Article] Stress, sex and evolution

Stress, sex and evolution
Pete Moore

Despite the widespread view that the effects of mutations are exacerbated by
environmental stresses, some stresses have been found to alleviate the
effects of mutations in bacteria.

Open any textbook on evolutionary genetics and you are bound to find phrases
such as 'stress reveals genetic variation'. The implication is that while
many stresses have a slight effect on an 'average' organism, if the organism
has a mutation then the stress can have greater impact. For bacteria this
would be revealed in reduced growth. At the very least you would expect
growth of mutated organisms to be inhibited by stress to the same extent as
that stress inhibits the growth of the non-mutated progenitor strain.

Compare this with an engineering example and it makes obvious sense. A car
drives faster down an urban road than across the stressful environment of
rough terrain. If you 'mutate' the vehicle by removing a screw at random and
it impedes the vehicle's ability to cope with the urban road, you would
expect the effects of this 'mutation' to be similar, or exacerbated, when it
is driven off-road. You would certainly not expect the deleterious effect to
be reduced when driving off-road.

But according to results published in this issue of Journal of Biology [1],
Escherichia coli appears not to have read the textbooks. Working in the
Laboratory of Living Matter at Rockefeller University, New York, Roy Kishony
and Stanislas Leibler have found that if some stresses are applied to
previously mutated organisms, the effect of each stress is less pronounced
than when it is applied to wild-type bacteria (see 'The bottom line' box for
a summary of their work). Kishony and Leibler emphasize that these are
bacteria with random deleterious mutations, not rare mutants that manage to
do better than their wild-type parents; their intention was to see the
effect of the average mutation as opposed to studying specific rare ones.
The conclusion from this study is that if you were to pick a mutation at
random, the chances are that some of its lost performance would be restored
under particular stresses. This really is surprising. It is saying that if
you take a damaged biological system and push it close to the extreme,
somehow the damage becomes less deleterious.

There had been previous hints at this effect. "This is part of a growing
body of data that shows that we don't understand mutational effects in
different environmental conditions," says botanist Jeffrey Blanchard, who
works at the National Center for Genomic Resources, Santa Fe, USA. "I am not
very surprised by the results," adds James Fry, of the Biology Department at
the University of Rochester, USA. "It goes against conventional wisdom, but
then I wasn't very sure I believed the conventional wisdom in the first
place. We had some results in our Genetics 2002 paper cited by Kishony and
Leibler - it wasn't a major emphasis of the paper, but one of the
implications is that there probably were mutations in which there probably
were smaller proportional effects under stressful conditions." Fry's paper
[2] gave an inkling, but Kishony and Leibler's has much more power to see
what is going on. "Working with Drosophila means that our study was more
crude than theirs," Fry notes.

Read the rest at the Journal of Biology
http://jbiol.com/content/2/2/10
[Requires free registration]

Kind Regards,
Robert Karl Stonjek.