In a new study published in Proceedings of the National Academy of Sciences, researchers detail their sequencing of 39 nuclear genes from specimens of Capsella rubella – commonly known as Pink Shepherd’s Purse – and its genetic predecessor, Capsella grandiflora. They estimate the species developed the ability to fertilize itself within the past 20,000 years.

Previously, little had been known about the speed at which plants shift from outcrossing (the practice of introducing unrelated genetic material into a breeding line) to self-fertilization.

“We were surprised as to the time period over which this speciation event seems to have occurred. In terms of evolutionary time, 20,000 years ago is extremely recent,” says the study’s co-lead author, John Paul Foxe, a graduate student in York’s Department of Biology. The study’s joint lead author, Tanja Slotte, is a former postdoctoral researcher at York; she is now at the University of Toronto.

“By contrast, a speciation event between two species of Arabidopsis, a close relative of Capsella, is estimated to have occurred somewhere between 5 and 8 million years ago,” he notes.

Scientists theorize that plants undertake this process in a trade-off between the benefits of greater self-reliance and the disadvantages inherent in inbreeding.

The study hypothesizes that Capsella’s reproductive abilities evolved in response to a major environmental shift following the last ice age. This environmental extreme persisted for several thousand years, followed by agriculture spreading across Europe, producing new habitats that would have been favourable to plants more capable of colonization.

As a result, Capsella reached a near-complete population bottleneck causing major reduction in population size.

“One of the advantages of self-fertilization is the ability to colonize new habitats from a very small foundling population, and that’s precisely what we saw with Capsella,” Foxe says.

The study, “Recent speciation associated with the evolution of selfing in Capsella,” was published on February 19, 2009. It is co-authored by Eli A. Stahl, University of Massachusetts; Barbara Neuffer, Universität Osnabrück; Herbert Hurka, Universität Osnabrück, and Stephen Wright.

The research was conducted at York and the University of Toronto and supervised by Wright, a member of York’s graduate program in Biology and Assistant Professor in the Department of Ecology and Evolutionary Biology at U of T.

Media contact:

Melissa Hughes, Media Relations, York University: 416 736 2100 x22097, mehughes@yorku.ca .

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