For decades, scientists have puzzled over how a certain sea slug acquires the ability to photosynthesize after ingesting algae. An advanced imaging technique now confirms that the slugs are literally stealing genes from the algae. It's considered the first example of "horizontal gene transfer" in a multicellular organism.
Researcher Sidney Pierce and colleagues have shown that the horizontal gene transfer of functional nuclear genes — which code for both chloroplast proteins and chlorophyll synthesis — from the ingested remains of the alga V. litorea is what's enabling the sea slug, E. chlorotica, to engage in photosynthesis. The process is aptly named "kleptoplasty."
It's considered the first direct evidence that the emerald green sea slug's chromosomes acquired some "functional" genes from the algae. What's more, it's an extremely rare — if not the only known — example of functional gene transfer from one multicellular species to another.
The blog of the Marine Biological Laboratory explains:
[The transferred 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 E. chloritica "steals" chloroplasts from V. litorea...and embeds them into its own digestive cells. Once inside the slug cells, the chloroplasts continue to photosynthesize for up to nine months—much longer than they would perform in the alga. 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," 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.
The gene that was transferred, called prK, plays an important role in photosynthesis by both maintaining the chloroplast and in allowing for carbon fixation. Without it, the sea slugs would not be able to perform photosynthesis. The researchers used an imaging technique called fluorescence in situ hybridization (FISH) to confirm the process.
Interestingly, the researchers speculate that other genes may contribute to the process as well. They chose prK from a variety of possible genes because, in their words:
several previous studies have found evidence for its horizontal transfer between V. litorea and E. chlorotica using both polymerase chain reaction (PCR) experiments in adult and larval slug DNA (Rumpho et al., 2009; Schwartz et al., 2010) and adult slug transcriptome sequence data analysis (Pierce et al., 2012). Also, prk encodes a protein used exclusively in the Calvin cycle and has no known homolog in non-photosynthetic organisms.
The researchers say the exact mechanism of the gene transfer is unknown. And in fact, there's still much to learn about this process, including further study of the "transferome" and tests to determine the chromosomal positions of other algal genes.
Check out the entire scientific study here.
Top image: Patrick Krug.