Friday, March 21, 2008

Giardia not so primitive after all

(Readers not interested in biology are advised to skip this post.)

In the mid-1990s, shortly after beginning work as biology assistant, I spent a lot of time learning as much as possible about the things we were teaching. So I read that protozoan Giardia lamblia, an intestinal parasite in humans, represented a group of primitive, early-branching unicellular eukaryotes that had never had mitochondria.

Though I was glad that an otherwise banal parasite turned out to be so interesting, its primitivity seemed to me somewhat weird. I mean, you expect to find a primitive organism hiding from competition in some out-of-the-way or extreme habitat, not inside a highly advanced organism. What on Earth had Giardia been doing before its mammalian hosts evolved, and where had it been living?

Back in those days, I had also the good habit to visit the university library at least twice a month to browse the new issues of Science journal. So at some point I learned that genes of apparent mitochondrial origin were found in the nuclei of Giardia and other amitochondrial protozoans, which clearly showed that their lack of mitochondria was secondary. (Giardia lives in anaerobic environment and, hence, doesn't need mitochondria - their function is cellular respiration.) However, these protists were still considered primitive and early-branching.

Last week, while working on my textbook chapter introducing eukaryotes, I found it necessary to refresh my knowledge on their evolution. To my delight, Giardia and the other similar organisms are no longer considered very primitive. It is now recognized that in some lineages the "molecular clock" of evolution produces artifacts, making the branch seem longer than it actually is. (In fact, taking into account the speeding effect of temperature on biochemical processes, we must expect such artifactual branch elongation in mammals and birds, and also in their endoparasites such as Giardia.)

Another curious theory shaping now is that major eukaryotic traits appeared almost simultaneously, and the absence of any of these traits in some extant eukaryotic species must be considered secondary unless proven otherwise. When mitochondrial genes were found in nuclei of amitochondrial protists, scientists began to suspect that mitochondria were acquired at the same time as nuclei (if not even earlier). So the quest to find a primarily amitochondrial eukaryote was all but cancelled. The same considerations seem to be valid for sexual reproduction - at any rate, I wouldn't devote my career to looking for a primarily asexual eukaryote.
Current concepts about eukaryote evolution are briefly described in this TreeOfLife document. Mention that in recent times they are changing literally every year, so don't try too hard to learn the latest theory - it is unlikely to survive for long.

The bottom line: If you find something fishy in a theory, keep an eye on subsequent developments in that field - science may give merit to your doubts.

Update: I am discussing Giardia and some other protozoans of medical importance here.

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