Evolution: Meet the sea slugs which can choose to be a plant or an animal!

Sea slugs, can function like a plant, photosynthesising with the chloroplasts it obtains from feeding algae. Credit: Patrick Krug
Sea slugs, can function like a plant, photosynthesising with the chloroplasts it obtains from feeding algae. Credit: Patrick Krug

Survival of the fittest. That is one evolutionary concept which has withstood the test of time. Imagine if you could fly like a bird, if you ate one? If you could swim under water like a fish, if you ate one? Do you think its far-fetched? Not if you are an emerald Sea Slug- Elysia chlorotica! A latest finding by researchers from University of South Florida and University of Maryland, College Park led by Dr. Sydney Pierce has shown that seas slugs, a marine invertebrate which feeds on algae for its nutrition, manages to acquire both the genes and the cell organelles which are responsible for photosynthesis from algae. This enables it to receive nourishment from the sun when the going gets tough and allows the slug to give up its life as an animal to live temporarily ‘like a plant’. The findings published in The Biological Bulletin represents the first known example of horizontal gene transfer in multicellular organisms. It is also the only known example of functional gene transfer from one multicellular species to another.

Scientists used advanced imaging techniques to confirm that a gene from the alga V.litorea is present on the slug E.chlorotica’s chromosome. This particular gene is essential for the functioning of photosynthetic machines called chloroplasts, which are found in algae and plants. Since 1970s, scientists were aware of the fact that sea slug ‘steals’ the chloroplasts from algae and embeds them into its own digestive cells(a process termed kleptoplasty). Once they are within the sea slugs, the chloroplasts perform photosynthesis and provide nourishment for upto 9months, much longer than they would perform in the algae. Scientists have been baffled ever since, trying to understand how does the slug manage to maintain these chloroplasts for so long.

“This paper confirms that one of several algal genes needed to repair damage to chloroplasts, and keep them functioning, is present on the slug chromosome,” Pierce says. “The gene is incorporated into the slug chromosome and transmitted to the next generation of slugs.” While the next generation must take up chloroplasts anew from algae, the genes to maintain the chloroplasts are already present in the slug genome, Pierce says.

“Is a sea slug a good [biological model] for human therapy? Probably not. But figuring out the mechanism of this naturally occurring gene transfer could be extremely instructive for future medical applications,” says study co-author Sidney K. Pierce, an emeritus professor at University of South Florida and at University of Maryland, College Park.

“There is no way on earth that genes from an alga should work inside an animal cell,” Pierce says. “And yet here, they do. They allow the animal to rely on sunshine for its nutrition. So if something happens to their food source, they have a way of not starving to death until they find more algae to eat.” This biological adaptation is also a mechanism of rapid evolution, Pierce says. “When a successful transfer of genes between species occurs, evolution can basically happen from one generation to the next,” he notes, rather than over an evolutionary timescale of thousands of years.

A landmark finding in the field of evolutionary biology, indeed!

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The original paper can be accessed here.