Gene Editing in the Natural World

Humans are a bit lucky. We are constantly studying and discovering. Because of this, we are able to tweak biology. Although we may not be there yet, these techniques have been developed with what appears to be the long term goal of eliminating diseases. But what about other organisms? We know animals adapt, it's one of the keys to survival. But what do other organisms use if they can't use CRISPR-Cas9?

I'd like to talk about an adaptation, and then the same adaptation taken to an extreme: an organism taking genes from another organism. A natural case of genome editing. The adaptation I'm talking about is an animal or single celled organism taking in algae, or another photosynthesizing organism and using their chloroplasts. This gives the animal the ability to use sunlight as a resource for food.

The first example is of a single cell organism called Mesodinium chamaeleon. This organism ingests an algae in the genus cryptomonas. The algae is stored in the body of M. chamaeleon where it undergoes photosynthesis. This provides sugar for M. chamaeleon and protection for the algae. The algae is stored for weeks before it has to be replaced again.

More recently, we saw the first example of a vertebrate using chloroplasts to their advantage. Spotted salamander (A. maculatum) eggs contain algae that enter the blastopore during an early developmental stage. This symbiotic relationship decreases mortality in the salamander and helps it grow in the egg for longer, while the algae is allowed to safely grow and receive nutrients from the egg. Researchers even found there may be some passage of algae from the oviducts of salamanders onto their offspring, although this hasn't been proven.

These examples of endosymbiosis are fascinating, but not surprising, it happens somewhat often in nature. However, a recent discovery brought endosymbiosis to a new level. The organisms mentioned before either need to replenish the chloroplasts in the algae, or grow out of the need for them. What if an animal wanted to maintain the chloroplasts they bring in from the environment?

Meet the sea slug Elysia chlorotica. It has acquired genes from the algae it has been eating for years. These genes help maintain the algal chloroplasts taken up by and stored in the slug's digestive system. The chloroplasts can now survive for months because of the genes. The sea slug has acquired these genes through horizontal gene transfer. The scientists showed this using a technique called fluorescent in situ hybridization (FISH). They develop a probe that binds to a specific DNA sequence. Attached to this probe is a fluorescent tag, so they can see whether the probe is bound using a fluorescent microscope. They then add this probe directly to the chromosomes. The probe they made was specific for chlorophyll maintaining genes in the algae. They even did this with larval chromosomes that had never been exposed to algae, and the signal remained. There is corroborating evidence from genome sequencing that came out several years previous to this study, and, although it was met with some disagreement, the FISH data puts the story to rest. This means that pieces of the algae's genome has made its way it the sea slugs, and has been passed on to future generations.

E. chlorotica still isn't able to make its own chloroplasts, but having the machinery in place is certainly the first step in having that happen. This adaptation allows it to live longer when food is scarce and, thus, increases its fitness. We may be many years away from an animal that produces endogenous chlorophyll, but this is amazing in its own right. It is a perfect example of genome editing occuring naturally, and is brought to you by an organism that is blurring the lines between plant an animal.

References:

Schwartz, J.A., Curtis, N.E., Pierce, S.K. FISH Labeling Reveals a Horizontally Transfered Algal (Vaucheria litorea) Nuclear Gene on a Sea Slug (Elysia chlorotica) Chromosome. Biological Bulletin, 227, 300-312 (2014).

Kerney, R., Kim, E., Hangarter, R.P., Heiss, A.A., Bishop, C.D., Hall, B.K. Intracellular invasion of green algae in a salamander host. Proceedings of the National Academy of Science, 108, 6497-6502 (2011).

Venn, A.A., Loram, J.E., Douglas, A.E. Photosynthetic symbioses in animals. Journal of Experimental Botany, 59, 1069-1080 (2008).

Moestrup. Ø., Garcia-Cuetos, L., Hansen, P.J., Fenchel, T. Studies on the Genus Mesodinium I: Ultrastructure and Description of Mesodinium chamaeleon n. sp., a Benthic Marine Species with Green or Red Chloroplasts. The Journal of Eukaryotic Microbiology, 59, 20-39 (2012)

http://newsinfo.iu.edu/news-archive/17995.html - article on spotted salamander eggs

Darwin, C. On the Origin of species. New York: P.F Collier & Sons Company, 1909. - For the quote in the first paragraph

Marshall, M. " Zoologger: Unique life form is half plant, half animal." News Scientist. January 13, 2002 vol. 105 no. 46 > Mary E. Rumpho, 17867-17871, doi: 10.1073/pnas.0804968105

Rumpho, M.E., Worful, J.M., Lee, J., Kannan, K., Tyler, M.S., Bhattacharya, D., Moustafa, A., Manhart, J.R. Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica. Proceedings of the National Academy of Science, 105, 17867-17871 (2008).

Image credits:

The first and last image come from the Schartz et al. paper from Biological Bulletin. The picture of Elysia Chlorotica come from Animalspace.net