Weeds lost interest in sex and started self-pollinating much earlier than previously thought, new studies have found.

The studies which contradicted a 2004 North Carolina State University research that a mustard-like plant Arabidopsis thaliana began self-pollinating in the last 400,000 years was conducted by a team of researchers led by Magnus Nordborg at the University of Southern California.

Arabidopsis thaliana, commonly called thale cress, or mouse-ear cress, a small flowering plant related to cabbage and mustard, is one of the model organisms for studying plant sciences, including genetics and plant development.

As part of the studies researchers analysed common combinations of genetic variants in A. thaliana to study whether the transition to self-pollination was recent through linkage disequilibrium (LD).

LD studies variants at different points on the genome that tend to go together and is important as it helps predict how a particular genome looks like depending on the information from selected locations.

In their Science Express study, to estimate when selfing started in the plant, researchers used its genome-wide pattern of LD.

Researchers noted that a recent transition to selfing should have left a mark in the LD pattern for A. thaliana since cross-pollination disrupts some combinations of variants in the genome of two plants.

The researchers hoped to find a comparatively high level of LD, as after the shift to self-pollination the interference of variant combinations would have slowed.

The study however found that self-pollination started 'on the order of a million years ago or more' and perhaps soon after the species evolved.

In their Nature Genetics study researchers used the LD pattern of A. thaliana to device a method to map the links between genetic variants and corresponding physical traits.

"This is of broad interest, as Arabidopsis is likely to become an important model for identifying the genetic basis of evolutionary change," Nordborg said.

Chris Toomajian, co-author of the studies suggested that recent shift of the plants to selfing can be ruled out.

"We can rule out a very recent change to self-fertilization," Nature quoted Toomajian, as saying.