A new study in the Proceedings of the National Academy of Sciences breaks down the genetic history of 1,700 species of ants and 10,000 plant genera, and the researchers found that the long history of ant and plant co-evolution started with ants foraging on plants and plants later responding by evolving ant-friendly traits.
“There are a number of different structures plants make that are specific for ant use,” explains Nelsen, who led the study with his fellow Field Museum researchers and co-authors Rick Ree and Corrie Moreau. “Some plants have evolved features that persuade ants into defending them from attack from other insects and even mammals. These include hollow thorns that ants will live inside, or extra nectar on leaves or stems for the ants to eat. Some ants will just cheat and take the nectar and run, but some will stick around and attack anything that tries to hurt the plant,” explains Nelsen. Other plants get ants to help them move their seeds around, by bribing them with rich food packets attached to seeds called elaiosomes. “The ant will pick up the seed and carry it away, eat the food packet, and discard the seed — often in a nutrient-rich area where it’ll grow better, and since it’s farther away from its parent, they won’t have to compete for resources.
But how did these complex relationships get started?
The team mapped the history of plant’s ant-friendly traits and of ants’ plant use onto these family trees — a process called ancestral state reconstruction. They were able to determine when plants began relying on ants for defense and seed distribution — and it looks like ants have relied on plants for longer than plants have directly relied on ants, since plants didn’t evolve these specialized structures until long after ants had been relying on them for food and habitat.
So it looks as though the plants came up with the idea. Which makes sense because they are stuck in one place but ants are mobile.
And while there has been a mutually beneficial relationship between ants and plants over the years, from an evolutionary standpoint, groups of ants that eat, forage on or nest in plants don’t seem to be any better off than those that do not. “We don’t see parts of the ant family tree that includes ants relying on plants for food or habitat diversifying or growing any faster than those parts of the tree that lack these interactions,” says Nelsen. “This study matters because it provides a glimpse into how these widespread and complex interactions evolved.” Paper. (paywall) – Matthew P. Nelsen, Richard H. Ree, Corrie S. Moreau. Ant–plant interactions evolved through increasing interdependence. Proceedings of the National Academy of Sciences, 2018; 201719794 DOI: 10.1073/pnas.1719794115 More.
Bet we haven’t heard the last of this story.
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