Summary Life inside ant colonies is orchestrated with diverse pheromones, but it is not clear how ants perceive these social signals. It has been proposed that pheromone perception in ants evolved via expansions in the numbers of odorant receptors (ORs) and antennal lobe glomeruli. Here we generate the first mutant lines in the clonal raider ant, Ooceraea biroi, by disrupting orco, a gene required for the function of all ORs. We find that orco mutants exhibit severe deficiencies in social behavior and fitness, suggesting they are unable to perceive pheromones. Surprisingly, unlike in Drosophila melanogaster, orco mutant ants also lack most of the approximately 500 antennal lobe glomeruli found in wild-type ants. These results illustrate that ORs are essential for ant social organization and raise the possibility that, similar to mammals, receptor function is required for the development and/or maintenance of the highly complex olfactory processing areas in the ant brain.

A major aim of sociogenomic research is to uncover common principles in the molecular evolution of sociality. This endeavor has been hampered by the small number of specific genes currently known to function in social behavior. Here we provide several lines of evidence suggesting that ants have evolved a large and novel clade of odorant receptor (OR) genes to perceive hydrocarbonbased pheromones, arguably the most important signals in ant communication. This genomic expansion is also mirrored in the ant brain via a corresponding expansion of a specific cluster of glomeruli in the antennal lobe. We show that in the clonal raider ant, hydrocarbon-sensitive basiconic sensilla are found only on the ventral surface of the female antennal club. Correspondingly, nearly all genes in a clade of 180 ORs within the 9-exon subfamily of ORs are expressed exclusively in females and are highly enriched in expression in the ventral half of the antennal club. Furthermore, we found that across species and sexes, the number of 9-exon ORs expressed in antennae is tightly correlated with the number of glomeruli in the antennal lobe region innervated by odorant receptor neurons from basiconic sensilla. Evolutionary analyses show that this clade underwent a striking gene expansion in the ancestors of all ants and slower but continued expansion in extant ant lineages. This evidence suggests that ants have evolved a large clade of genes to support pheromone perception and that gene duplications have played an important role in the molecular evolution of ant communication.sociogenomics | chemosensation | social evolution | antennal lobe | Formicidae T he field of sociogenomics seeks to understand the molecular basis of sociality, asking how social life evolved and how it is governed at the genomic level (1). A particularly pertinent question in the field is the role of novel genes vs. conserved genes in the evolution and regulation of social behaviors, including communication (2, 3). Communication is important to a broad range of organisms, yet relatively little is known about the genetic components of communication systems and how they evolve. A particularly interesting question in the field is how social signals are perceived and what sort of genetic and physiological specializations facilitate signal perception in highly social organisms (4, 5). In the most tractable sociogenomic models, the eusocial hymenopteran insects, communication is largely chemical, and the genetics and physiology of chemosensation in these species may hold the key to understanding their advanced communication (4,6, 74).Much is already known about the insect chemosensory system in general (8,9). Briefly, chemicals are detected by porous sensory hairs (sensilla) located on chemosensory organs such as the legs, wings, palps, and especially the antennae. The sensilla house the dendrites of olfactory and/or gustatory receptor neurons (ORNs/GRNs) with receptor proteins in the olfactory receptor (OR), the gustatory receptor (GR), or the ionotropic receptor (IR) gene ...

Summary Social insects are important models for social evolution and behavior. However, in many species experimental control over important factors that regulate division of labor, such as genotype and age, is limited [1, 2]. Furthermore, most species have fixed queen and worker castes, making it difficult to establish causality between the molecular mechanisms that underlie reproductive division of labor, the hallmark of insect societies [3]. Here we present the genome of the queenless clonal raider ant Cerapachys biroi, a powerful new study system that does not suffer from these constraints. Using cytology and RAD-Seq, we show that C. biroi reproduces via automixis with central fusion and that heterozygosity is lost extremely slowly. As a consequence, nestmates are almost clonally related (r=0.996). Workers in C. biroi colonies synchronously alternate between reproduction and brood care, and young workers eclose in synchronized cohorts. We show that genes associated with division of labor in other social insects are conserved in C. biroi and dynamically regulated during the colony cycle. With unparalleled experimental control over an individual’s genotype and age, and the ability to induce reproduction and brood care [4, 5], C. biroi has great potential to illuminate the molecular regulation of division of labor.

Queens and workers of eusocial Hymenoptera are considered homologous to the reproductive and brood care phases of an ancestral subsocial life cycle. However, the molecular mechanisms underlying the evolution of reproductive division of labor remain obscure. Using a brain transcriptomics screen, we identified a single gene, (), which is always up-regulated in ant reproductives, likely because they are better nourished than their nonreproductive nestmates. In clonal raider ants (), larval signals inhibit adult reproduction by suppressing , thus producing a colony reproductive cycle reminiscent of ancestral subsociality. However, increasing ILP2 peptide levels overrides larval suppression, thereby breaking the colony cycle and inducing a stable division of labor. These findings suggest a simple model for the origin of ant eusociality via nutritionally determined reproductive asymmetries potentially amplified by larval signals.