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Sex Isn't Everything

Sex Isn't Everything
The ants, Mycocepurus smithii, amongst their fungus garden. Photo: Alex Wild.

The ants, Mycocepurus smithii, amongst their fungus garden. Photo: Alex Wild.

In the end, sex might not be everything. At least, that’s the story for a fungus-farming ant that seems to be doing just fine without it. In fact, they’ve done away with males entirely, a “first ever” for the ant world.

Most social insects—the wasps, ants and bees—are relatively used to daily life without males. Their colonies are well run by swarms of sterile sisters, lorded over by an egg-laying queen. But, eventually, all social insect species have the ability to produce a crop of males who go forth in the world to mate and propagate. All, that is, but one.

Dr. Ulrich Mueller and former graduate student Dr. Anna Himler (now at the University of Arizona) have shown that the fungus-farming ant, Mycocepurus smithii, is completely asexual. They report their findings in Proceedings of the Royal Society B.

The ants are found commonly (though deep in the ground) from northern Mexico to Argentina and throughout the Caribbean islands. They certainly aren’t new to science, but their male-less status is.

Tightening the Screws

To confirm the ants’ atypical asexuality, Mueller and Himler collected a mound of observational and experimental evidence.

They raised the ants in captivity for many years and never once saw a male. They and their many colleagues have also never seen any males in the wild.

Dr. Ulrich Mueller. Photo: Marsha Miller.

Dr. Ulrich Mueller. Photo: Marsha Miller.

In the lab, Mueller can propagate the ants like cells in culture. He can pluck a virgin female from a colony, place her in a new container with polenta (to use as fodder for her fungus garden), and she will eventually lay eggs and produce a new colony without ever seeing the likes of a male or his sperm.

Using DNA fingerprint analysis, he and Himler confirmed that all of the daughters in the colonies are genetically identical to their mothers. And part of the female’s mating apparatus—a critical lock-and-key mechanism in the female reproductive tract that holds the male penis—has degenerated.

“We also hammered them with antibiotics to see if we could make them produce males,” says Mueller.

He explains that some bacteria are known to turn their hosts asexual, an adaptation that helps transmit the bacteria from generation to generation. Take away the bacteria, and effected species are known to revert to their old sexually reproducing ways. But no amount of antibiotics made these fungus-farming ants embrace a sexual lifestyle.

In the evolutionary scheme of things, sex is the norm, so Mueller says this is an interesting finding. Sex provides the genetic recombination that helps species adapt and avoid pressures from parasites, predators and changing environments.

Mueller says most asexual animal species are but a flash in the evolutionary pan. “I would be surprised if this lineage is more than one million years old,” he says.

Whether or not their asexual strategy will become an exception to the rule is a question for an evolutionary biologist two million years from today.

Not Monoculturists

These ants may eschew diversifying their progeny through the mingling of the sexes, but they don’t shun all diversity.

Just as organic farmers grow a diverse array of crops for market, this particular species of fungus-farming ant grows many varieties of certain type of fungus.

“Many of the species of fungus-farming ants specialize on growing one type of fungus, like a farmer who only grows a crop of rutabaga or corn,” says Mueller. “This species grows a variety of fungi, even within the same close area.”

The ability of this ant species to switch to new fungal crops could mitigate the typical evolutionary “asexuality handicap,” report Mueller and Himler in their paper. Rather than diversifying their own genes through sex, the ants diversify their crops. Mueller says this might help the species avoid (in the long term) losing the fungal food they totally depend on to fungal pathogens.

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Friday, 27 January 2023

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