A couple days I introduced an awesome sea slug that eats algae and uses them to become photosynthetic. I thought it would be worth revisiting this marvelously plant-like animal for a couple reasons. One is that I’d like an excuse to post this excellent photo, which is on the cover of the latest issue of the Proceedings of the National Academy of Sciences, where a new paper on the sea slug is being published (photo by Mary Tyler). Another reason is that I wanted to relay an email exchange I had with the lead scientist on the study, Mary Rumpho.
Rumpho discovered that the sea slug has incorporated a key gene for photosynthesis from the algae into its own DNA. That means that the slugs don’t just passively let the photosynthesizing structures from the algae (called plastids) harness sunlight. The slugs themselves actually make proteins that are essential for photosynthesis.
I wondered how in the world a gene from algae got into the slug’s own DNA. Rumpho responded (my notes in brackets):
Our thoughts are that the nuclei break open as they go through the guy, releasing their DNA. The DNA is then phagocytosed [eaten] along with the chloroplasts into cells lining the digestive system. The digestive system expands throughout the growing sea slug and interestingly it is found to branch right next to the reproductive organs. In addition, the invertebrate has an open “blood” system, so if the DNA was exchanged from the gut into the blood, it also would come in direct contact with the reproductive organs.
We are doing high-throughput sequencing now on the sea slug transcriptome [the genes expressed in specific cells] and hopefully soon, the genome, so we should learn a lot more about which genes are there. I will also mention that we frequently/always? find a virus in the sea slug that really takes off as the sea slugs age. We believe it might be a retro-virus, but the support is not strong at this time. It’s possible the DNA is moving via a viral vector through the blood to the germ line.
Yet another example of a discovery in evolution leading to a new hypothesis, leading in turn to a new experiment. And yet another wrinkle for Hollywood to consider (alien race of plant people who evolved photosynthesis when a deadly virus picked up DNA from their salads inserted it into their DNA…you get the idea…)
Originally published November 18, 2008. Copyright 2008 Carl Zimmer.