BY swapping chunks of DNA on a yeast鈥檚 genome, researchers have turned one species into another. Their genetic tinkering has not only answered a vexing question about how species form but has shown that it is possible to instantly undo, and thus illuminate, thousands of years of evolutionary change.
Speciation is an excruciatingly slow process 鈥� so slow in fact, that it has been impossible to directly test how it works. So researchers play detective by poring over fossil records and sifting through the genomes of organisms. 鈥淚t鈥檚 a retrospective and passive analysis,鈥� says Stephen Oliver of the University of Manchester.
Until now, that is. Oliver and his team have engineered the chromosomes of brewer鈥檚 yeast Saccharomyces cerevisiae to make it indistinguishable from a related species S. mikatae. Essentially, they have reversed a process by which one species splits into two.
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The hallmark of different species is their inability to mate and reproduce. One way this reproductive isolation is thought to occur is when fragments of DNA accidentally swap between an individual鈥檚 chromosomes: this 鈥渃hromosomal translocation鈥� makes them incapable of reproducing with others of the same species.
In yeast, for instance, if a mutated individual mates with a normal one, then half the spores formed will be missing parts of the genome, and will be less successful in reproducing than their parents (see Graphic). This means that the mutated individual has become somewhat isolated, in a reproductive sense, setting it on the path to becoming a new species.
S. cerevisiae and S. mikatae are thought to have diverged in this way. Their genome sequences are very similar, but parts of their chromosomes have been swapped in one species relative to the other. When they mate, they produce sterile hybrids. But there was no way of verifying if chromosomal translocation is what actually drove the species apart.
To find out, Oliver鈥檚 team moved chunks of DNA on S. cerevisiae鈥檚 chomosomes, placing them in the same order as they appear in the genome of S. mikatae. They then mated the two. The big question was, 鈥淲ere they [still] separate species or not?鈥� says Oliver.
When the two species mate naturally, less than 2 per cent of the spores are viable enough to form colonies. But after the genetic tinkering, the species were having quality sex like never before, with 20 to 30 per cent of the spores producing colonies, Oliver says. By rearranging their genomes, his team had in effect turned the two species into one, proving that chromosomal translocation is partially responsible for keeping the species reproductively isolated (Nature, vol 422, p 68).
鈥淭his study is the first that has directly tested a hypothesis related to speciation,鈥� says evolutionary geneticist Ken Wolfe of the University of Dublin in Ireland. 鈥淲hat鈥檚 so nice is that it鈥檚 a completely controlled experiment, where the only difference between the species is this chromosomal rearrangement.鈥� This is a radical change in how evolutionary processes such as speciation are studied in the laboratory, says Oliver. Wolfe agrees. Previously, researchers had to infer how complex behavioural traits drive insect species, such as the fruit fly, apart.
Oliver鈥檚 team is now using the technique to create new yeast strains which they are then pitting against natural strains. By doing so, they hope to understand whether the exact location of genes on the chromosomes is an accident of evolution, or if it confers a selective advantage on the organism. 鈥淚t鈥檚 a much more proactive way of studying evolution,鈥� says Oliver.