Recent research published in Nature shows that a sexually competent strain of yeast is better able to adapt to a stressfull environment than an asexual strain (
Goddard, 2005).
A normally sexual strain was genetically modified, by the deletion of 2 genes, to reproduce asexually. Strains were then cultured in isolation in either benign or stressful conditions for 300 vegetative generations. They were than put in competition with a stored ancestral form of the same strain in the conditions they had adapted to.
In the benign environment no change was seen in fitness for either the sexual or the asexual strain. In contrast the harsh environment saw an increase in fitness, relative to the ancestral strains, for both the sexual and asexual strains with fitnesses improved by 94% and 80% respectively. The differences between the levels of improvement was found to be statistically significant.
I think this experiment works well to obviate a number of problems often seen in trying to explore the advantages of a sexual reproductive strategy. The genetic homogeneity of the two strains, other than at the 2 manipulated loci, in particular removes any problems associated with using already established methods such as starvation to change reproductive strategies, which tend to affect the selective regimes the organisms are subject to.
One caveat about this research is that although the yeast reproduce sexually they do not have distinct sexes. So any issues about the cost of sex, usually calculated as a twofold cost, are avoided in tis system. The authors also note that in 'higher' eukaryotes the effects of deletrious mutations are usually more pronounced so any weeding out effect that sexual reproduction confers may be of more benefit to the 'higher' taxa.
TTFN,
WK
Nature 434, 636 - 640 (31 March 2005); doi:10.1038/nature03405
Sex increases the efficacy of natural selection in experimental yeast populations
MATTHEW R. GODDARD*, H. CHARLES J. GODFRAY & AUSTIN BURT
Why sex evolved and persists is a problem for evolutionary biology, because sex disrupts favourable gene combinations and requires an expenditure of time and energy1. Further, in organisms with unequal-sized gametes, the female transmits her genes at only half the rate of an asexual equivalent (the twofold cost of sex)2. Many modern theories that provide an explanation for the advantage of sex incorporate an idea originally proposed by Weismann more than 100 years ago: sex allows natural selection to proceed more effectively because it increases genetic variation3-5. Here we test this hypothesis, which still lacks robust empirical support, with the use of experiments on yeast populations. Capitalizing on recent advances in the molecular biology of recombination in yeast, we produced by genetic manipulation strains that differed only in their capacity for sexual reproduction. We show that, as predicted by the theory, sex increases the rate of adaptation to a new harsh environment but has no measurable effect on fitness in a new benign environment where there is little selection.
This message has been edited by Wounded King, 04-04-2005 04:02 AM
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