Daniel Kronauer, PhD
Stanley S. and Sydney R. Shuman Associate Professor
Laboratory of Social Evolution and Behavior
The Rockefeller University
(September 25, 2020)
Differentiation, Communication, and Emergence in Ant Societies
For insects that live in colonies, such as ants or honeybees, roles are sharply defined. Each individual is part of the success of the larger colony, rather than success as an individual (perhaps humans could learn something here). As Dr. Kronauer explained, increasing the size of an ant colony led to workers becoming more specialized while behaviorally more diverse. Dr. Kronauer also explored how genes differ and change within an ant society, such as the importance of insulin (the ant version) to reproductive signals. His work also demonstrates the importance of odor signals in ant colonies. Without these signals, ants were asocial and unable to communicate, an essential part of living in a colony.
Eusocial insects, like ants and honeybees, have served a pivotal role in the fields of evolution, development, ethology, and ecology for over a century. What makes eusocial insects stand out from an evolutionary perspective is that natural selection has shifted to the level of the colony, and acts on individual colony members as parts of a larger entity, the ‘superorganism’. Eusocial insects are therefore central to modern evolutionary thinking, especially in the context of inclusive fitness theory and multilevel selection. The organismality of insect societies in turn allowed for the evolution of extreme specialization among its members, reminiscent of cell-type differentiation in multicellular organisms. Queen and worker ants, for example, differ dramatically in size, morphology, physiology, longevity, and behavior, despite being developmentally derived from the same genome. Workers, in turn, specialize on different behavioral tasks, such as nursing and foraging. Eusocial insects are therefore excellent subjects to study developmental plasticity and behavioral individuality. Analogous to the neural and endocrine systems of multicellular organisms, eusocial insects have evolved sophisticated channels of communication, from a multitude of pheromones to the honeybee waggle dance, allowing researchers to probe the neural basis of complex social behavior. Finally, the intimate interactions between colony members give rise to adaptive emergent properties at the group level. This strength in numbers not only underlies the ecological dominance of eusocial insects, but it also constitutes a fascinating example of distributed computing and collective behavior in complex biological systems.
We study these interconnected topics using an unconventional model system, the clonal raider ant Ooceraea biroi. This species uniquely combines experimental accessibility with the fascinating biology of eusocial insects. Clonal raider ants are small (2.5mm) and can be kept in large numbers (tens of thousands in a Tupperware box). All individuals can reproduce asexually and clonally. Even small colonies of ~10 ants are fully functional. This combination of traits is unique among eusocial insects and allows us to propagate and work with well-defined genetic lines, to scale up and replicate experiments, and to efficiently create and maintain stable genetic knockout and transgenic lines. My group has developed an arsenal of resources for this species, including high-quality reference genomes and associated datasets, CRISPR/Cas mutagenesis, as well as custom hardware and software for high-throughput automated behavioral tracking. Here I summarize some of our major recent findings, and provide a brief outlook on ongoing work.
The origin of eusociality remains a major problem in evolutionary biology, partly because we know very little about the genetic and molecular innovations that made this major transition possible. The defining feature of insect eusociality is the division of labor between reproductive queens and non-reproductive workers. To better understand how these caste differences are regulated, we conducted an unbiased screen of brain gene expression between queens and workers of several species across the ant phylogeny. Among the thousands of genes considered, only one, insulin-like peptide 2 (ILP2), the ant version of human insulin, was significantly and consistently more highly expressed in queens across all species. We then carried out functional characterizations of the ILP2 peptide in the clonal raider ant and showed that it strongly upregulates ovarian activity. Maybe most surprisingly, we found that the expression of the gene in adults is regulated by signals from the larvae: when larvae are present, the gene is downregulated and the ants do not lay eggs but care for the brood instead. Egg-laying only resumes in the absence of larvae. This suggested a simple model in which parental behavior in ants has evolved via coopting an ancient metabolic pathway, insulin signaling, and putting it under partial social control. However, individuals with higher baseline insulin levels are less responsive to larval suppression, suggesting that, in a second step, phenotypic plasticity in baseline levels was exaggerated over evolutionary time to ultimately give rise to queens, which are insensitive to larval suppression, and workers, which are reproductively inactive. This study was able to reconstruct major molecular evolutionary events that occurred over 100 million years ago. It also provides a solid foundation from which we continue to explore the causal molecular differences between the castes of eusocial insects.
Ant eusociality also entailed the evolution of complex pheromone communication, which has been studied from a chemical ecology perspective for decades. However, until recently, we knew almost nothing about the underlying neural mechanisms. We and others have since shown that ants have evolved extremely complex chemosensory systems. Specifically, ant genomes harbor massive expansions of odorant receptors (ORs), a particular class of chemosensory receptors. Based on our most recent genome assembly we found that, with ~500 genes, the clonal raider ant has ten times more ORs than Drosophila, making ORs prime pheromone receptor candidates. Insect ORs form ligand-gated ion channels that consist of one of many tuning ORs and a conserved co-receptor, Orco. We therefore created Orco gene knockouts in which the function of all odorant receptors was disrupted. These ants did not respond to pheromones and were asocial, conclusively demonstrating the importance of ORs in pheromone communication. We are now conducting additional experiments to study how the ant olfactory system is organized, and how it perceives and processes the sophisticated pheromone language.
In addition to understanding the proximate mechanisms that underlie the organization of insect societies, we also use the clonal raider ant to study the adaptive benefits of eusociality. This is an important problem because, intuitively, one might assume that, at the onset of group-living, animals of the same kind should compete over resources, rather than benefit from each other’s company. We found that, as we gradually increased group size, individual workers became more specialized, while colonies became behaviorally more diverse, even at very small group sizes. In parallel, fitness increased via effects on survival, reproduction, and developmental timing, seemingly due to increases in colony homeostasis. This demonstrated that sociality can be adaptive from the outset, with the benefits of group living outweighing the costs. We have since improved the behavioral tracking approaches required for this kind of work, and are now studying how even the most complex collective phenomena arise from local interactions between ants, and how these dynamics depend on group composition and communication.