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Most species on Earth exist as two biological sexes, and the exchange of genes between males and females is vital for their survival. Yet, what actually determines whether an individual develops into a male or female varies widely across species.

In humans, sex is determined by the X and Y chromosomes: individuals with XY become male, while those with XX become female. A single gene on the Y chromosome, SRY, acts as the switch that triggers male development.

However, for many other species, the mechanism is completely different. Among insects such as bees, wasps, and ants, sex is determined through a system known as haplodiploidy, which is found in around 12% of animal species. In this mechanism of sex determination, females develop from fertilized eggs and have two sets of chromosomes, while males develop from unfertilized eggs and possess only one set.

In honeybees, researchers discovered more than two decades ago that a gene called csd controls this remarkable process. Individuals carrying two different versions of csd develop into females, whereas those with only one version become males. Yet until now, it was unclear whether the other 20,000 species of bee use the same genetic switch.

To uncover this mystery, Matthew Webster's research group at Uppsala University sequenced the entire genome of the red mason bee (Osmia bicornis), a common species frequently found in bee hotels in gardens across Europe. They carefully screened hundreds of nests to identify unusually large males—rare individuals that carry two identical copies of the sex-determining gene.

By comparing their genomes, they were able to pinpoint the exact location of the gene responsible for sex determination in this species. Their findings are this week in the journal PLoS Biology.

They were surprised to find that the gene identified, ANTSR, is the same one that was recently shown to control sex in a species of ant. ANTSR is a long-noncoding RNA gene, which means it is not converted into a protein.

Depending on the number of copies present, it acts as a switch to initiate the developmental process towards male or female. The finding that this gene has a conserved function in bees and ants suggests that it has been working as a sex-determination gene for more than 150 million years.

"This is really surprising because it means ANTSR could be the main sex-determining gene used by more than 150,000 species of bees, wasps and ants," said Matthew Webster, senior author of the study. "We still don't understand how variation in this gene generates the signal triggering male or female development. The reason why honeybees use a different sex-determining gene is also still a mystery."

This discovery brings researchers a major step closer to understanding how sex-determination systems evolved across bees and ants, and the molecular mechanisms underpinning the haplodiploid system of sex determination. This is relevant for understanding how genetic diversity within these systems helps maintain healthy populations. The findings may also have implications for bee conservation and breeding programs, as they provide key insights into the genetics underlying reproduction in these essential pollinators.