Unusual reproductive behavior of strange ants surprises scientists

COLUMBUS, Ohio — A genetically unusual ant population is changing some of the fundamental ways researchers think about insect colonies.

Social insects, such as ants and bees, thrive on the caste system – a precise division of labor among the members of the colony. In most of these societies, the environment is thought to influence whether larvae develop into queens or sterile female workers, said Steve Rissing, a professor of evolution, ecology and organismal biology at Ohio State University.

But in a new study, Rissing and his colleagues found some genetically odd colonies of harvest ants (Pogonomyrmex), which don’t seem to adhere to the traditional rules of caste development. They found that genetics – not the environment – determines the fate of a developing ant, and consequently the role it will play in the colony.

The researchers report their findings in the latest issue of the journal Current Biology. The team was led by Sara Helms Cahan, an assistant professor of biology at the University of Vermont.

A typical ant colony includes one queen and, in the case of harvester ants, hundreds or thousands of sterile female workers (worker ants are always female and, with a few exceptions, sterile. Soldier ants are larger versions of worker bees.) During her lifetime, which can be as many as 20 or 30 years, a queen mainly produces worker eggs.

Male ants, which come from unfertilized eggs, usually have one goal: to mate with a queen. Males are usually scarce and a queen will only produce male eggs when it is time to make more colonies. Then a queen produces eggs from which both males and queens (reproductive females) emerge. The males and new queens swarm out of the nest, mate, and the young queens attempt to establish a new colony. Males, which have a short lifespan, die shortly after mating.

The type of ants in this study—harvest ants—are one of the largest insect communities in the western United States, with a range of hundreds of miles and nests so large they can be seen from airplanes.

“This is the ant that controls the west — it’s everywhere,” Rissing said.

The researchers had noticed that in certain areas — mainly in southeastern Arizona and New Mexico — some male harvester ants looked different. So they collected several dozen pairs of queens and males and brought these pairs back to the lab for genetic testing, with surprising results.

“The DNA of some of these ants was just weird — we certainly didn’t expect to get the results we did,” Rissing said. “It seems that the queens in these colonies mate with males from two different genetic lines. And when a queen and a male of the same lineage usually mate, it usually produced a reproductive female — another queen. But when a queen and a male of different genetic sexes mated, that mating overwhelmingly produced a sterile worker.

“This kind of reproductive behavior is very different from what we expect to see in ant communities,” he continued. “We would expect the same DNA sequence from all ants in a given colony, but that didn’t happen here.”

Never mind that the lab experiments mimicked the creation of a new colony, which relies heavily on workers and needs only one queen: When a queen and a male of the same line mate, they produced eggs that would lead to many queens. The results also showed that all eggs produced became workers when a queen mated with a male from an alternate line.

Traditional interpretations of social insect colonies would dictate that the need for workers would influence the fate of an ant’s role, negating any genetic predisposition; this research shows that this is not always the case.

These harvest ants had two different genetic lines that the researchers called H1 and H2.

Each H1 and H2 queen was mated to an H1 or H2 male. In nature, queen ants mate for only one period, but during that time they mate with many males (this causes the queen to build up a sizable sperm bank.) Queens can store sperm and lay eggs throughout their lives.

All queens in the study laid about 60 fertilized eggs, but only 0.3 percent of the eggs from the same lineage mating (H1 queen and male; H2 queen and male) developed to adulthood. The queen raised these reproductive females — genetically queens themselves — as workers, although the researchers noted that these ants apparently struggled to fulfill their roles as workers.

“The ants of the same lineage that reached adulthood had almost completely lost their ability to develop into functional workers,” Rissing said.

In contrast, 87 percent of the eggs from the alternative line became successful adult workers.

“It’s obvious what the queen has to do — she has to mate with more than one male,” Rissing said.

The researchers suspected that harvester ant queens can probably tell the difference between the males they mate with. The researchers also noted that the males had different colors depending on which lineage they belonged to. In the wild, a queen can use this information to ensure she has enough sperm from males of both sexes, which would ensure the success of the colony.

Interestingly, men apparently cannot tell the difference between women. (Rissing said he and his colleagues could only tell the difference using modern molecular lab technology.)

“If males could do that, it could mean the end of a colony, because males may prefer to mate with a queen of the same lineage,” Rissing said. In theory, a male ant wants his genes to live on. If his sperm fertilizes an egg from a queen of a different lineage, his genes will die with the sterile female worker produced.

“This population of harvester ants depends on this two-line system for survival,” Rissing said. “The hybrid worker caste is what ties the two otherwise independent H1 and H2 genome-based populations together. So far, this is quite an unusual finding, but as we gain access to more and more tools that help us understand what’s going on level, we will likely find that many social insects and other animals do not live up to our predictions and expectations.”

Rissing and Cahan conducted this study with Glennis Julian of the University of Arizona, and Tanja Schwander and Joel Parker, both of the University of Lausanne in Switzerland.

This work was funded by grants from the Durfee Foundation, the Swiss Society of Naturalists, and the Swiss National Science Foundation.

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