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How plants evolved with ants to make them their servants

Plants are so boring. They just sit there all day photosynthesizing while animals have all the fun, right? Apparently, not so much. Take a look at these interactions between ants and plants. Plants have evolved their features specifically to make them enticing to ants, like juicy nectar for the insects to eat and hollow thorns for them to take shelter in. In exchange, plants use the ants to spread their seeds and even act as their 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 plant genera, and researchers found that plants were later responding by evolving ant-friendly traits.

Our main interests are in studying how interactions between organisms have evolved, and how these interactions shape their evolutionary history. When did ants first start using plants, and when did plants start making structures for ants to use?

There are a number of different structures that plants make are specific for ant use. Some plants have evolved features that force ants into defending them from attack from other insects and even mammals. These include hollow thorns that ants would live inside or extra nectar on leaves or stems for the ants to eat. Some ants would just cheat and take the nectar and run, but some will stick around and attack anything that tries to hurt the plant. Other plants get the ants to help them move their seeds around, by bribing them with rich food packets attached to seeds called elaiosomes. The ant would pick up the seed and carry it away, eat the food packet, and throw the seed, often in a nutrient-rich area where it’ll grow better.

But scientists were not sure how the evolutionary relationship between ants and plants actually got started. If evolution is an arms race between species developing ways to profit off their neighbors, then scientists want to know whether plants or ants fired the first shot. It was sort of a chicken-and-egg question, whether things started with ants developing behaviors to take advantage of plants, or plants evolving structures to take advantage of ants.

The history of plants and ants evolving together goes back to the time of the dinosaurs, and it’s not that easy to tell from fossils how the organisms interacted. There are very few fossil records of these structures in plants, and they don’t extend far back in time. And there are tons of ant fossils, but they don’t show these ant behaviors, we don’t necessarily see an ant preserved in amber carrying a seed.

So, in order to determine the early evolutionary history of ant-plant interactions, we turned to large amounts of DNA data and ecological databases. In our study, we linked these behavioral and physical features with family trees of ants and plants to determine when the ants started eating and living on plants, and when the plants developed the capacity to produce structures that ants use.

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