Plantimals!

E. chlorotica. Image kindly provide by Dr. S.K. Pierce.

We all know what animals are. We all know what makes plants and animals different. These are things we learned in elementary school, right? Wrong. It turns out that these categories, like many things one learns in elementary school, are not so cut-and-dry in reality.

The neatest SICB talk we saw was given by Dr. Sydney Pierce of the University of South Florida. He and his colleagues are researching the transfer of genes from algae¹ to sea slugs² that eat the algae. They found that many algal genes appear in the genome of a sea slug called Elysia chlorotica^3-5. It's important to note that E. chlorotica doesn't gain algal genes every time it feeds; the original acquisition took place some time in evolutionary history. These algal genes are passed down from one sea slug generation to the next; they are even found in sea slug larvae that haven't yet fed on the algae. Yes, that is as strange as it sounds—as though one of your distant ancestors ate broccoli, incorporated some of the broccoli's DNA into his/her reproductive cells, and eventually passed it down to you, thereby making you part broccoli!

There is another cool part to this story: E. chlorotica slurps the contents out of algal cells and then incorporate the chloroplasts from the algal cells into some of their own cells. (Chloroplasts are the parts of plant cells that contain chlorophyll and carry out photosynthesis.) The slugs can then use the chloroplasts for photosynthesis for awhile. So, that strange scenario above just got stranger: not only would you have broccoli DNA passed down to you, but when you eat broccoli, you could capture some of the broccoli's machinery for photosynthesis. That means you could make your own food (like a plant), rather than having to eat other organisms—like an animal! [Note that sea slugs are animals just as unquestionably as we humans are. They even have many of the same parts: brains, muscles, hearts, etc.]

Among closely related sea slug species, there is great variation in how long they can keep the chloroplasts. Most can only maintain the chloroplasts for a short time; however, E. chlorotica appears to be able to photosynthesize for months, so that in the lab they need only one meal in a lifetime. In yet another twist, Pierce et al. found that the slugs had obtained the ability to make chlorophyll^4,5—not just to use the chlorophyll taken from the algae! This shows something astounding: the sea slugs did not acquire a couple of random algal genes in evolutionary history; they actually acquired an entire complex pathway, including several genes for different enzymes.

To demonstrate that it was indeed the slugs who were actively making the chlorophyll, Pierce et al. gave the slugs radioactively-labeled building blocks for chlorophyll synthesis. Sure enough—they found that some of the radioactive material showed up as chlorophyll in the slugs' bodies.

This is a fascinating example of the transfer of genes between very different species—in this case, allowing an animal to take on some of the characteristics and abilities of plants.

Humans love to put things in nice neat bins and categories. One of the most fun parts about biology, though, is finding out how messy those bins really are. We use the metaphor of a branching tree to represent evolution, but this sea slug research shows us that sometimes the branches can come back together in unexpected ways.

1) Technically, algae are not in the same group as plants according to modern taxonomy; however, like plants, they are eukaryotes (their cells contain nuclei) that obtain energy through photosynthesis. Therefore, they are often informally called plants.

2) "Sea slug" is the informal name for members of the group of exceptionally beautiful molluscs called opisthobranchs. Land slugs (which are also cute) are a completely separate group of molluscs.

3) Pierce, S.K., Curtis, N.E., Hanten, J.J., Boerner, S.L., and Schwartz, J.A. (2007) Transfer, integration and expression of functional nuclear genes between multicellular species. Symbiosis. 43, 57–64

4) Pierce, S.K., Curtis, N.E., and Schwartz, J.A. (2009) Chlorophyll a synthesis by an animal using transferred algal nuclear genes. Symbiosis. 49, 121-131

5) Pierce, S.K., Curtis, N.E., and Schwartz, J.A. Chlorophyll synthesis by a sea slug (Elysia chlorotica). Presented at the Society for Integrative and Comparative Biology meeting, Seattle, Jan. 3-7th, 2010

We're back!

We recently returned from Seattle, where the annual meeting of the Society for Integrative and Comparative Biology (SICB) took place. We'd like to summarize a few of our favorite things from the meeting in the next few posts.