Structure of evolving populations of Saccharomyces cerevisiae: adaptive changes are frequently associated with sequence alterations involving mobile elements belonging to the Ty family.

Haploid a and diploid a/alpha and a/a populations of Saccharomyces cerevisiae evolving in laboratory environments for up to 300 generations were analyzed for sequence rearrangements associated with the Ty family of transposable elements. In contrast to results with Escherichia coli, evolving populations of yeast exhibit a high frequency of sequence rearrangements associated with mobile genetic elements. In particular, adaptive shifts in these populations are often associated with such sequence rearrangements. The results are most compatible with the explanation that there is direct selection for some of the sequence rearrangements. In addition, the pattern of changes suggests that the structure of evolving microorganism populations may be more complex than expected.