February 11, 2011 | 4
Tens of millions of years before humanity sowed its first crops, a somewhat humbler organism was starting up its own large-scale agricultural operations. Leaf-cutter ant species depend on actively managed fungus farming to feed their teaming colonies.
The ant’s newly sequenced genome, based on three male Atta cephalotes ants collected in Gamboa, Panama, and described in PLoS Genetics‘s February 10 issue, reveals a carefully culled genetic profile that has been adapted over the millennia to survive and thrive in symbiosis with its prize fungus.
The leaf-cutter ant joins a handful of other ants—including the Argentine ant, the Florida carpenter ant and Jerdon’s jumping ant—to have had its genome sequenced.
In comparison with other insect genomes, the fungus-farming ant’s genome (estimated to have just over 18,000 genes) appears to have been pruned back over the years. "Presumably, the ants have lost these genes because of their dependency on the fungus," Garret Suen, a bacteriologist at the University of Wisconsin-Madison, and study co-author, said in a prepared statement. "They don’t need these proteins kicking around any more."
The genetic regions that seem to have been most heavily trimmed are those related to food digestion. The ants are lacking, for instance, "genes associated with nutrient acquisition and amino acid biosynthesis," the researchers noted in their paper. "These genes appear to be no longer required because the fungus may provide these nutrients."
The fungus that the ants grow in their colonies is made into handy food packets, known as gongylidia, full of amino acids, carbohydrates, lipids, proteins and vitamins. Work is currently underway to sequence the genome of the farmed fungus as well as other microbes that are known to exist in the symbiotic system.
"The genome allows us to understand interactions in this insect-microbial system," Cameron Currie, also a UW bacteriologist and paper co-author, said in a prepared statement. "It improves our ability to understand the evolution of symbiosis."
Researchers also hope to delve deeper into the ants’ DNA to better understand the vast morphological differences that A. cephalotes exhibits. Diminutive farmer ants have head sizes as small as 0.8 millimeters wide, where as large soldier leaf-cutter ants from the same colony can have heads that are more than four times that size.
Aside from their intriguing growth and feeding patterns, the leaf-cutter ants are a major player in tropical ecosystems in the Western Hemisphere. Their individual colonies can contain millions of members and take up as much volume as 600 cubic meters. The industrious insects are responsible for shearing off at least 12 percent of tropical forests’ foliage each year—which they feed to their fungal fruits, rather than consuming directly themselves. This appreciable harvesting makes them key links in the carbon cycle as well as powerful pests when human agriculture encroaches on their own.
Image of leaf-cutter ants courtesy of Wolfgang Hoffmann/University of Wisconsin-Madison; video courtesy of UW-Madison College of Agricultrual and Life Sciences