Scientists have partially transformed brewer's yeast into another species by manipulating pieces of its chromosomes. The finding, reported in the 6 March issue of Nature, is the first direct evidence that evolution can use chromosomal rearrangement to create new species.
New species arise after two individuals can no longer produce fertile offspring together. Geneticists have long suspected that the cutting and pasting of chromosome fragments that occurs naturally, and rather haphazardly, is important for speciation. But the evidence was always indirect, and conclusions were a best guess based on the family tree of a set of related species. For example, previous research by geneticist Stephen Oliver of the University of Manchester, U.K., and colleagues suggested that chromosomal flip-flopping in yeast was not the primary means of speciation, since past rearrangements did not appear to correspond with the branching off of new species.
Now, instead of trying to reconstruct the evolutionary past, Oliver's team has recreated it with genetic tinkering. Using standard techniques, the researchers rearranged chromosomes VI and VII of brewer's yeast, Saccharomyces cerivisiae, so their gene order matched that of the same chromosomes in its close relative, S. mikatae. Then they crossed S. mikatae with the altered version of S. cerivisiae. That yielded hybrids that could reproduce up to 30% of the time, a success rate edging toward that of same-species crosses. Hybrids of the unmanipulated versions could reproduce in roughly 1% of cases.
Because only some of the hybrids were fertile some of the time, Oliver says there must be some other genetic difference that distinguishes the two species. One likely explanation is that genes that have evolved separately, even without any chromosomal rearrangement, produce proteins incompatible with the proteins of other species, he says.
"This is the first time anyone has genetically engineered one of the big steps in the formation of a new species," says evolutionary geneticist Clifford Zeyl of Wake Forest University in Winston-Salem, North Carolina. But he agrees that chromosome rearrangement is probably not the initial trigger for speciation.