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August 30, 2006

Biological complexity: not what you think

I've long been skeptical of the idea that life forms can be linearly ordered in terms of complexity, with mammals (esp. humans) at the top and single-celled organisms at the bottom. Genomic research in the past decade has shown humans to be significantly less complex than we'd initially imagined, having only about 30,000 genes. Now, along comes the recently sequenced genome of the heat-loving bug T. thermophila, which inhabits places too hot for most other forms of life, showing that a mere single-celled organism has roughly 27,000 genes! What are all these genes for? For rapidly adapting to different environments - if a new carbon source appears in its environment, T. thermophila can rapidly shift its metabolic network to consume and process it. That is, T. thermophila seems to have optimized its adaptability via accumulating, and carrying around, a lot of extra genes. This suggests that it tends to inhabit highly variable environments [waves hands], where having those extra genes is ultimately quite useful for its survival. Another fascinating trick it's learned is that it's reproductive behavior shows evidence of a kind of genetic immune system, in which foreign (viral) DNA is excised before sexual reproduction.

From the abstract:

... the gene set is robust, with more than 27,000 predicted protein-coding genes, 15,000 of which have strong matches to genes in other organisms. The functional diversity encoded by these genes is substantial and reflects the complexity of processes required for a free-living, predatory, single-celled organism. This is highlighted by the abundance of lineage-specific duplications of genes with predicted roles in sensing and responding to environmental conditions (e.g., kinases), using diverse resources (e.g., proteases and transporters), and generating structural complexity (e.g., kinesins and dyneins). In contrast to the other lineages of alveolates (apicomplexans and dinoflagellates), no compelling evidence could be found for plastid-derived genes in the genome. [...] The combination of the genome sequence, the functional diversity encoded therein, and the presence of some pathways missing from other model organisms makes T. thermophila an ideal model for functional genomic studies to address biological, biomedical, and biotechnological questions of fundamental importance.

J. A. Eisen et al. Macronuclear Genome Sequence of the Ciliate Tetrahymena thermophila, a Model Eukaryote. PLOS Biology, 4(9), e286 (2006). [pdf]

Update Sept. 7: Jonathan Eisen, lead author on the paper, stops by to comment about the misleading name of T. thermophilia. He has his own blog over at Tree of Life, where he talks a little more about the significance of the work. Welcome Jonathan, and congrats on the great work!

posted August 30, 2006 02:20 PM in Evolution | permalink


You mentioned that Tetrahymena thermophila is a "heat-loving bug", which "inhabits places too hot for most other forms of life". Well, I just want to clarify something here. As implicated in its scientific name, T. thermophila is heat-loving indeed. But the question is -- how hot it loves?
T. thermophila is a fresh-water, single-celled ciliated protozoan. Most freshwater ciliates live in ponds, lakes, rivers and etc, so the optimal temperature for these ciliates is the "regular" environmental temperature, say, 15 to 25 C. Wild T. thermophila lives in ponds and lakes, too. However, somehow it grows better in A LITTLE higher temperature when researchers grow it in lab. It divides rapidly at 30 C (which is the standard culturing condition we do in the lab), and it can live in an environment as warm as 38 C. But T. thermophila will die if you heat it up higher than 42 C.
Therefore, T. thermophila loves warmness, and I believe that many organisms don't like to live at 30 C. But I also believe that there are still quite a few other life forms that can live in 38 C. So maybe saying that T. thermophila can inhabit places “too hot for most other forms of life" is not incorrect, but I have to point out that it's a fairly strong statement.

Posted by: tetra at September 1, 2006 06:12 AM

Actually, my statement was even worse than you think, because I actually know very little about T. thermophila - I think I must have confused it with another thermo- critter I was reading about recently - the kind that lives near the heat vents at the bottom of the ocean. So, thank you very much for the correction :)

Posted by: Aaron at September 4, 2006 01:20 AM

I have even been trying to get the species renamed to something like Tetrahymena tepidophila, but nobody seems to listen to me about this. I spend most of my time working on organisms that grow at > 85C, so the Tetrahymena name is particularly annoying to me.

Posted by: Jonathan Eisen at September 6, 2006 11:29 PM