A new study reported in the Biological Bulletin provides the first direct evidence that the chromosomes of a sea slug called the Eastern emerald elysia (Elysia chlorotica) have some genes that come from the algae it eats. The genes help sustain photosynthetic processes inside the sea slug that provide it with all the food it needs.
The Eastern emerald elysia (Elysia chlorotica). Image credit: Patrick Krug.
A team led by Prof Sidney Pierce from the University of South Florida and the University of Maryland, College Park, used an advanced imaging technique to confirm that a gene from the food alga Vaucheria litorea is present on the slug’s chromosome.
This gene makes an enzyme that is critical to the function of photosynthetic machines called chloroplasts, which are typically found in plants and algae.
It has been known since the 1970s that the Eastern emerald elysia steals chloroplasts from the algae and embeds them into its own digestive cells.
Once inside the slug cells, the chloroplasts continue to photosynthesize for up to 9 months. The photosynthesis process produces carbohydrates and lipids, which nourish the slug.
How the slug manages to maintain these photosynthesizing organelles for so long has been the topic of intensive study and a good deal of controversy.
“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. The gene is incorporated into the slug chromosome and transmitted to the next generation of slugs,” Prof Pierce said.
“While the next generation must take up chloroplasts anew from algae, the genes to maintain the chloroplasts are already present in the slug genome.”
“There is no way on Earth that genes from an alga should work inside an animal cell.”
He added: “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.”
“When a successful transfer of genes between species occurs, evolution can basically happen from one generation to the next, rather than over an evolutionary timescale of thousands of years.”
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Julie A. Schwartz et al. 2014. FISH Labeling Reveals a Horizontally Transferred Algal (Vaucheria litorea) Nuclear Gene on a Sea Slug (Elysia chlorotica) Chromosome. Biol. Bull., vol. 227, no. 3, pp. 300-312
