Friday, November 14, 2008
Our Closest Single-Celled Cousin
Usually,when we want to understand the biology of humans and other complex animals, we use complex animal models--like primates, or canines, or fruitflies. A recent article I read pointed out that there are certain phenomena that are most easily understood with a much simpler model.
Recent work on how multicellularity evolved has focused in on a single-celled organism called a choanoflagellate. (I have shown two images here--one a highly magnified photograph, the other an artist's rendering.) Choanoflagellates have, according to the article, "a distinctive form: a cell with an apical flagellum, a kind of propeller that can move it through the water or drive a flow of water over it, and a ring or collar of microvilli, thin projections that act like a net to capture bacteria for food."
One interesting thing about choanoflagellates is that they have evolved cell-adhesion and signalling proteins to enable them to interact with other single-celled organisms. It is precisely these mechanisms, claims the article, that allowed for the development of multicellular life, because the same proteins that choanoflagellates use to interact with other cells are the proteins that more "advanced" forms of life use to negotiate between their own cells. Although the article does not make this point, this would seem to be a classic case of exaption--a mutation favorable to an earlier form for one reason (inter-organism interaction) being coopted by a later form for an entirely different purpose (integrity of a multicellular organism).
Two classes of proteins in particular are shared by choanoflagellates and multicellular animals: cadherins and integrins. Cadherins regulate cell adhesion though interaction with environmental calcium. And integrins help cells stick to the extracellular matrix. Without them, our cells would not be able to cohere into integral bodies.
In some senses, we may learn as much about complex animal life by studying these "simple" single-celled organisms as we do by looking at more obvious animal models.
The link to the article is http://www.seedmagazine.com/news/2008/10/the_masons_apprentice_1.php.