In the 1960s, Penn biologist Dan Janzen, as part of earning his Ph.D., re-described what has become a classic example of biological mutualism: the obligate relationship between acacia-ants and ant-acacia trees. The acacia trees produce specialized structures to shelter and feed the ant colony, and the ants, in turn, defend the tree against herbivores.
In a new study in Proceedings of the National Academy of Sciences, colleagues of Janzen's in the Penn Biology Department uncover a genetic mechanism that programs the plant side of the ant-acacia relationship. Scott Poethig, a plant biologist, and Aaron Leichty, who earned his Ph.D. working under Poethig and is now a postdoctoral researcher at the University of California, Davis, showed that these species of acacia develop the traits necessary to feed the ant colony--hollow swollen thorns to house them, and nectaries and nutrient-rich leaflet tips called Beltian bodies to feed them--as part of an age-dependent phenomenon in plant development.
"There is a cost associated with making these traits," says Poethig, senior author on the report, "but the plant needs them, otherwise it's a goner. Dan showed: no ants, no plants. The plant is eaten by everything from grasshoppers to mice.