How plants evolved to make ants their maids

The plants are boring. They simply sit down during photosynthesis while the animals are having fun. Right? Not really. Examine the interactions between ants and plants – plants have evolved to make them attractive, such as the juicy nectar that insects eat and the hollow spines to shelter. In exchange, plants use ants for seeds and even act as bodyguards. 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 kinds of plants, and researchers have discovered that the long history of co-evolution ants and plants began with the search for ants on plants and that the plants reacted later by developing ant-friendly characteristics.

"My main interest is to study the evolution of interactions between organisms and how these interactions shape their evolutionary history." When did ants begin to use plants and when did plants start? started creating structures to use? " says Matt Nelsen, postdoctoral researcher at Field Museum and lead author of PNAS study.

"There are a number of different structures made by plants and designed to be used by ants," says Nelsen, who led the study with fellow researchers and co-authors of the Field Museum, Rick Ree and Corrie. Moreau. "Some plants have developed characteristics that persuade ants to protect them from attacks by insects and even mammals, which include hollow spines in which ants will live inside, or an extra nectar on them. the leaves or stems so that the ants can feed, take the nectar and run, but some will stay there and attack anything that could harm the plant, "says Nelsen. The other plants get ants to help them move their seeds, corrupting them with rich food packets attached to seeds called elaiosomes. "The ant will pick up the seed and carry it, eat the food pack and throw it away – often in a nutrient-rich area where it will grow better, and as it is further away from its parent, compete for resources ".

But scientists did not know how the evolutionary relationship between ants and plants had begun. If evolution is an arms race between species that develop ways to take advantage of their neighbors, scientists then wanted to know if the plants or ants had fired the first shot. "The question was whether ants were developing behaviors to take advantage of plants or if their structures were evolving to take advantage of ants," says Ree, plant curator at the Field Museum.

The history of plants and ants moving together dates back to the time of the dinosaurs, and it's not easy to tell from fossils how the organisms interacted. "There are very few fossil traces of these structures in plants, and they do not go back very far in the past, and there are tons of ant fossils, but they do not usually show these ant behaviors – we do not let's not necessarily see an ant kept in amber carrying a seed, "says Nelsen.

Thus, to determine the evolutionary history of ant interactions, Nelsen and his colleagues turned to large amounts of DNA data and ecological databases. "In our study, we associated these physical and behavioral traits with ants and plant family trees to determine when ants started to eat and live on plants and when plants developed the ability to produce structures. used by ants, "says Moreau, curator of Field. ants.

The team traced the history of its respective characteristics for ants and its use by these family trees – a process called ancestral state reconstruction. They were able to determine when plants started to rely on ants to defend themselves and for seed distribution – and it seems that ants relied on plants longer than plants directly counted on ants, since plants have developed these specialized structures only long after the ants relied on them for food and habitat.

"Some ants do not use a lot of plants directly, others depend on them to feed, feed and nest.We have discovered that to invest fully in the use of plants, the ants started feeding in the trees and then incorporated plants into their diet Then, from there, they started to nest in trees, so if this progressive change towards greater dependence on Plants is intuitive, it still surprised us, "said Nelsen.

And although there has been a mutually beneficial relationship between ants and plants over the years, from an evolutionary point of view, groups of ants that eat, stuff or nest in plants do not seem to not be better off than those who do not. "We do not see any parts of the ants' family tree that include ants that depend on plants to feed or diversify or grow their habitat faster than parts of the tree that n? do not have these interactions, "says Nelsen. "This study is important because it gives insight into how these extensive and complex interactions have evolved."

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More information:
Matthew P. Nelsen et al., "Ant-plant interactions have evolved through increasing interdependence" PNAS (2018). www.pnas.org/cgi/doi/10.1073/pnas.1719794115