Saturday, July 26, 2008

Choanoflagellates II

No sooner than we do an article on a topic that may seem esoteric to some – namely tyrosine kinase signaling in choanoflagellates – than new information comes along to add to the picture.

The previous discussion was about how remarkable it is that a complete set of sophisticated intercellular signaling proteins exists in a single-celled organism. The new research partially echoes the previous findings:

Primitive Single-Celled Microbe Expert In Cellular Communication Networks (7/7/08)
When it comes to cellular communication networks, a primitive single-celled microbe that answers to the name of Monosiga brevicollis has a leg up on animals composed of billions of cells. It commands a signaling network more elaborate and diverse than found in any multicellular organism higher up on the evolutionary tree, researchers at the Salk Institute for Biological Studies have discovered.

Their study, which will be published during the week of July 7-11 in the online edition of the Proceedings of the National Academy of Science, unearthed the remarkable count of 128 tyrosine kinase genes, 38 more than found in humans.

But it also points out that M. brevicollis actually has a more extensive tyrosine kinase system than metazoa do:
"We were absolutely stunned," says Manning. "Based on past work, we had expected maybe a handful of these kinases but instead discovered that this primitive organism has a record number of them. Two other essential parts of the tyrosine kinase network - PTP and SH2 genes - are also more numerous than in any other genome, showing that it is the whole network that is elaborated here." ...

The Monosiga kinases are more divergent than anything previously seen in animals, which may help scientists understand the fundamentals of how all tyrosine kinase signaling works. Despite their extreme diversity, Monosiga kinases time and again arrive at the same solution to a problem, as do animal kinases, but using a distinct method for instance to create a sensor structure that emerges from the cell, or to target a kinase to a specific part of the cell. "This convergent evolution suggests that there are only a limited number of ways build a functional network from these components," says Manning.

And as was pointed out before, this discovery merely suggests new questions that need to be answered:
With all this new information, one obvious question remains unanswered: what is a single-celled organism doing with all this communications gear? "We don't have a clue!" says Manning, "but this discovery is the first step in finding out."

A possible answer, though one for which no evidence exists, as far as I know, is that M. brevicollis actually developed from the type of cell it strongly resembles ("collar cells") found in sponges. Perhaps the tyrosine kinase signaling system actually evolved in sponges, but then some of the constituent cells decided to go it alone. It would still need to be explained why the kinases didn't disappear in subsequent evolution. Since they are still present, they must serve some useful purpose.

Further reading:

The protist, Monosiga brevicollis, has a tyrosine kinase signaling network more elaborate and diverse than found in any known metazoan – research article from PNAS that reports the research discussed (open access)

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Anonymous Anonymous said...

The notion that choanoflagellates might have evolved from sponges comes up from time to time, but in fact has been soundly refuted through the analysis of multiple independent lines of evidence. It is worth checking out the phylogenomic (phylogenetics based on genome-scale data) work of Antonis Rokas as well as the comparative genomics of mitochondria conducted by Dennis Lavrov. In addition, analyses using concatenations of multiple genes (e.g. by Herve Philippe) demonstrate that all animals, including sponges, form a monophyletic clade that does not include choanoflagellates. In fact, there is no reliable data suggesting the derivation of choanoflagellates from sponges. (Indeed, there is some controversy about whether the morphology of choanoflagellates and sponge choanocytes is homologous, although I believe that this too is from a lack of consideration of the existing data.)

8/01/2008 11:16:00 AM  
Blogger Charles Daney said...

Looks like pretty good information. Thanks for the comment.

8/01/2008 11:45:00 PM  

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