Incipient speciation by divergent adaptation and antagonistic epistasis in yeast

Understanding the conditions that promote the evolution of reproductive isolation, and thus speciation. Here we empirically test some of the key predictions of speciation theory (Coyne 2004; Kohn 2005) by experimentally evolving the initial stages of speciation in yeast. After allowing replicate populations to adapt to two divergent environments, we observed the consistent, de novo evolution of two forms of postzygotic isolation: reduced rate of mitotic reproduction and reduced efficiency of meiotic reproduction. In general, divergent selection resulted in greater reproductive isolation than parallel selection, as predicted by ecological speciation theory. Our experimental system allowed for the first controlled comparison of the relative importance of ecological and genetic mechanisms of isolation, and the novel ability to quantify the effects of antagonistic epistasis. For mitotic reproduction, hybrid inferiority was conditional upon the selective environments and was both ecological and genetic in basis. In contrast, isolation associated with meiotic reproduction was unconditional and was caused solely by genetic mechanisms. Overall, our results show that adaption to divergent environments promotes the evolution of isolation through antagonistic epistasis, providing evidence of a plausible common avenue to speciation and adaptive radiation in nature (Schluter 2000,2001: Funk 2006) Keywords: Speciation, antagonistic epistasis, divergent adaptation We measured gene expression effects due to interaction of divergently adapted genomes in hybrid cells. Reference RNA was a mix of parental lines adapted to the different environments. Test RNA was from the hybrid. There were four independent replicates for each comparison. The goal was NOT to measure the changes in expression associated with adaptation, which would have used the progenitor as a source of reference RNA.