First-Time Discovery: Social Structure Of Ant Colonies Could Have Important Role In Gene Flow And Origin Of Species

October 17, 1996

**EDITORS: EMBARGOED UNTIL 4 P.M WEDNESDAY, OCT. 16, 1996**

ATHENS, Ga. -- A team of researchers from the University of Georgia and the University of Rochester has discovered the first evidence that social behavior apparently dictates genetic flow within a single species of social insects.

The discovery, announced today in the journal Nature, could have a significant impact on how scientists view the role of social organization in the origin of species.

"The greatest significance of this work is it demonstrates that alternations or variations in social behavior within a species can very profoundly influence patterns of interbreeding," said Dr. Kenneth Ross, an entomologist from the University of Georgia. "It suggests that mating may not be random at all, and that there may be constraints on who mates with whom."

The new model of speciation, which Ross admits is sure to be controversial, was discovered among two social forms of the fire ant, Solenopsis invicta. One form contains a single egg-laying queen in a nest, while the other contains 200 or more queens per nest. The two forms live close to each other across the ant's native range in South America and its introduced range in the Southeastern United States.

Since the two social forms differ in several features of their breeding biology, Ross and Dr. DeWayne Shoemaker of the department of biology at the University of Rochester were able to use genetic markers to examine how genes flow between the forms. Their data demonstrated four different potential routes of gene flow between single-queen and multiple-queen colonies. But three of the routes did not lead to the movement of genes between the social forms of S. invicta.

Instead, they found that queens from multiple-queen nests commonly mate with and use the sperm of males from single-queen colonies. (This may help explain why multiple-queen populations through the introduced range in the United States closely resemble adjacent single-queen populations genetically.)

"This is the first good genetic evidence that such a thing could happen among these ants," said Ross.

The new tests are contrary to much of what is known about the way species arise through the transfer of gene flow. Scientists have often theorized that changes in genetic structure can lead to changes in social organization but have rarely argued that the model works the other way around.

For the study, fire ants were sampled from several test areas in north Georgia where the two social forms occur together. The use of a genetic technique called RFLP mapping allowed the team to discover a maternally inherited fragment of mitochondrial DNA that in turn led to the detection of four composite haplotypes. By tracing the presence or absence of each type at the test colonies, Shoemaker and Ross was able to reveal just how queens are involved with the gene flow between colonies of the different social forms.

There are several reasons for the new theory of gene flow in social insects, Ross said. First, queens from single-queen nests dispersed widely during mating flights and founded new colonies without the assistance of worker ants, relying on extensive nutrient reserves acquired during adult maturation to accomplish their task. In contrast, queens from multiple-queen mounds mate in the nests where they are born and attempt to become egg-layers or participate in mating flights and seek adoption into other multiple-queen nests.

"The queens from the multiple-queen colonies thus don't need the excessive nutrient reserves, and in fact these reserves are a disadvantage," said Ross, "because we have found that workers in the multiple-queen colonies are intolerant of queens with these high nutrient reserves."

Of crucial importance is the fact that the single gene-flow path between the two social forms could make S. invicta vulnerable to an eventual stopping of all genetic exchange between them. This could occur if mating flights were abandoned by queens from multiple-queen colonies in favor of within-nest mating and could lead to complete reproductive isolation between parts of the same species that have diverged for reasons of social organization.

The new findings published today in Nature add to earlier discoveries about fire ants that Ross has published in Science and in the Proceedings of the Natural Academies of Sciences, which showed, among other things, that certain fire ant queens have a gene that causes them to signal to the colony for their own destruction. Ross's research also showed that there are a high number of unrelated queens in fire ant mounds -- a discovery that demonstrated genetics may be less crucial than previously thought in the colonization of social insects.


University of Georgia

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