Here's the latest in cutting edge research from Harvard University's Origins Of Life Initiative.
This entry was posted on Tuesday, October 16th, 2012 at 7:56 pm and is filed under Origin of Life.
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I'm not quite sure what you mean by "the putative ancestor could have been very similar doing exports for attachment," since the modern flagellum does in fact export proteins, albeit generally not for attachment.
The authors of the paper argue that the NF-T3SSs evolved from the flagellum in the following steps:
1. Deletion of certain flagellum-specific genes would result in a system that exports proteins but does not provide motility.
2. The ancestral NF-T3SS then gained and lost several genes, and diversified. Independent gain of secretins several times "allowed the formation of ancestral contact-dependent protein secretion systems, and the concomitant ability to subvert eukaryotic cells by direct delivery of effectors in their cytosol."
3. The NF-T3SSs adapted to specific host cells.
The model begins with a fully-fledged flagellum that functions as a motility device.
To be fair, Nick Matzke's model for flagellum evolution is compatible with the hypothesis that the TTSS evolved from the flagellum, since his model invokes a currently-non-existing-export system. However, the TTSS has often been used as evidence that the flagellum was once an export system, based on the grounds that the TTSS and flagellum share a common ancestor. But if the TTSS evolved from the flagellum, as this paper suggests, then the TTSS cannot be evidence that the flagellum is basically a modified secretion system.
I don't intend to speak for Nick Matzke, but from what I've seen he's pretty much always been arguing for the hypothesis that the flagellum and NF-T3SSs share a common ancestor, and that therefore this is evidence that that flagellum evolved from a secretion system. That hypothesis has been significantly weakened by this piece of research.
I can see why, what difference does it make? If the flagellum and t3ss had a common ancestor, the debate remains. Was that ancestor simpler than the flagellum? Of the same complexity? Or more complex?
If the t3ss evolved from the flagellum, the debate remains. Was there kind of a bell curve? A simpler ancestor for the flagellum, the t3ss derived from a more complex ancestor? Or was the ancestor much more complex than both the flagellum and t3ss? Since some bacteria , even some considered more ancient, don't even have a flagellum, I tend to go with the notion that the entire system evolved. But I haven't been following this for quite some time.
Here's why the phylogenetic relationship of the flagellum and NF-T3SSs matters:
Firstly, if the two systems form monophyletic groups (i.e., the NF-T3SSs do not nest within the flagellar sequences), then parsimony dictates that the common ancestor of these two clades was a secretion system, simply because you cannot assume that the common ancestor had any more protein parts than the number of parts in common between flagellar systems and TTSSs. Thus, such a phylogeny (e.g., the phylogenies reported by Gophna et al.) would be evidence that the flagellum arose from a secretion system.
Secondly, if the NF-T3SSs nest within the flagellar sequences (as this recent study shows), then this means that the NF-T3SSs arose from a full-fledged flagellar system – and this in turn means that the NF-T3SS is not evidence that the flagellum was ever a secretion system.
I'd also like to offer a quick comment on this: "Since some bacteria , even some considered more ancient, don't even have a flagellum, I tend to go with the notion that the entire system evolved."
Some bacteria phyla do indeed lack flagellar genes – perhaps most strikingly IMHO the Cyanobacteria. However, the majority of bacteria phyla do have flagellar genes, and you can't really say that some of the more ancient phyla lack flagellar genes since the whole phylogeny at the root of bacteria is pretty confused. I.e., since we don't really know what the most ancient bacteria are, you can't really say one way or the other if the most ancient bacteria lineages have flagella.
I wish these people the best of luck – my guess is that very little progress will be made over the next couple decades. Maybe new tidbits and hunches about what they think happened but certainly nothing along the lines of determining an undirected means of forming functional proteins or assembling cellular constituents.
Glad this blog is still up and running! Any idea what Mike Gene has been up to now?