1. Department of Genetics, University of Dublin, Trinity College, Dublin 2, Ireland

Correspondence to: Kenneth H. Wolfe¹ Correspondence and requests for materials should be addressed to K.H.W. (e-mail: Email: khwolfe@tcd.ie).Additional details of the dataset and results are available on the World-Wide Web at URL http://acer.gen.tcd.ie/~khwolfe/yeast.

Gene duplication is an important source of evolutionary novelty¹,². Most duplications are of just a single gene, but Ohno¹ proposed that whole-genome duplication (polyploidy) is an important evolutionary mechanism. Many duplicate genes have been found in Saccharomyces cerevisiae, and these often seem to be phenotypically redundant^{3, 4, 5, 6, 7}. Here we show that the arrangement of duplicated genes in the S. cerevisiae genome is consistent with Ohno's hypothesis. We propose a model in which this species is a degenerate tetraploid resulting from a whole-genome duplication that occurred after the divergence of Saccharomyces from Kluyveromyces. Only a small fraction of the genes were subsequently retained in duplicate (most were deleted), and gene order was rearranged by many reciprocal translocations between chromosomes. Protein pairs derived from this duplication event make up 13% of all yeast proteins, and include pairs of transcription factors, protein kinases, myosins, cyclins and pheromones. Tetraploidy may have facilitated the evolution of anaerobic fermentation in Saccharomyces.