The brilliant emerald green sea slug, Elysia chlorotica, spends months living on sunlight just like plants. It’s been called the photosynthesizing sea slug in the past, but how it manages to do this as well as it does is a complete mystery. In a new study appearing in the Biological Bulletin, researchers reveal that the sea slug has incorporated genes from the algae that it eats.
"There is no way on earth that genes from an alga should work inside an animal cell," says Sidney Pierce from the University of South Florida. "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."
Chloroplast are plant organelles that contain chlorophyll, the green photosynthetic pigment. Researchers have known since the 1970s that this sea slug steals chloroplasts from the alga Vaucheria litorea. The sea slugs embed the chloroplasts into their own digestive cells, where the organelles continue to photosynthesize for up to nine months—that’s even longer than they would perform in algae. The sea slugs stay nourished thanks to the carbohydrates and lipids produced with photosynthesis.
But until now, no one knew for sure how the slugs manage to maintain these pilfered chloroplasts. Using DNA amplification, sequencing, and advanced imaging techniques, Pierce and colleagues revealed that the sea slug’s chromosomes contain genes from the algae that code for both chloroplast proteins and chlorophyll synthesis.
"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 explains in a news release. "The gene is incorporated into the slug chromosome and transmitted to the next generation of slugs." And even though future slugs will need to snatch up new chloroplasts from algae, the genes to maintain them are already in the slug genome.
These sea slugs are one of the only known examples of functional gene transfer from one multicellular species to another. "When a successful transfer of genes between species occurs, evolution can basically happen from one generation to the next," Pierce adds, rather than over thousands of years.