Chromosomal evolution in Saccharomyces

The chromosomal speciation model invokes chromosomal rearrangements as the primary cause of reproductive isolation 1 . 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 sterile 2 , 3 . Reciprocal translocations are thought to be the main form of large-scale rearrangement since the hypothesized duplication of the whole yeast genome 10 8 years ago 4 , 5 . To test the chromosomal speciation model in yeast, we have characterized chromosomal translocations among the genomes of six closely related species in the S accharomyces ‘sensu stricto’ complex 6 . 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.