Chromosomal rearrangements and speciation

Abstract

Reproductive isolation in the Saccharomyces genus is the result of post-zygotic barriers, as many species mate successfully but the hybrids are sterile. To test whether chromosomal rearrangements were the cause of reproductive isolation (and therefore speciation) in yeast, Fischer et al.1xChromosomal evolution in Saccharomyces. Fischer, G. et al. Nature. 2000; 405: 451–454Crossref | PubMed | Scopus (198)See all References1 characterized chromosomal translocations among the genomes of six closely related species belonging to the ‘sensu stricto’ group of the Saccharomyces genus. All species studied had the same number of chromosomes. The translocations were mapped using single-gene probes from Saccharomyces cerevisiae. The authors showed that rearrangements occurred between closely related species, whereas more-distantly related species had colinear genomes. The translocations appeared to be clustered in a few species and each translocation was species specific. Mapping these translocation breakpoints suggested that the rearrangements were likely to be the result of ectopic recombination between Ty elements or other repeated sequences. The rate of formation of translocations was not constant, which is consistent with the idea that bursts of translocations have occurred during the evolution of some species.Such ectopic recombination normally occurs at very low frequency under laboratory conditions, but it can be stimulated by inactivation of the mismatch-repair system. During adaptation to a new environment, mismatch-repair-deficient mutants can be selected in microbial populations, under strong selective pressure. Similarly, this could happen during yeast evolution, allowing the bursts of translocations. However, these data suggest that the chromosomal rearrangements do not necessarily promote speciation, although they might have contributed to reproductive isolation when it occurred.