subscribe to our mailing list:
|
SECTIONS
|
 |
|
|
Letters
[Write a Reply]
[Letters Index]
| Title |
Author |
Date |
| I was just wondering why we don't see flagellum motors in the process of evolving. |
Matzke, Nicholas |
May 23, 2004
|
Hi Rod,
A few brief points:
1. At least 99% of all the species that have ever lived are extinct. This presumably applies to bacteria also (with the usual caveats about the different definition of "species" in bacteria, lack of data, etc.). The critters we observe will be descendents of the successes in a particular niche -- e.g., in the case of motility, the strains that evolved high-functioning swimming would be much more likely to persist than the strains with poorly-functioning swimming.
2. The exception would be critters who have uses for "lower-functioning" motility structures or nonmotile structures because of the particular niches they live in. For example, some bacterial crawl rather than swim, and they use a pilus that extends and retracts. It just so happens that this "twitching motility" structure is homologous to both Type IV secretion and to archaeal flagella. Although the structure is not completely understood, it is at least representative of a transitional motility structure.
Twitching motility was mentioned in my paper, as were various secretion systems (at last count, nonmotile Type I secretion, Type II/IV secretion, Type III secretion, and (perhaps) slime secretion systems all had homologous motility systems. So it looks like many secretion systems are "transitional" motility systems in a sense, although once motility has evolved in a lineage there may not be much selective pressure to evolve it again.
Finally, various bacterial flagella such as spirochete flagella (which rotate inside the outer membrane rather than outside) and gram-positive flagella (which have no outer membrane and thus lack the outer components) may represent "transitional flagella". We don't really know since deep bacterial phylogeny is unresolved. In my paper I chose to assume that these were derived rather than transitional, mostly to be conservative, but they could be early offshoots during the evolutionary process. Certainly they show that large deviations from the canonical E. coli flagellum are possible.
3. The biggest point (also, I might add, covered in my paper) is that we are almost wholly ignorant of actual prokaryote diversity. Only a small fraction of prokaryotes are readily cultured on agar plates in the lab, and only a small fraction of the bacteria that have been cultured have had their genomes sequenced (mostly pathogens, model organisms, and industrial organisms -- not a random sample of nature by any means), and of those only some have really been studied in extensive detail. To say, in our present ignorance, that we have much idea of what is really out there in the microbial world, seems as foolish as someone believing in special creation based on the tame biota of pre-Age of Exploration Europe.
Long answer to a short question, I know, but it is easier than reading the paper I suppose...
======
Nic Matzke
|
| Related Articles: |
Evolution in (Brownian) space: a model for the origin of the bacterial flagellum
|
| Title |
Author |
Date |
| I was just wondering why we don't see flagellum motors in the process of evolving. |
Downard, James |
Jul 20, 2004
|
Two problems: (1) you are drawing on a false assumption (that everything is evolving all the time), which leads to more general problem (2) a failure to think historically (that changes may oocur in a set of circumstances that no longer pertain).
Incidentally, since every organism on earth is not catalogued and variants studied, it actually can't be stated categorically that even existing flagella aren't substantially changing. But more importantly, the putative precursor core to the flagellum (a TYPE III-style pump) apparently no longer exists in any extant organism. Later bacteria borrowed parts of the pump feature for the current Type III system, but that occurred long after the flagellum system originated.
Trying to retrocalculate what the circumstances of a system's origination were is not often easy, but this problem applies to more than just ID thinking. Some Young Earth creationists made great hay of the supposed fact that chert deposits weren't being formed today, inferring that large chert layers from ancient times had to have been formed in the Flood. The creationists had again failed to thinking historically: it turned out large chert fields could only be produced when the ocean chemistry was right, and chert layers stopped being produced when diatoms appeared and altered the way minerals were held in ocean solution.
Some clues to what may have been going on with the flagellum's evolution would certainly involve the role of the motor proteins that turn it (how they do this is still unclear, and I know of no studies that have laid out their possible interactions with other molecules, so this is still virgin research turf). Flagellin itself can trigger an interleuken response, so that filament may have been related to a pathogen predator/prey relationship. Not having the original ecological system to look at naturally complicates things in much the same way as trying to explain chert deposits if you didn't know about diatoms.
|
| Related Articles: |
Evolution in (Brownian) space: a model for the origin of the bacterial flagellum
|
|
|
