Friday, September 28, 2007

The evolutionary biology of Giardia - no mitochondria

Giardiasis is an annoying and fairly common infection, but we don't think it's not a big deal for a healthy adult. Turns out that the Giardiasis bug, Giardia, has some very interesting biology ....
The Loom : Carrying Ancient History In The Gut

... Giardia, many researchers suspected, was one of those early-branching eukaryotes. This suspicion was generated at first by simply eyeballing the creatures. They are quite weird. Their teardrop-shaped bodies have eight tails for swimming and a suction pad to clamp onto the wall of the intestines. They also carry two nuclei, each with its own DNA. How Giardia manages to keep all those genes coordinated--and why it even has two nuclei--remain mysteries. Bizarre single-celled eukaryotes are pretty easy to find. What set Giardia apart from most other eukaryotes was what it lacked. Scientists could not find a lot of those compartments in which the business of most eukaryote cells takes place.

Most significantly, it was missing mitochondria. Lots of things take place inside these sausage-shaped structures, most importantly the generation of ATP, the energy-bearing molecule found in all living things. Mitochondria started out as free-living bacteria and later evolved into permanent symbionts inside the eukaryote cell. (Mitochondria still carry some DNA of their own, which bears a strong resemblance to one group of free-living bacteria.) The fact that Giardia seemed to be missing mitochondria hinted that it was a transitional eukaryote...
Alas, the story is more complex than this, so you do need to and read Carl Zimmer's essay. Fascinating stuff.

I probably invited this bug onboard during a particularly rough college train trip through Mexico. I didn't realize I was carrying a possible echo of biology's "big bang".

Incidentally, speaking of parallels to cosmology, there's apparently a suspicion among some biologists that there's a kind of "dark matter" component to biology. They're looking for terrestrial life forms so bizarre that we don't even recognize them as "living" ...

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