The reproductive system of most ants is pretty freaky, by human standards. Unlike in our species, where all individuals have a diploid set of chromosomes, nearly all ant species utilize a “haplodiploid” system in which females are diploid and males are haploid, with only one chromosome for each chromosomal site. So, in order to produce males, a queen lays unfertilized eggs, while in order to produce females, the eggs must be fertilized.
But it gets even weirder. In a few rare cases, ant species reproduce via a system termed “social hybridogensis”. In a particular version of this reproductive system, in order to produce new queens, a queen must mate with males of it’s own genetic lineage. But – and here’s where thinks get freaky – in order to produce workers, the queen must mate with a male of a different genetic lineage. Therefore, fertile colonies arise only via reproduction that requires not two but three mating partners.
This type of freaky ant lovin’ was previously only known in populations derived from two harvester ant species, Pogonomyrmex rugosus and P. barbatus. But recent work by Jonathan Romiguier and colleagues, published in Molecular Ecology, reveals that social hybridogensis is more widespread than previous believed – at least in harvester ants. Their study shows that this system has evolved independently at least four times in harvester ants, with their results demonstrating three origins in the genus Messor.
One striking aspect of this finding is that this highly unusual mating system has repeatedly evolved in ant groups characterized by a fairly unique ecology – all four species exhibiting this form of social hybridogensis live in dry climates and subsist almost entirely on seeds. Romiguier and colleagues suggest that the short period of summer rainfalls allows for fairly tightly-timed mating flights, a likely requirement for this freaky ant lovin’ to successfully evolve. A non-mutually exclusive hypothesis is that the seed diet disrupts the usual reproductive determination system in ants, as previous work has shown that larvae receiving a diet with a higher insect/proteins to seeds ratio are more likely to become queens. Without this rationing flexibility, harvester ant colonies may be unable to regulate queen/worker production ratios, and thus must more strictly rely on a gene-based determination system.