The ability of plants to self-pollinate a big factor in the spread of weeds is much older than previously thought in one widely studied species, leading biologists say.

The findings show that at least in plant evolution, sex with others may be more trouble than its worth.

The mustard-like plant Arabidopsis thaliana lost interest in sex and started self-pollinating at least a million years ago, said plant geneticists led by Magnus Nordborg, associate professor of molecular and computational biology at the University of Southern California.

The results contradict a 2004 estimate from North Carolina State University that A. thaliana began self-pollinating in the last 400,000 years.

We can rule out a very recent change to self-fertilization, said Chris Toomajian, USC research associate in molecular and computational biology and co-author of two new papers on A. thaliana in Science Express and Nature Genetics.

Self-pollination, or selfing, confers a major advantage to weedy species. A selfing plant can invade new territory by itself and colonize it alone.

The potential downside -- a nasty case of inbreeding depression -- is averted by rare sexual breeding. According to an older study, 1 percent of all A. thaliana have received pollen from other plants of the species.

A little sex goes a long way, Nordborg said.

The researchers arrived at their findings by studying common combinations of genetic variants.

Certain variants at different points on the genome tend to go together, like blond hair and blue eyes in humans.

This phenomenon, called linkage disequilibrium, is important because it helps predict what an individuals genome looks like based on information from selected locations.

In their Science Express study, published online July 26, the researchers used the genome-wide pattern of LD to estimate the time at which sel
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Contact: Carl Marziali
marziali@usc.edu
213-740-4751
University of Southern California
7-Aug-2007