Chromosomal evolution in Saccharomyces
G. FISCHER*, S. A. JAMES†, I. N. ROBERTS†, S. G. OLIVER‡ & E. J. LOUIS*
* Department of Biochemistry, University of Oxford, South Parks Road, Oxford
OX1 3QU, UK
† National Collection of Yeast Cultures, Institute of Food Research, Norwich
Research Park, Colney, Norwich NR4 7UA, UK
‡ School of Biological Sciences, University of Manchester, 2.205 Stopford
Building, Oxford Road, Manchester M13 9PT, UK
Recent issue of Nature
Correspondence and requests for materials should be addressed to E.J.L.
(e-mail: elouis at molbiol.ox.ac.uk).
The chromosomal speciation model invokes chromosomal rearrangements as the
primary cause of reproductive isolation1. In a heterozygous carrier,
chromosomes bearing reciprocal translocations mis-segregate at meiosis,
resulting in reduced fertility or complete sterility. Thus, chromosomal
rearrangements act as a post-zygotic isolating mechanism. Reproductive
isolation in yeast is due to post-zygotic barriers, as many species mate
successfully but the hybrids are sterile2, 3. Reciprocal translocations are
thought to be the main form of large-scale rearrangement since the hypothesized
duplication of the whole yeast genome 108 years ago4, 5. To test the
chromosomal speciation model in yeast, we have characterized chromosomal
translocations among the genomes of six closely related species in the
Saccharomyces 'sensu stricto' complex6. Here we show that rearrangements have
occurred between closely related species, whereas more distant ones have
colinear genomes. Thus, chromosomal rearrangements are not a prerequisite for
speciation in yeast and the rate of formation of translocations is not
constant. These rearrangements appear to result from ectopic recombination
between Ty elements or other repeated sequences.
Available in PDF format on request
Roger
