Asexual reproduction bucks trend for unique ant species

Genetic diversity is essential for a species’ survival. Maintaining genetic diversity is easy when a species reproduces sexually: an egg and sperm combine the genetic material of two organisms together to form genomically robust offspring with two distinct versions of the species’ genome.

Without a combination of different genetic makeups, asexually reproducing species typically suffer from a lack of diversity, limiting their population on Earth. One such animal is the clonal raider ant, which produces successive genetically identical daughters directly from unfertilized eggs through parthenogenesis, a method of asexual reproduction in which offspring inherit two sets of genetically identical chromosomes from their mother.

Over time, the random inheritance of these chromosomes should repeat endlessly, leading to a catastrophic loss of genetic traits and ultimately to the extinction of the species. Yet these blind, queenless insects, native to Bangladesh and now found in tropical regions around the world, seem to be surviving just fine. How is this possible?

As researchers at Rockefeller University recently discovered, clonal invasive ants don’t gamble when it comes to inheriting genes: Instead, they ensure that their offspring inherit two different versions of their entire genome, preserving much of the genetic diversity present in the ancient founders of each clonal lineage.

In theory, this shouldn’t work: chromosomes are thought to be shuffled randomly during meiosis (the type of cell division used to make sperm and eggs during reproduction in all animals, plants, and fungi). But as reported in Nature Ecology & Evolution, in this animal the process seems to be far from random.

“We think we’ve found a way that clonal invasive ants avoid the loss of genetic diversity that typically occurs through parthenogenesis,” says Kip Lacy, a graduate researcher in Daniel Cronauer’s Social Evolution and Behavior Laboratory and lead author of the paper. “Maybe this diversity is what allows the species to persist.”

The asexual conundrum

Parthenogenetic species are rare, but they appear in a variety of organisms, including reptiles, amphibians, worms, fish, and birds. They’re unlikely to survive long-term: “Completely asexual species tend to go extinct pretty quickly,” Lacey says.

“Cloning is like a one-way road to degradation,” Kronauer adds. “You can’t remove mildly harmful mutations from the genome every time they arise. You just end up accumulating more and more mutations over time.”

The problem starts with two challenges that asexual species must overcome at the cellular level: First, they need to create a diploid genome, containing two sets of chromosomes, to pass on to their offspring.

“But in the case of clonal invasive ants, reproduction doesn’t involve sperm, so where do they get the extra chromosomes?” Lacey said.

Second, the offspring must have a genetic makeup compatible with development and reproduction, something that many asexual species, which have two genetically identical sets of chromosomes, lack.

Lacey became interested in parthenogenesis while studying tropical fire ants during his master’s degree at the University of Georgia. Some colonies of fire ants reproduce asexually to produce queens. These ants, Lacey found, had almost completely lost genetic diversity. When he joined Cronauer’s lab in 2019, he set out to understand how clonal invasive ants avoid such pitfalls.

Mother and daughter

During meiosis, chromosomes break down and recombine to create new combinations of gene copies. After these so-called crossover events occur, chromosomes are randomly shuffled throughout cell division.

With parthenogenesis, clonal lineages are drawn from two identical sets of chromosomes, so “you would expect a lot of diversity to be lost with each cycle,” Kronauer says, which is like diluting the gene soup.

To understand why this wasn’t the case for the cloned invading ants, the researchers looked at mother-daughter and sister-ant pairs. To make sure they were truly family pairs, they tracked genetically modified ants that glowed red under a microscope — a groundbreaking genetic engineering technique developed by researcher Taylor Hart in Kronauer’s lab. These pairs were the only animals in the colony that glowed.

Using linked-read gene sequencing, which allows researchers to reconstruct entire chromosome sequences, the researchers found that there was no loss of genetic diversity from mother to daughter. However, there was evidence of crossing over in the daughter’s genome. In total, the researchers recorded 144 crossing over events, but only one showed loss of genetic diversity.

“This is because chromosomes that have recombined with each other are always inherited together,” Lacey says. “This co-inheritance explains why the species continues to survive. In the case of clonal invasive ants, this occurs 800% more often than if the genetic dice were rolled randomly.”

New Tactics

According to Kronauer, this strategy for maintaining genetic diversity has never been documented before. Its existence suggests that there may be more ways to avoid random genetic succession than we know. For example, one well-known deviation from random succession is when a “selfish” gene promotes its own reproduction over other genes, essentially manipulating the game to its advantage. But this deviation cannot explain the reproduction of clonal invasive ants, which is “unselfish” because no gene has an advantage and all gene copies are co-inherited. It is unclear whether this strategy of unselfish succession occurs in other animals, including sexually reproducing species.

The discovery highlights the utility of studying species with unusual reproductive biology, Lacey said: “If we hadn’t studied these asexual ants, we might never have learned about this reproductive mode.”