A specific collection of genes has been identified as the reason why red imported fire ant nests have either one or multiple queens.
Red imported fire ants, Solenopsis invicta, exist as two different colony types: monogyne colonies that contain a single reproductive queen, and polygyne colonies that contain multiple reproductives. This difference in nest and colony structure is driven by a single supergene.
A supergene is a collection of genes located in close proximity on a chromosome and so are inherited together. Researchers have identified a supergene in red imported fire ants – the so-called ‘social chromosome’ – that determines whether the developing queens will leave the nest (example pictured above) to form a monogyne colony or develop to form part of a polygyne colony. Queens with the Sb supergene develop into queens that can form part of polygyne colonies. Queens lacking the Sb supergene develop into queens that form monogyne colonies.
Furthermore, it appears that queens carrying the supergene have a different cuticular chemical profile, which can be detected by workers that also carry the supergene (Sb workers). If these Sb workers encounter a queen lacking the supergene they will attack and kill it, whilst accepting reproductives carrying the supergene. In contrast, workers from monogyne colonies that lack the supergene do not appear to be able to tell the difference between queens carrying or lacking the supergene. It appears this discrimination is not solely due to the detection of differences in cuticular hydrocarbons, but also requires the presence of additional chemicals that signal queen fertility.
It is hypothesised that this supergene is also responsible for a range of behaviours that drive the differences between the monogyne and polygyne colonies. Understanding this further may help develop new ways to manage and control this invasive pest.
Further reading: Arsenault et al. (2020). Simple inheritance, complex regulation: Supergene-mediated fire ant queen polymorphism. Molecular Ecology. 29. 10.1111/mec.15581.