Greg Laden's Blog

... well, OK, maybe that is a slight exaggeration.

You know about giardia. Giardia intestinalis. It causes a nasty gut infection, and you get it by drinking water pretty much anywhere in the US (potentially). It is very hard to get rid of.

Giardia adapt to immune system attacks (of their host) in a way that passes that adaptation down to their offspring without genes. It is a Lamarkian process. Giardia have no mitochondria, yet many of the genes known to be in mitochondria in eukaryotes are found in the giardian nucleus. So, ancestral giardia probably had mitochondria, but all those genes got transferred over to the nucleus.

The absence of mitochondria and the significant reduction of some other organelles has led people to, probably falsely, believe that giardia is some kind of intermediate between prokaryotes and eukaryotes. Again, this is probably a misinterpretation. Giardia, as a eukaryote which has lost specific organelles (yet still does just fine) would be in a sense "more evolved" than any eukaryote. Including, dear reader, you.

And now, there is even a newer twist to the story.

In a forthcoming article in Current Biology, Cooper et al (2007) will report on evidence that giardia have been having sex all along and we did not even know it. Right there in our intestines!

The recent recognition of many of the genes known to be required for meiosis in the [giardian] genome has ... cast doubt on the idea that Giardia is primitively asexual, but so far there has been no direct evidence of sexual reproduction in Giardia, and population data have suggested clonal reproduction. We did [a bunch of fancy scientific stuff, and the results] provide genetic data supportive of sexual reproduction in Giardia.

Certain things happen when sex happens. For example, chromosomes may bet reassorted. If there was no sexual reproduction, genes should almost always stay put in relation to each other on a particular chromosome. Mitosis does not involve a mechanism for mixing up genes across different locations on the chromosome. But when you look at the giardia genome, there are different arrangements of genes across different chromosomes in different populations, strongly suggesting that some time in the past on each of these lineages, the giardia had sex. Likely, not only once, either.

There is evidence as well of recombination of genes within individual chromosomes.

So the evidence is farily strong that meiotic events occur in giardia. However ...

Although evidence from genomic and population genetics suggests that meiosis occurs in Giardia, sexual reproduction has never been directly observed. Sex in Giardia could be infrequent, furtive, or cryptic [19], and it could occur during trophozoite replication or during encystation or excystation.

The authors go on to suggest research strategies to catch them in the act.

Cooper, Margarethe A., Rodney D. Adam, Michael Worobey and Charles R. Sterling. (2007) Population Genetics Provides Evidence for Recombination in Giardia. In press. doi:10.1016/j.cub.2007.10.020.