Topic: Evolution
Quote: Under these circumstances [stress-induced hyper-mutation], as per EAM,[above], it has been repeatedly demonstrated [since 1988, at Corning, by that scientist whose name escapes me] that the number of adaptive mutations outnumbers the number of of ineffectual or maladaptive mutations by a strongly significant statistical percentage.
Response: On the face of it, the implication is that this kind of thing happens all the time; that everywhere you look, stress-induced hypermutation is occurring, and along with it adaptive mutations are occurring disproportionately often - that this is part and parcel of everyday evolution.\nThe ardour with which this evidence is brandished is, however, misleading. In only a handful of studies, using specific strains of bacteria with specific genetic features under specific environmental circumstances, have specific adaptive mutations seemed to occur with anomalous frequency. There is no suggestion that these cases are in any way generally representative, and even if the results were being interpreted correctly, the idea that they constitute evidence for a pan-species mechanism of internally directed evolution is, unfortunately, wishful thinking.\nBut what of these rare anomalous cases? The most famous was reported by J. Cairns and P. L. Foster, in which a dish of starving, non-growing E-coli (a particular strain with a Lac- mutation) appeared to preferentially induce a reversion mutation (to Lac+) in order to enable growth when the only source of energy was lactose. Moreover, the mutation did not occur with such frequency if lactose was not present, if there was another growth-inhibiting factor in play, or if food sources other than lactose were available. On top of that, although the adapted bacteria showed evidence of accumulating other non-selected mutations, it was nowhere near enough to explain the rate of adaptive mutation, and the bacteria which didn't adapt showed no accumulation of unselected mutations at all!\nUnderstandably, this looked to them, and other people at the time, like a pretty cut'n'dried example of an environmentally-or-organismically induced adaptive mutation.\nHowever, the nagging problem remained that this event was completely unprecedented, and was not observed in the general case of bacteria under starvation conditions. Why was this strain of bacteria so good at inducing this particular reversion mutation in just those circumstances?\nThe answer, as it turned out, was that the experimental assumption of 'no growth' taking place in the e-coli colony was in error.\nThe strain of e-coli used in the experiment had a 'leaky' (inefficient) mutant Lac allele, with only 1% to 2% of the normal ability to process lactose. That 1% to 2% was then deliberately 'smothered' by the experimenters through the addition of scavenger cells which mopped up the product of what little lactose the e-coli could process, effectively halting their growth. Because the later, visible growing colonies of e-coli had a reversion mutation to lac+, the experimenters deduced that mutation was required for growth to be possible.\nHowever, what they did not take into account at the time was that during normal growth prior to incubation on the test medium, a small proportion of the e-coli colony experienced a duplication of the leaky lac gene. This mutation conferred no advantage - until the colony was plated on lactose. Now, the second copy of the gene increased the total amount of lactose they could process beyond the ability of the scavenger cells to inhibit growth, and the e-coli could (slowly) grow and divide.\nWithin these slow-growing microcolonies, selection favoured those cells which experienced further lac- duplications. And the more lac- duplications a cell has, the more likely one of them will experience a reversion mutation to lac+, explaining the apparently anomalous frequency of that particular mutation. When this happens, cell growth rate jumps considerably - but now it is the additional lac- genes which are holding the organism back, destabilising the chromosome and tying up resources which could be more efficiently processed by the lac+ gene. Selection now favours those lac+ revertants who lose the unstable lac- duplications. By the time the microcolonies become visible to the naked eye, they consist almost entirely of lac+ revertants.\nThis hypothesis agrees with all observations related to the earlier experiments, and was verified in a number of ways, including repeating the experiment with a colony composed entirely of duplicate lac- bacteria, increasing the number of microcolonies 100-fold, which tallies with the ~1% background rate of chromosomal duplications in those bacteria. Also, bacteria in the late-arriving microcolonies were observed to possess residual duplicate lac- genes.\nIn short, the results of the experiment agree completely with classical evolutionary theory.\nIt should be noted, by the way, that in addition to the late-arriving 'directed mutation' microcolonies that causes such a stir, the original experimenters also observed the expected number of early-appearing colonies from pre-plating revertant mutants, a la L&D.\nEDIT: Reading further around the subject, it seems no stress-induced hypermutable state was involved either. Apparently, 10-20% of the bacteria with lac- duplications also possessed 'dinB' (a mutation-promoting gene apparently, though it's beyond the scope of my research here) on the same plasmid. The mutagen 'came along for the ride' so to speak, as selection favoured the duplication of that section of DNA, and induced hgher rates of mutation in - yep - 10-20% of the adaptive population.\nSo not only was the hypermutational state of that subpopulation not the cause of the adaptive mutation (clearly, since the other 80-90% of adaptation happened without it), it was actually harmful.\nLive n learn!\n2ND EDIT: The same experiment yields similar results with lac- situated within the chromosome itself, only with 100-fold fewer colonies, reflecting the lower incidence of chromosomal duplication and reversion events in comparison with plasmids.
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