their colony independently. Moreover, polygyny is also often associated with short-range dispersal strategies, although the relationship between the number of queens and the dispersal strategy in ants is not clear-cut. Here, dispersal strategies were investigated in C. pygmaea, a highly polygynous and polydomous ant species from northeastern Brazil. Field observations and laboratory experiments show that this ant exhibits a suite of traits that are more commonly associated with long-range dispersal and independent colony foundation: functional wings in both males and females, high queen/worker dimorphism, strong weight loss in mature queens, nuptial flights and, in the lab, ability of young queens to found new colonies in haplometrotic conditions. On the other hand, this species shows a high degree of polygyny with a strong seasonal component, and, at least under laboratory conditions, mature queens seem able to develop propagules if they are accompanied by at least 10 workers. These features strongly suggest that (1) some of the gynes do not engage in a long-range dispersal but become new queens in their mother colony and (2) that budding events are possible in this species. We therefore speculate that C. pygmaea has a dual dispersal strategy probably related to environmental conditions: some gynes engage in long-range dispersal followed by independent colony foundation at the beginning of rainy season, while others mate in the parental colony and are re-adopted leading to high polygyny. During the rainy season, budding events can lead to colony extension and increased polydomy. Polydomy is commonly thought to improve resource discovery and exploitation through decentralized foraging behavior, a significant advantage during the rainy season when food ressources (mainly floral/extrafloral nectaries and hemipteran honeydew) are more abundant and when colony needs for food supplies are highest.

Nesting habits shape feeding preferences and predatory behavior in an ant genus

We tested if nesting habits influence ant feeding preferences and predatory behavior in the monophyletic genus Pseudomyrmex (Pseudomyrmecinae) which comprises terrestrial and arboreal species, and, among the latter, plant-ants which are obligate inhabitants of myrmecophytes (i.e., plants sheltering so-called plant-ants in hollow structures). A cafeteria experiment revealed that the diet of ground-nesting Pseudomyrmex consists mostly of prey and that of arboreal species consists mostly of sugary substances, whereas the plant-ants discarded all the food we provided. Workers forage solitarily, detecting prey from a distance thanks to their hypertrophied eyes. Approach is followed by antennal contact, seizure, and the manipulation of the prey to sting it under its thorax (next to the ventral nerve cord). Arboreal species were not more efficient at capturing prey than were ground-nesting species. A large worker size favors prey capture. Workers from ground- and arboreal-nesting species show several uncommon behavioral traits, each known in different ant genera from different subfamilies: leaping abilities, the use of surface tension strengths to transport liquids, short-range recruitment followed by conflicts between nestmates, the consumption of the prey's hemolymph, and the retrieval of entire prey or pieces of prey after having cut it up. Yet, we never noted group ambushing. We also confirmed that Pseudomyrmex plant-ants live in a kind of food autarky as they feed only on rewards produced by their host myrmecophyte, or on honeydew produced by the hemipterans they attend and possibly on the fungi they cultivate.

Surveying the microbiome of ants: comparing 454 pyrosequencing with traditional methods to uncover bacterial diversity

We are only beginning to understand the depth and breadth of microbial associations across the eukaryotic tree of life. Reliably assessing bacterial diversity is a key challenge, and next-generation sequencing approaches are facilitating this endeavor. In this study, we used 16S rRNA amplicon pyrosequencing to survey microbial diversity in ants. We compared 454 libraries with Sanger-sequenced clone libraries as well as cultivation of live bacteria. Pyrosequencing yielded 95,656 bacterial 16S rRNA reads from 19 samples derived from four colonies of one ant species. The most dominant bacterial orders in the microbiome of the turtle ant Cephalotes varians were Rhizobiales, Burkholderiales, Opitutales, Xanthomonadales, and Campylobacterales, as revealed through both 454 sequencing and cloning. Even after stringent quality filtering, pyrosequencing recovered 445 microbe operational taxonomic units (OTUs) not detected with traditional techniques. In comparing bacterial communities associated with specific tissues, we found that gut tissues had significantly higher diversity than nongut tissues, and many of the OTUs identified from these groups clustered within ant-specific lineages, indicating a deep coevolutionary history of Cephalotes ants and their associated microbes. These lineages likely function as nutritional symbionts. One of four ant colonies investigated was infected with a Spiroplasma sp. (order Entomoplasmatales), a potential ant pathogen. Our work shows that the microbiome associated with Cephalotes varians is dominated by a few dozen bacterial lineages and that 454 sequencing is a cost-efficient tool to screen ant symbiont diversity.

Host plant use by competing acacia-ants: mutualists monopolize while parasites share hosts

Protective ant-plant mutualisms that are exploited by non-defending parasitic ants represent prominent model systems for ecology and evolutionary biology. The mutualist Pseudomyrmex ferrugineus is an obligate plant-ant and fully depends on acacias for nesting space and food. The parasite Pseudomyrmex gracilis facultatively nests on acacias and uses host-derived food rewards but also external food sources. Integrative analyses of genetic microsatellite data, cuticular hydrocarbons and behavioral assays showed that an individual acacia might be inhabited by the workers of several P. gracilis queens, whereas one P. ferrugineus colony monopolizes one or more host trees. Despite these differences in social organization, neither of the species exhibited aggressive behavior among conspecific workers sharing a tree regardless of their relatedness. This lack of aggression corresponds to the high similarity of cuticular hydrocarbon profiles among ants living on the same tree. Host sharing by unrelated colonies, or the presence of several queens in a single colony are discussed as strategies by which parasite colonies could achieve the observed social organization. We argue that in ecological terms, the non-aggressive behavior of non-sibling P. gracilis workers — regardless of the route to achieve this social structure — enables this species to efficiently occupy and exploit a host plant. By contrast, single large and long-lived colonies of the mutualist P. ferrugineus monopolize individual host plants and defend them aggressively against invaders from other trees. Our findings highlight the necessity for using several methods in combination to fully understand how differing life history strategies affect social organization in ants.