The Further Adventures of the Emerald Green Sea Slug

By Carl Zimmer | November 18, 2008 6:07 pm

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…)


Comments (23)

  1. Matt

    I mean, seriously, I need to know if this is half as mind-blowing as it sounds. How much rethinking is going on in biology departments right now?

  2. Ole

    It’s quite mind-blowing, but it’s not really that revolutionary. The usual processes behind integration of genetic material in a eukaryotic genome just incorporated something unusual, genetic material from an algae. Just think of all the viruses that are in the human genome. There have been large incorporations of genetic material before, the process behind the endosymbioses of mitochondria and chloroplast are good examples of this. Several algae linages have undergone secondary and even tertiary (some might argue that there have been quaternary also) endosymbioses where a heterotrophic organism has eaten an red or green algae and kept transferred a lot of genes from the algae to its own genome and kept the chloroplast and become an autotrophic organism.

    The details of the process that leads to incorporation of large amounts of foreign DNA is not completely known, and I’d be interested in seeing what had gone on here. The promotors had to be correct, it had to be inserted into euchromatic areas of the genomes. Maybe the most interesting is how many and what genes have been transferred.

  3. Very interesting….

    Of course, it doesn’t answer the question:
    “where did the gene for photosynthesis in the algae originally come from?”

  4. As a silly aside, no kid, regardless of how he or she hates green things, would ever have the patience to pick out all the plastids. And thus serving these slugs as entrees would be hugely popular with parents who can’t get their kids to eat salads.

  5. amphiox

    Colin Purrington: the next challenge would be to get the kid to eat the slug.

  6. Kermit the Frog, meet Ollie the Slug. It’s still not easy being green.

  7. “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…”

    The popular Hong Kong Sci-Fi writer, Ni Kuang, published in the 80’s a novel titled “A second kind of human” about a group of “people” evolved from plants. The novel concludes with an intriguing moral point: being a vegetarian won’t ease your guilt of destroying life. The only moral lifestyle is to synthesize your own energy. Elysia chlorotica is almost a moral animal.

  8. John A. Davison

    Isn’t it possible that the necessary genes for the slug to become photosynthetic were always there masked in the slug’s genome? For a further discussion of this alternative view of phylogeny I refer you to my weblog –

    and in particular to my “Prescribed Evolutionary Hypothesis” thread.

  9. Owlmirror

    I note that the paper was edited by Lynn Margulis.

    Dr. Margulis wrote a book with her son Dorion Sagan about exactly this sort of genetic mixing; it is called “Acquiring Genomes“, and discusses the potential and known examples of genetic mixing.

    Another fascinating example of this was the discovery that a few species of fruit flies had incorporated a huge portion of the genome of a bacterium, Wolbachia, inside their own genomes.

      Serendipitous discovery of Wolbachia genomes in multiple
    Drosophila species

    Acquiring Genomes also discusses other odd and fringe-y biological hypothesis, including larval transfer, which is the idea that the reason insect larvae (and other larvae) look so different from the adult form is because they represent an ancient genetic transfer and combination from a different class of organisms.

    Some of it is very far out near the fringe indeed, but strange as the ideas are, they are at least testable scientific hypotheses, which can be falsified or supported by comparative genomics. And as the green sea slug shows, some organisms are themselves far out on the fringe.

    PS: This is an interesting and potentially relevant paper from 1969:

      Uptake of Isolated Chloroplasts by Mammalian Cells

  10. John A. Davison

    Are there any experimentally verified examples of the Margulis hypothesis for the symbiotic origin of cellular organelles? I know of none myself. Surely if her hypothesis is correct it should be easy to verify. Until that has been demonstrated I will stick with the Prescribed Evolutionary Hypothesis myself.

    “An hypothesis does not cease being an hypothesis when a lot of people believe it.”
    Boris Ephrussi

  11. John A. Davison

    Well I guess this prolonged silence suggests that there is not a shred of experimental evidence supporting Margulis’ symbiosis hypothesis. That suits me just fine.

    “A past evolution is undeniable, a present evolution undemonstrable.”

  12. John,

    Margulis’ theory regarding the symbiotic development of organelles during the early evolution of life is now very well established, even though it was very controversial only a decade ago. I spoken to people that were at scientific conferences where she was pretty much laughed of the stage, but now solid genetic evidence seems to be backing her up.

    That said, she also maintains that symbiotic fusion wasn’t isolated to just a few rare events in early evolution, but continues to play an important role. This is still a very controvesial idea, but is slowly gaining ground as more evidence (like that above regarding the sea slug) continues to appear. In fact, the Human Genome project seems to confirm that we ourselves seem to have aquired genes from viruses and bacteria within relatively recent times.

    I think Wikipedia does a fairly good job of summing up all this, including some of the points of controversy.

