From cross to self-pollination

From time to time, it happens that self-incompatibility is naturally lost and that plants acquire the ability to self-pollinate. Known cases include, for example, some cruciferous species. In this plant family, the genes that are crucial for making a plant self-incompatible are well known. Until now, researchers assumed that loss-of-function mutations at these "S-genes" explained loss of self-incompatibility. In a crossing study published in Nature Communications, evolutionary ecologists from Konstanz present evidence for a different mechanism in sand cress (Arabidopsis lyrata), involving what is known as a modifier gene.

"Self-pollinators have an increased potential to establish self-sustaining populations outside their natural range as invasive species and can survive without insect pollinators. Hence, a better understanding of the mechanisms that can lead to cross-pollinators becoming self-pollinators is of high ecological relevance," explains Marc Stift, evolutionary ecologist at the University of Konstanz and one of the study authors.

Something didn't quite fit

That a modifier gene -- i.e. a gene that affects the expression of another, spatially separated gene -- might play a role in sand cress had already been suspected. However, so far there were no data to confirm this idea. The Konstanz-based researchers therefore conducted an extensive experiment: They crossed individuals from self-incompatible and self-compatible sand cress populations in all combinations possible. Afterwards, they determined the breeding system of more than 1500 progeny and linked this to genetic data about the S-gene variants (S-alleles) that these progeny inherited.

One of the researcher's findings was that crosses between cross-pollinators and self-pollinators can lead to both forms of breeding system in the progeny, with the S-alleles from the self-incompatible partner being crucial. Thus, also in sand cress, the S-gene plays a role in the loss of self-incompatibility. However, the researchers did not find evidence that this functional role was explained by a loss-of-function mutation.

In other words, the self-recognition genes were somehow involved in the breakdown of self-incompatibility in selfing sand cress, but not by the same mechanism as known from other species. On the contrary: "In fact, our experiments revealed progeny with identical self-recognition genes, of which some were self-incompatible, and others were completely self-fertile," says Yan Li, who conducted the crossing experiments for her doctoral studies in Konstanz. This provides strong evidence for the previously unproven alternative mechanism involving a modifier gene. "Now we will have to find out if this mechanism is unique to sand cress, or if it has also led to the transition from cross-pollinator to self-pollinator in other plant species," Stift adds.