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NICHE EXPANSION IN BACTERIA: CAN INFECTIOUS GENE EXCHANGE AFFECT THE RATE OF EVOLUTION?
Ralph Evans 1
1 Department of Zoology, University of Massachusetts, Amherst,
Massachusetts 01003
Recombination occurs by infectious gene transfer in bacteria,
at rates much lower than recombination by sexual reproduction in other organisms.
Thus, recombination may accelerate evolution in bacteria only under restricted
conditions, such as occur when mutations at several loci are required for
the evolution of an expanded ecological niche. Mathematical ("chemostat")
models of several such cases—evolution of independence from three limiting
essential or "interactive-essential" resources; evolution of the ability to
use three new substitutable resources; and evolution of resistance to three
growth inhibitors—were analyzed by computer simulation. All combinations
of three mutation rates (U) and four values for the "infectious gene
transfer rate parameter" (
) were considered. Recombination accelerated
evolution most when U was low and was high, but was unlikely
to have large effects when was low enough to be realistic for natural
populations of Escherichia coli. Recombination had the largest effects
when resources were substitutable, and in that case could have substantially
reduced the chance of random loss of the favored "triple mutant" while it
was still rare. The simulations also revealed some interesting features of
selection for an expanded niche. Evolution of independence from essential
resources occurred more rapidly when the resources were weakly complementary
than when they did not interact. Selection for the ability to use all substitutable
resources was weak after all intermediate types that used only one or two
of the resources had arisen.
Accepted on March 21, 1986
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