Turning back the evolutionary clock to reveal how new species arise

Scientists at the University of Manchester have turned back the evolutionary clock to reveal a hidden mechanism for differentiation between species of the same family, according to an article published in the journal Nature this week. The finding sheds new light on how different species may have arisen and questions the very notion of how we define individual species.

The work, funded by the Biotechnology and Biological Sciences Research Council (BBSRC) and the Wellcome Trust, was carried out on baker’s yeast Saccharomyces cerevisiae, one of six species in the Saccharomyces genus (family). Each of the Saccharomyces species can mate with each other to produce hybrids that are usually infertile (rather like mating a horse and a donkey to produce a mule). At some point in the evolution of the different species, parts of the chromosomes swapped around, resulting in different chromosome arrangements in the six different Saccharomyces species.

The researchers used a new technique to alter the arrangement of baker’s yeast chromosomes so that they look like the chromosomes of another member of the genus. They were then able to mate the engineered organism with a different species to produce healthy hybrids that were 20 or 30 times more likely to be fertile. This finding suggests that the arrangement of the chromosome is very important in separating between different species.

“Our engineered baker’s yeast calls into question how we define species,” says Professor Steve Oliver from the University of Manchester who led the research team. “We have shown that chromosome arrangement plays a key role is differentiating between species, so by simply changing the arrangement of the chromosomes are we effectively creating a new species?”

“How species arise has been a subject that has fascinated biologists since Darwin’s time. Necessarily, the studies of Evolutionary Biologists are retrospective in nature – by examining the relationships between present-day species, they hope to infer how they arose over geological time. Together with colleagues in Leicester and Norwich we have been able to take a more interventionist approach and engineer the chromosomes of the humble baker’s yeast in order to dissect out different contributors to the evolutionary process.”

The chromosome rearrangements are made by inserting special ’targets’ at precise locations in the two chromosomes that you want to exchange DNA. A bacterial enzyme is then produced in the living yeast cell which acts as a molecular ’scissors’ to cut the two yeast chromosomes at these targets. The yeast cell repairs the cuts by rejoining the DNA. Sometimes it joins pieces from different chromosomes, thereby creating the required rearrangement.

In further experiments the researchers found that the progeny of the hybrids often had extra copies of many of the chromosomes – thus suggesting a route for the development of genetic redundancy that is thought to be an essential source of innovation in the evolution of new species.