Study on leaf-cutter ants sheds light on species evolution

In a feat not easy for some birds, the Amazonian leaf-cutter ant can fly across the 2-mile width of the Amazon River, reveals UT graduate Scott Solomon and other researchers in a study on the ant's biodiversity.

That fact debunks the "riverine barrier hypothesis," at least in regard to leaf-cutter ants. The hypothesis states that species diversity comes from an inability to cross the width of a waterway, causing a divide in the genetic pool.

In their study, the researchers, from the State University of Sao Paulo in Brazil, found there are genetically similar types of the leaf-cutter ants on both sides of the Amazon River.

However, the study supports similar, yet distinct, concepts of evolution called "pleistocene refugia hypothesis" and "marine incursion hypothesis" to explain the range of diversity in the leaf-cutter ant. The hypotheses state that spatial separation leads to divergent genetic evolutions.

The "pleistocene refugia hypothesis" states that genetic differences grow out of species isolation, which could have occurred through movement of the earth's plates. The "marine incursion hypothesis" says that because of changing sea levels, rising tides could have forced the ants to disperse in order to survive.

Historical flooding in the Amazon and Ice Age climate shifts also contributed to the wide range of ant speciation, according to the study, although more unknown factors are involved.

Solomon, who earned a Ph.D. in ecology, evolution and behavior while at UT, became involved with the research group through UT professor and biologist Ulrich Mueller in 2001. For the study, leaf-cutter ants were collected everywhere from southern Mexico to the Amazon region, he said. Leaf-cutter ants are also found in Austin.

The ants are named "leaf-cutters" because they cut leaves and gather the pieces in order to create compost that grows a specific type of fungus they eat.

Solomon is based in Brazil for the next year conducting post-operation research on the study. He said he is trying to understand the evolution of the ant and will try to re-sequence the events of its evolution.

"I thought it was really interesting, really fun," he said. "All the conditions were right, and the more I learned about [the ants], the more fascinating I found that they were."