More thoughts on the Cit+ E. coli evolution experiment
Prospector B: The Lost Dutchman Mine. But finding it is not a goal.
I have established a new post label for the Citrate-eating E. coli evolution experiment. Post labels are listed in the sidebar.
Whether evolution of Cit+ (citrate-eating) E. coli bacteria was an initial "goal" (I use the standard definition of "goal," and there are such things as secondary goals, longshot goals, incidental goals, etc.) when the long-term E. coli evolution experiment began in 1988 has not been established, but it occurred to me that Cit+ evolution must at least have become a "goal" sometime along the way because the experimenters must have been checking for the appearance of Cit+ bacteria because the Cit+ bacteria were detected when they were only 0.5% of the population at about 31,500 generations.
Also, someone suggested that the popular name for the first mutation (~20,000 generations), the "potentiating" mutation, is wrong, because the name implies that this first mutation was a pre-requisite for the second mutation and there is no evidence (at least so far as I know) that it is a pre-requisite. Below is part of a comment that I posted on another blog in response to another comment:
"If the cit+ mutation had occurred first, wouldn't we be calling the later potentiating mutation the cit+ mutation instead? "
I prefer saying that a mutation "became established" as opposed to saying that a mutation "occurred" -- some mutations, particularly mutations that occurred in one of the last generations of a daily population, might have been lost when only one percent of each daily population was used to start the next day's population.
The first mutation, the so-called "potentiating" mutation, was a "silent" (unexpressed) mutation that became established at around 20,000 generations (~ 9 years); the second mutation, which was expressed as the Cit+ trait, became established at around 31,500 generations (about 5 years later). The "potentiating" mutation is apparently very unusual because the Cit+ trait appeared in only one of the 12 lines of bacteria -- the second mutation is apparently relatively common because the Cit+ trait appeared repeatedly when the experiment was re-run starting with frozen samples from generations containing the "potentiating" mutation (generations numbered 20,000 or later). Because the second mutation is fairly common, the Cit+ trait probably would have appeared in the other 11 lines of bacteria if the first mutation were common. IMO there is no evidence that the first mutation was really a "potentiating" mutation in the sense of being a pre-requisite for the second mutation, so IMO you are right in questioning use of the term "potentiating" to describe the first mutation.
It also occurs to me that it was wrong to call the first mutation "silent" and the second mutation "expressed," because either mutation would be silent in the absence of the other. So maybe it would be best to use the names "mutation A" and "mutation B," respectively, for the first and second mutations.
Labels: Citrate-eating E. coli