Credit: P. Morgan/Macmillan Publishers Limited

Eusocial insects display complex social behaviours, but the underlying molecular mechanisms are largely unknown. Now, a trio of papers in Cell decribe two genes (orco and corazonin) that control social behaviour in ants. Furthermore, two of the studies describe the first mutant lines in ants, which were generated by CRISPR–Cas9 genome editing.

Until now, functional genetic studies have not been possible in ants

Most social behaviours in ants are mediated by pheromones. An obvious difference between the chemosensory receptor repertoires of ants and non-social insects (such as Drosophila spp.) is that ants have a much greater number of olfactory receptors (ORs). To test whether ORs mediate the complex social behaviours that differentiate ants from non-social insects, two different groups of researchers used CRISPR–Cas9 genome editing to generate mutations in the orco gene, which encodes a highly conserved olfactory co-receptor that is essential for the function of all ORs. Until now, functional genetic studies have not been possible in ants because, by definition, only the queen can reproduce in eusocial species. Trible et al. bypassed this problem by generating orco mutant lines in Ooceraea biroi, a parthenogenetic ant species; germline modifications are transmitted to all progeny because they are maternal clones. By contrast, Yan et al. harnessed the unusual reproductive strategy of Harpegnathos saltator to increase the number of reproducing ants to enable them to establish orco mutant lines. In the absence of a queen, non-reproductive H. saltator workers can become 'gamergates', which lay fertilized eggs. This caste transition can be replicated in the lab simply by isolating workers.

Despite O. biroi and H. saltator having diverged 120 million years ago, both orco mutant lines had strikingly similar phenotypes. As expected, workers displayed a range of behavioural phenotypes consistent with a general loss of olfactory perception. However, unexpectedly, the number of glomeruli in the antennal lobes of the brain was dramatically reduced in orco mutant ants, as was the number of olfactory sensory neurons in the antennae. This dramatic neuroanatomical phenotype was consistent in both ant species but was not observed for Drosophila melanogaster orco mutants, which points to a possible role for orco in antennal lobe development and/or maintenance specifically in social insects.

In the third study, Gospocic et al. exploited the worker–gamergate transition of H. saltator to investigate the mechanisms regulating caste-specific behaviours. They identified corazonin, which encodes a neuropeptide, as a highly expressed worker-biased gene and showed that its caste-specific RNA expression is conserved in other insect species with different social systems. In ants undergoing the worker–gamergate transition, high corazonin peptide levels promoted worker-specific behaviour and inhibited behaviours associated with progression to the gamergate caste; as expected, short interfering RNA (siRNA) knockdown of the corazonin receptor (CrzR) gene had the opposite phenotypic effect. The researchers went on to identify the vitellogenin gene as a key regulatory target of corazonin; its expression is consistently downregulated in response to increased corazonin levels, suggesting that corazonin and vitellogenin have opposing effects on caste identity. Consistent with this hypothesis, siRNA knockdown of vitellogenin gene expression promoted worker-specific behaviours. Based on these observations, the authors propose that caste-specific behaviours and caste identity in H. saltator, and possibly other social insects, are regulated by a corazonin–vitellogenin axis.

The genetic uniformity of ant colonies, combined with the diverse social systems within ant species, makes them an excellent model for studying the molecular underpinnings of complex social behaviours. The development of functional genetic tools in two ant species further expands their utility as model organisms. O. biroi allows fast and efficient generation of multiple mutant lines, while H. saltator paves the way for future implementation of more sophisticated genetic tools that, for example, allow inducible gene expression.