Ferns and flowers bribe helpful ant defenders with nectar, but ferns developed this ability much later – our study shows why

If you are taking a better have a look at a plant in your park, your garden, and even your kitchen, you'll probably see damage. Whether a caterpillar has gnawed off a part of a leaf or a mealybug is sucking sap, animals feed on plants on a regular basis.

Of course, herbivory, or eating plants, is just not ideal for a plant's survival, so plants have evolved many various defense mechanisms to counteract this threat, including physical and chemical weapons. Cacti, for instance, are equipped with spines that pierce skin. Herbs reminiscent of mint, lavender, and rosemary produce volatile scents that might help deter herbivores.

Other plants bribe their bodyguards by secreting thick, sweet nectar.

Nectar is mostly related to flowers, where it’s used to draw bees, birds or butterflies to move pollen from one flower to a different. But other plants also produce other forms of nectar glands, called extrafloral nectariesPlants produce these glands to bribe ants with a sweet reward; in return, the ants defend the plant from herbivorous insects.

I study Evolutionary biology of plantsand recently worked with Fern biologist Fay-Wei Li At the Boyce Thompson Institute and Cornell University Ant biologist Corrie Moreau to research the evolution of ant defense mechanisms in plants.

We found something amazing: almost 130 million years ago, throughout the Cretaceous period, ferns and flowering plants Ant bribery nectar glands developed independently at concerning the same timeWe found this out by utilizing complex algorithms to estimate the evolutionary origins and history of ferns, flowering plants, and ants.

This cut-off date is sort of interesting since it was very early within the evolutionary history of flowering plants, but quite late in Evolutionary history of the fernOur work has shown that old dogs can learn some recent tricks – and, more importantly, how this happened in ferns.

Meeting above ground

Plants are the first producers of just about all food on Earth, so virtually all living things rely upon them for survival. For this reason, herbivory is a component of life. But it also imposes significant costs on many industries, from houseplants to crops. Major pest infestations may even threaten global food security.

For all these reasons, understanding how plants defend themselves against predators is an important challenge.

The evolution of ant-based defense strategies inextricably linked two lineages in the dominion of life. This meant that ants and plants ultimately evolved together – a process called coevolutionWhen one species changes, the opposite may change in response, and these changes may even be encoded of their genes.

Flowering plants emerged within the Cretaceous period about 150 million years ago, and our analyses showed that they formed close bonds with ants early on. These flowering plants and their ant partners appear to have co-evolved over time.

However, this was not the case with ferns. Although they’d the potential to evolve nectaries concurrently flowering plants, they only began to rapidly evolve nectaries once they learned to live amongst trees.

Ferns were originally land plants, but after flowering plants evolved into large trees, jumped onto their branches as epiphytes – Plants that grow on other plants, often with none connection to the soil.

A drop of moisture on a fern leaf where it is connected to the stem
Drynaria pilosacommonly generally known as basket fern, secretes nectar.
Jacob Switzerland, CC BY-ND

Ferns may also climb up trees, like ivy, or form their very own trunk, like Tree ferns. This also helped the ferns to succeed in the treetops.

The undeniable fact that ferns didn’t produce nectar for ants until they moved to trees puzzled me as a fern biologist. That was until my co-author Corrie Moreau identified that the majority ants live in treetops.

This was entirely logical. When ferns became cover dwellers, they became closer to ants, which were already related to nectar-bearing trees. By interbreeding with these ants, the ferns eventually took advantage of the established, mutually useful relationship between the ants and flowering plants.

The evolution and ecology of fern-ant relationships

While our study uncovered recent features of ant-mediated plant defense, it left many questions unanswered. For example, are some ants specialized on ferns, or are they generalists that may feed on nectar from a greater diversity of plants? How exactly did plants originally evolve the physical ability to provide extrafloral nectar? Are the genes encoding nectar development the identical in ferns and flowering plants? Is the chemistry of nectar from ferns and flowering plants the identical?

Our study lays the inspiration for further research into the evolution and ecology of those nectaries. This is significant basic research. It can be conceivable that research on this area could contribute to breeding programs that promote nectary-mediated ant defense, thereby reducing the necessity for pesticides to guard plants from predators.

image credit : theconversation.com