Photograph by John C. McMichael.
Photography contest, Honorable Mention.
Moraxella bovis -- evolution in a Petri dish.
Mr. McMichael writes
Almost two decades ago, I was studying the bacterium Moraxella bovis [which causes keratoconjunctivitis in cattle] when I discovered that if I stabbed through the agar to the bottom of the Petri dish, the bacterium would form very large flat colonies at the interface of the agar with the Petri dish. This is a Coomasie Brilliant blue stained colony prepared by squeezing out most of the moisture of the agar and using the same procedure used to stain acrylamide electrophoresis gels. When I examined a large number of these colonies, I discovered that the wedge sectors of new variants in the colony appeared more frequently during the colony's early growth when the colony's expansion rate is at its slowest. You can see several of these sectors in the photograph. I speculate that the frequency of new variant expression depends on how fast the parent culture is expanding. The slower the parent expands, the greater the frequency of new variant expression, and the faster the parent grows, the lower the frequency of new variant expression. I call this phenomenon "parental suppression." I am not an expert in evolution, but I have often wondered if this applies to the expression of new mutations of larger species such as in the Cambrian explosion. For more information, see the Journal of General Bacteriology, 138: 2687-2695 (1992).
While I'm sure the new variants remain the same species, has anybody tried segregating the most variant variants (?) and repeating the experiment a few times (grin), to see how long it takes to get a new species?
Venus Mousetrap · 30 November 2009
Paul Burnett said:
While I'm sure the new variants remain the same species, has anybody tried segregating the most variant variants (?) and repeating the experiment a few times (grin), to see how long it takes to get a new species?
That's sounding dangerously like science, there.
Mike · 30 November 2009
Mr. McMichael wrote:
...I have often wondered if this applies to the expression of new mutations of larger species such as in the Cambrian explosion.
No. If it happens every time its a pretty clear indication that it isn't due to a mutation. Also, the Cambrian "explosion" is misleadingly named, as far as the general public is concerned. Its a period of time over tens of millions of years. Hard to say how a bacterial culture growing a couple of days relates to that and multicellular eukaryote evolution.
Who is this Mr. McMichael character?
eric · 30 November 2009
Mike said:
Who is this Mr. McMichael character?
The photographer. See the text above the picture.
If young strains variated/speciated more rapidly when the parent strain grows slowly, IMO that would support the punc-e hypothesis. Because what the photographer is speculating about is essentially whether there is more rapid variation in a relatively "empty" ecosystem, absent parental strain competitors.
nohoval turrets · 1 December 2009
Mr McMichael said:
You can see several of these sectors in the photograph
I can see one obvious wedge at top right. Are there others? Am I looking at the right thing?
John · 1 December 2009
Eric is right about my view of how M. bovis variants arise in a punctuated manner and he is more elegant in stating it. My speculation is that this might serve as a model to think about evolutionary events, particularly what happens when a species colonizes areas that are relatively bereft of life—think of the finches’ arrival on the Galapagos or the mass extinctions marking the ends and beginnings of geological eras.
Nohoval. There is another easily observed wedge on the very left. There are others, but they were observed before staining. Type McMichael and bovis into Google and it will take you to the paper where you can see a B&W picture of this same colony.
I also think we need to reconsiders Lenski's long-term E. coli experiment. The phenomenon of parental control of mutant expression in Lenski’s experiment may have been over looked. In this experiment they searched for cit+ mutants by serially passaging the bacteria. Unfortunately, they emphasized the number of bacterial generations before the mutant was expressed but neglected to state how many passages were needed. When you passage bacteria in liquid media the bacteria go through three growth phases—lag, exponential, and plateau. In two of these, the parent bacteria are growing at less than optimal rate. In my thinking, the number of passages may be more critical to the emergence of the cit+ mutant than the number of generations of bacteria.
stevaroni · 1 December 2009
The slower the parent expands, the greater the frequency of new variant expression, and the faster the parent grows, the lower the frequency of new variant expression.
This makes sense, if the colony is expanding quickly, resources are probably abundant, and competition less intense. In an environment where resources grow scarce, competition increases, and suddenly minor variations between organisms become significant.
Michael Buratovich · 2 December 2009
I know I might be strange, but to me that picture is pure beauty.
8 Comments
Paul Burnett · 30 November 2009
While I'm sure the new variants remain the same species, has anybody tried segregating the most variant variants (?) and repeating the experiment a few times (grin), to see how long it takes to get a new species?
Venus Mousetrap · 30 November 2009
Mike · 30 November 2009
eric · 30 November 2009
nohoval turrets · 1 December 2009
John · 1 December 2009
Eric is right about my view of how M. bovis variants arise in a punctuated manner and he is more elegant in stating it. My speculation is that this might serve as a model to think about evolutionary events, particularly what happens when a species colonizes areas that are relatively bereft of life—think of the finches’ arrival on the Galapagos or the mass extinctions marking the ends and beginnings of geological eras.
Nohoval. There is another easily observed wedge on the very left. There are others, but they were observed before staining. Type McMichael and bovis into Google and it will take you to the paper where you can see a B&W picture of this same colony.
I also think we need to reconsiders Lenski's long-term E. coli experiment. The phenomenon of parental control of mutant expression in Lenski’s experiment may have been over looked. In this experiment they searched for cit+ mutants by serially passaging the bacteria. Unfortunately, they emphasized the number of bacterial generations before the mutant was expressed but neglected to state how many passages were needed. When you passage bacteria in liquid media the bacteria go through three growth phases—lag, exponential, and plateau. In two of these, the parent bacteria are growing at less than optimal rate. In my thinking, the number of passages may be more critical to the emergence of the cit+ mutant than the number of generations of bacteria.
stevaroni · 1 December 2009
Michael Buratovich · 2 December 2009
I know I might be strange, but to me that picture is pure beauty.