Supergenes to determine the sex of entire ant colonies

Sexually reproducing organisms usually invest equally in male and female offspring. But there may be a deviation.

This abnormality led researchers to study parent-offspring conflict, genome conflict, and cooperative breeding. Some social insect species exhibit the unusual population-level pattern of a split sex ratio, with some colonies specializing in producing future queens and others specializing in producing males.

However, previous studies assumed that the split sex ratio was not under genetic control.

Researchers have discovered the genetic basis for the split sex ratio – how queen ants produce all-male or female broods.

“It’s weird to have a parent who only produces one sex or the other,” said UC Riverside entomologist and study author Jessica Purcell.

Scientists have known for some time that ant colonies can specialize in producing all-male or all-female offspring. For the first time, scientists at UC Riverside have located a series of genes on a single chromosome that are related to this phenomenon.

They published their discovery in the new National Academy of Sciences paper.

When humans mate, both parents contribute one copy of the genome to their offspring. However, female ants are the only ones to carry two specimens like humans and most other animals while the males have only one specimen.

“Male ants develop from unfertilized eggs laid by their mothers,” said UCR evolutionary biologist and senior study author Alan Brelsford. “That’s why male ants, like bees and wasps, genetically have a mother but no father.”

Purcell and Brelsford found their study specimens in 2016 while traveling to collect and study ants from Riverside to the Arctic Circle. In the Yukon region of northern Canada, they found more than 100 colonies of two Formica ant species that seemed ready for their annual breeding flights. Back in Riverside, doctoral student of ecology German Lagunas-Robles analyzed the genomes of these ants, looking for differences between male and female colonies.

Formica ants examining equipment intended to study their habitat. (Alan Brelsford UCR)

A queen will mate, land, chew off her wings and find a place to burrow during mating flights. She will lay about a dozen eggs in that burrow, which then develop into her first brood of workers. These worker ants are always female, but they will not reproduce. Once mature, the worker bees take over foraging for food and the queen continues to reproduce, laying hundreds of eggs a day.

While the males live only a few hours after their mating flight, the queen will store their sperm and use it to produce new offspring for the next ten years. Most young in an ant colony are wingless workers, but in adult colonies queens will also produce offspring that can fly.

Although the researchers found genes linked to the sex of offspring, genetics may not be the only way queens can influence the sex of their colonies. They could decide not to use their stored sperm, which would result in male ants. Workers could also manipulate the sex ratio by not feeding or selectively killing certain larvae.

Other studies have shown that food availability also affects the sex of an ant colony.

“If extra food is dumped on a colony, it will produce fewer males,” said Brelsford. The research team wants to conduct additional research to find out when genes or environmental factors play a greater role in determining the sex of offspring.

The team also wants to study how these genes work in different environments to help conserve beneficial native North American ants. Mating flights tend to coincide with specific seasons and temperatures. Climate change can affect food availability and the timing of reproduction and can unbalance the sex ratio in general.

Unlike their invasive, non-native pesky relatives, these species rarely harass humans and perform essential environmental functions.

“Ants are truly an integral part of ecosystems as one of the most abundant insects,” Purcell said. “Gardeners love earthworms, but ants do similar things to improve soil health.”

Magazine reference

  1. German Lagunas-Robles, Jessica Purcell and Alan Brelsford; Linked supergenes underlie the split sex ratio and social organization in an ant. PNAS Nov 16, 2021 118 (46) e2101427118; DOI: 10.1073/pnas.2101427118

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