  13. John A. Davison

    Sorry Lonnie, but it will be established when it is produced experimentally and not before. My Prescribed Evolutionary Hypothesis and the Universal Genome Hypothesis offer alternative explanations which cannot be discounted.

    I do not regard Wikipedia as a valid source. It is very biased in favor of the Darwinian atheist model. Natural selection and random mutation have never been instrumental in either speciation or the generation of any of the higher taxa. Quite the contrary, both are anti-evolutionary, serving to prevent rather than promote evolutionary progress. So is Mendelian (sexual) reproduction. Besides, creative evolution is a phenomenon of the distant past.

    “A past evolution is undeniable, a present evolution undemonstrable.”
    John A. Davison

    I don’t expect agreement but I do feel compelled to present my thesis here or anywhere else I am still allowed to speak. EVERYONE is welcome at my weblog.

  14. John,

    Propenents of ID (in any form) tend to dislike Wikipedia because it is based on a process of peer review and citation of respected sources. As ID tend to not be published in reputable journals like Science or Nature, their ideas tend to not hold up within that kind of forum. That’s not bias but rather a good editorial process.

    In this case, Margulis’ Endosymbiotic theory has been published and peer reviewed in major scientific journals.

    As for testing, like many Creationists (of which ID is just the new flavor), you misunderstand what testing means from a sceintific perspective. It need not be something replicable in a laboratory (although there is an article cited above that suggest it may be able to be replicated in a laboratory). It can simply be a suggestion of what other discoveries we’d logically expect to find if it were true, and what we’d expect to find if it were false.

    For example, if we were to apply your theory to the slug, then we’d expect to find unexpressed/unused genes for photosysthesis in other animals. That is a testable falsifiable hypothesis. To my knowledge we have not yet found such genes. It is also problematic when you consider that genetic evidence can “date” genes. Given that, we’d also expect evidence that these genes were acquired long ago, and not recently. Once again, a testable hypothesis.

  15. John A. Davison

    Lonnie, whever that is.

    Lynn Margulis’ “Endosymbiotic theory” is not a theory . It is an unverified hypothesis. I know exactly what testing means as I am a physiologist and certainly not a traditional Creationist by any means. The simple truth is that lateral gene transfer has yet to be experimentaly demonstrated as the device by which symbiotic relationships have been established.

    I do not subscribe to Popper’s falsifiable nonsense either. Hypotheses are either verified or they are not. The Margulis hypopthesis has not been verified. Untl it is, I will stick to my Prescribed Evolutionary Hypothesis. Thank you very much.

  16. yutt

    “Untl it is, I will stick to my Prescribed Evolutionary Hypothesis. Thank you very much.”

    In that case, feel free to write an article about your hypothesis for the leading Creationist scientific journals.

  17. Sam Barns

    I must agree with Professor Davison. Wikipedia has a very questionable business and has been for sometime. It appears to take any joe’s input and state it as fact. I am sorry but if that is who your using as a source then I would disagree with you as would a lot of others.

  18. Sam Barns

    Also Lonnie,
    I would also argue that for a very long time Wikipedia has mislead many reader without utilizing their ability to simply look up the facts. “Wikipedia has come under fire from all sides amid claims that much of its content is unreliable and prone to internet vandals who deliberately print false information on the website”.

    Also ” Under the name Essjay, the contributor edited thousands of Wikipedia articles and was once one of the few people with the authority to deal with vandalism and to arbitrate disputes between authors.

    To the Wikipedia world, Essjay was a tenured professor of religion at a private university with expertise in canon law, according to his user profile. But in fact, Essjay is a 24-year-old named Ryan Jordan, who attended a number of colleges in Kentucky and lives outside Louisville.

    Let me continue ” is an important web resource that serves as an encyclopedia for far too many people. Wikipedia is open to editing by anyone, and is the largest encyclopedia in existence”.

    Therefore I would suggest you follow Professor Davison advice and actually submit REAL evidence.

  19. Sam Barns

    It was brought to my attention their was some controversy over Essjay but that is but one complaint among numerous others. But still the problem was well known as you can see from other related articles. Wikipedia’s certainly tried to defend the free encyclopedia that anyone can edit. But, it is that “anyone can edit” aspect that frightens me. It should frighten you, too.

  20. This slug look interesting. It is amazing that it eat eats algae and uses them to become photosynthetic. One more prof that we have many unique animals(creatures) on our planed that need to be discovered.

  21. barbara

    It is nature that gently reminds us when we think we have it all figured out, the sea slug informs that we do not know even half of it.


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The Loom

A blog about life, past and future. Written by DISCOVER contributing editor and columnist Carl Zimmer.

About Carl Zimmer

Carl Zimmer writes about science regularly for The New York Times and magazines such as DISCOVER, which also hosts his blog, The LoomHe is the author of 12 books, the most recent of which is Science Ink: Tattoos of the Science Obsessed.


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