Plantimals 1: A photosynthetic sea slug
In June 2008 I wrote the following:
On the way to work this morning I drove by an expanse of iceplant (Carpobrotus edulis) – considered by some to be a barbarian of the botanical world – and had the sudden longing for chlorophyll to reside within my skin.
Two years have passed and I remain fascinated by the idea of plant-animal hybrids. So it is with great delight that I am about to introduce two animals who use chloroplasts for energy production: a sea slug and a salamander. While the slug, as a mollusk, is a far cry from my vision of harnessing photosynthesis for myself, the discovery of solar salamanders is downright exciting. Salamanders, as vertebrates, are more closely related to humans and one step closer to the human branch on the taxonomic tree.
I’d intended to cover both animals in a single post, however given the length of the content, the post be split into a series. This first post will feature the sea slug, and the second will focus on the solarmander.
At first glance, the image to the left looks like a leaf, given its shape and color. However, this is a picture of a sea slug, Elysia chlorotica, packed so full of chloroplasts it is vibrantly green.
The slug E. chlorotica acquires chloroplasts from algae it eats and stores the stolen energy-makers in the lining of it’s gut. Afterward, a sea slug can live the remainder of it’s life without eating. Energy generated from chloroplasts alone is not sufficient to sustain our slug, so researchers were puzzled as to how it survived without eating.
Mary Rumpho of the University of Maine, has discovered how the sea slug gets this ability: it actually photosynthesizes with genes taken from the algae it eats.
Chloroplasts only contain enough DNA to encode about 10% of the proteins needed to keep themselves running. The other necessary genes are found in the algae’s nuclear DNA. “So the question has always been, how do they continue to function in an animal cell missing all of these proteins,” says Rumpho.
In their latest experiments, Rumpho and colleagues sequenced the chloroplast genes of Vaucheria litorea, the alga that is the sea slug’s favourite snack. They confirmed that if the sea slug used the algal chloroplasts alone, it would not have all the genes needed to photosynthesise.
They then turned their attention to the sea slug’s own DNA and found one of the vital algal genes was present (PNAS November 18, 2008 vol. 105). Its sequence was identical to the algal version, indicating that the slug had probably stolen the gene from its food.
While other animals may photosynthesize after eating plants, they do so by acquiring entire plant cells. Our sea slug is unique in that plant genes have been incorporated into the animal’s own DNA. Such a molecular trick is crafty and streamlined, as the slug cells take both chloroplasts and the genetic instructions required to operate them while leaving the remainder of the plant cell behind. As a result, E. chlorotica are singularly capable of both animal and plant functions: movement and photosynthesis.
Check out a short clip about these sea slugs here.
Check back for the next post in this series about photosynthetic animals. Next up is a solar-powered vertebrate!