Ants exploited plants long before the plants returned the favour
Plant symbiosis with ants developed long before the insects started chowing on leaves and stems. Tanya Loos reports.
A study of ants and plant evolution reveals that the former were relying on the latter as sources of food and habitat long before the plants evolved structures to attract them.
Interactions between ants and plants are diverse and abundant, with many complex symbiotic relationships.
Some 11,000 species of flowering plants across 77 families produce elaiosomes: lipid rich seed appendages that provide ants food in return for seed dispersal. Minute pockets of nectar on the leaves or stems, known as extra-floral nectaries, occur on close to 4000 species, and there are nearly 700 plants that supply homes, known as domatia, for ants in their stems – in return for the insects’ efficient defence against herbivores.
But according to molecular analysis and the fossil record, ants were once ground-foraging carnivores. How did today’s plant-ant mutualism and interdependence evolve? Previous studies have focused primarily on smaller groups, and often only on the ants or plants rather than both. Matthew Nelsen and colleagues from the Field Museum of Natural History in Chicago, US, decided to undertake a macro-evolutionary study of the two organism groups.
“Some ants do not rely on plants for food or habitat, while others rely heavily on plants,” Nelsen says.
“By linking pre-existing trait databases with large-scale phylogenies, we were able to provide a clearer picture of how interactions between plants and ants evolved.”
Nelsen and colleagues Richard Ree and Corrie Moreau examined the evolution of plant-dependence in ants by creating a time-scaled history of 1746 ant species. The process was repeated for plants, looking at the evolution of ant-associated traits across 10,785 genera.
The findings, published in the journal PNAS, reveal that ants used plants for food and nesting well before the evolution of specialised ant associated structures.
“Our work suggests that ants first began using plants as places to search for food; they then began relying on plant-based food sources, and then finally began nesting in the plants,” Nelsen explains.
The insects began foraging in trees and incorporating plant foods into their diets during the Early Cretaceous period, around 145 million years ago. Arboreal nesting originated near the end of the Late Cretaceous, roughly 80 million years later.
In plants, specialised structures such as eliaosomes and domatia evolved about 66 million years ago, long after ant-plant relationships began.
“So what were ants eating and where were they nesting before these plant structures evolved?” Nelsen asks.
“We think ants were relying on more generalised plant-based food sources, such as sap, and were using more generalised nesting habitats, such as cracks and holes in bark or under epiphytic plants prior to the evolution of these plant structures.”
The research also demonstrates that although interactions with ants likely facilitated plant diversification, living and feeding on plants did not affect ants in the same way.