article-5 - Vol 447 | 31 May 2007 | doi:10.1038/nature05856...

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LETTERS Incipient speciation by divergent adaptation and antagonistic epistasis in yeast Jeremy R. Dettman 1 , Caroline Sirjusingh 1 , Linda M. Kohn 1 & James B. Anderson 1 Establishing the conditions that promote the evolution of repro- ductive isolation and speciation has long been a goal in evolution- ary biology 1–3 . In ecological speciation, reproductive isolation between populations evolves as a by-product of divergent selection and the resulting environment-specific adaptations 4–6 . The leading genetic model of reproductive isolation predicts that hybrid infer- iority is caused by antagonistic epistasis between incompatible alleles at interacting loci 1,7 . The fundamental link between diver- gent adaptation and reproductive isolation through genetic incompatibilities has been predicted 1,4,5 , but has not been directly demonstrated experimentally. Here we empirically tested key pre- dictions of speciation theory by evolving the initial stages of spe- ciation in experimental populations of the yeast Saccharomyces cerevisiae . After replicate populations adapted to two divergent environments, we consistently observed the evolution of two forms of postzygotic isolation in hybrids: reduced rate of mitotic reproduction and reduced efficiency of meiotic reproduction. This divergent selection resulted in greater reproductive isolation than parallel selection, as predicted by the ecological speciation theory. Our experimental system allowed controlled comparison of the relative importance of ecological and genetic isolation, and we demonstrated that hybrid inferiority can be ecological and/or gen- etic in basis. Overall, our results show that adaptation to divergent environments promotes the evolution of reproductive isolation through antagonistic epistasis, providing evidence of a plausible common avenue to speciation and adaptive radiation in nature. The development of reproductive isolation is a key aspect of spe- ciation because it is important for both initial divergence and main- tenance of distinct species. If two species are specialized to different environments, interspecific hybrids might have reduced fitness in both environments because they exhibit maladaptive intermediate phenotypes, underscoring the inherent link between ecological isola- tion and divergent adaptation 5,6,8,9 . The link between genetic isolation and divergent adaptation can be explained by Dobzhansky–Muller genic incompatibilities 1,7 . A progenitor population enters and adapts to divergent environments, and in each of these environments certain new alleles are favoured by selection. Hybridization creates novel combinations of alleles untested by selection 10,11 , and negative inter- actions among such alleles can reduce hybrid fitness.
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This note was uploaded on 04/22/2009 for the course EEB 318 taught by Professor Cutter during the Spring '09 term at University of Toronto- Toronto.

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article-5 - Vol 447 | 31 May 2007 | doi:10.1038/nature05856...

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