A male (black, with wings) and several females (brown, no wings) of Ooceraea biroi ants. Image credit: Kip Lacy (CC BY-NC)
In most animals, sex is determined at conception. The factors that determine whether an embryo becomes male or female vary across animals, one example being sex chromosomes. In humans and other mammals, a gene on the Y chromosome triggers male development, whereas its absence allows female development.
A different system occurs in haplodiploid species, including wasps, bees and ants. In these animals, haploid eggs (which inherit one set of each chromosome from their mother) develop into males, while diploid eggs (which inherit two sets of chromosomes, one from each parent) develop into females. In many haplodiploid species, inbreeding can produce diploid males, leading to the idea of a system called complementary sex determination, in which having two different versions of a certain gene produces a female while having two identical versions (homozygous diploids) or just one version produces a male.
So far, genes or candidate genes responsible for this type of development have only been identified in honeybees and two ant species. Lacy et al. studied the clonal raider ant, Ooceraea biroi, which is evolutionarily distinct from those ants. The researchers sequenced the genomes of diploid males and females to look for regions of chromosomes where males had two identical alleles, but females had two different ones. They found such a region, a nearly 50,000 DNA base pair long stretch on chromosome 4 that was evolutionarily related to a non-coding RNA region found in the other two ant species.
In this region, they also found patterns of genetic diversity consistent with a complementary sex determination locus. These patterns were not found near genes related to the gene transformer, which are complementary sex determination loci in honeybees and possibly one of the ant species. This suggests that the long non-coding RNA–based locus may have been conserved for over 100 million years of ant evolution, while the transformer-derived system may have evolved later in some species, possibly independently adopting a similar function to that in honeybees.
Understanding complementary sex determination is crucial for the conservation of species, especially for pollinators. When populations shrink and inbreeding occurs, sterile diploid males can be produced, threatening population stability. Knowing which species use complementary sex determination can help track inbreeding and guide breeding programs. The discovery that a single complementary sex determination locus appears to be conserved across ants suggests that similar conservation strategies could apply to many species.
