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Titles of research papers usually don’t surprise us, but this one did. It is called Primitive eusociality in a land plant?. The reason for the surprise is that eusociality, so far as we know, is restricted to the animal kingdom. Many animals live in social groups of varying degrees of complexity. The most complex of these is called ‘eusocial’, meaning ‘truly social’. Members of eusocial colonies belong to multiple generations, exhibit cooperative care of the offspring, and only some reproduce, while others merely work for the welfare of the colony.

Now, Kevin Burns, a professor of biology at the Victoria University of Wellington, New Zealand, and his colleagues claim that they have discovered a eusocial plant. Burns’s research takes him, among other places, to Lord Howe Island, a small volcanic island located between Australia and New Zealand. Just over 14 sq. km wide, it was found and claimed by the British in 1788. It is currently administered by the state of New South Wales in Australia, and harbours stunted tropical dry forest, coral reefs and some mountains, with many endemic species of plants and animals, in addition to a resident human population of 328 people and a floating population of 400 tourists

A eusocial fern?

The object of Burns’s study is the epiphytic fern Platycerium bifurcatum. Epiphytes are plants that live on plants, but they are non-parasitic – they germinate on other plants but do not take nourishment from them. Surprising as it may seem, epiphytes derive all their nutrition from the air and rainwater. Such an austere lifestyle suggests interesting innovations that deserve greater attention from evolutionary biologists.

Ferns are vascular plants – those with xylem and phloem tissues to transport water and food – but they lack seeds or fruits. When I was in high school, ferns were called pteridophytes, but the pteridophytes have turned out to be a bunch of loosely related plants, and hence the true ferns now go by the more fancy name polypodiophyta. They have stems and leaves and produce their own food by photosynthesis, but reproduce with spores.

Though their eusociality is a new discovery, Platycerium bifurcatum itself is a well-known plant. Commonly known as elkhorn fern or the staghorn fern, because its leaves are forked like the antlers of deer, it is commonly cultivated as an ornamental plant in gardens and homes. It is easy to mount on wooden boards and needs little care.

Burns and his team found that Platycerium bifurcatum never occurred solo but was always clustered in distinct colonies, with 6 to 58 individuals per colony, well separated from other colonies.

The fern, as is well-known, produces two kinds of leaves, technically called fronds. The so-called ‘strap fronds’ are long, narrow and photosynthetically active. Some of them are also reproductively active. The other kind, called ‘nest fronds’, are reproductively inactive. The function of the non-reproductive nest fronds appears to be to absorb water and nutrients and share them with the reproductively active strap fronds.

Burns and his colleagues have provided two additional lines of evidence that are suggestive of eusociality. First, they find that the per-capita reproductive effort increased with colony size. Second, they found through DNA sequencing that although eight colonies were clones, at least two colonies consisted of unrelated individuals.

If all the individuals in a colony are clones of each other, they might simply be thought of as a single plant. But if some of the members are unrelated, they would have to be regarded as a colony, with the non-reproductive fronds being altruistic towards the reproductive fronds inasmuch as they share water and nutrients.

In another paper, Burns makes a strong argument for the evolution of eusociality and division of labour in Platycerium bifurcatum in response to ecological stress. His arguments are essentially indistinguishable from the arguments zoologists make for the evolution of eusociality and division of labour in their taxa.

It is entirely possible that purists will raise red flags and demand more stringent evidence for overlap of generations, cooperative brood care and reproductive caste differentiation – the three diagnostic characters of eusociality. To be fair to Burns and his colleagues, they frame the title of their paper as a question to be answered by their readers or by future work.

They also display admirable caution in their prose: “Here, we explore whether a major evolutionary transition to eusociality has occurred in plants…”. Nevertheless, this is already an exciting finding that should broaden the scope of eusociality or near-eusociality. More importantly, it should bring eusociality to the doorstep of the plant kingdom.

Excerpted from https://science.thewire.in/the-sciences/eusocial-fern-suzanne-batra-endosperm-frugal-science-raghavendra-gadagkar/