One of the basic and most widely accepted hypotheses in plant breeding systems is that flowering plant species that embrace self-fertilisation enter an evolutionary “dead end”, and are destined for extinction. The theory is based on the assumptions that it is impossible for species that exclusively self-pollinate to return to outcrossing, and that in such species the extinction rate is more rapid than the rate of speciation. Although more than 50 years old, the assumptions of the dead end hypothesis of selfing have been under-studied, partly due to the exceeding rarity of the events in question. In a recent Tansley Review for the New Phytologist (freely available at: http://dx.doi.org/10.1111/nph.12182), Boris Igic and Jeremiah Busch interrogate these claims, and make suggestions for potential lines of research that could uncover the history of self-pollination in flowering plants.
When selfing arises, it confers an immediate advantage, due to the fact that more of the selfing parent’s genes are passed on to its offspring compared with its outcrossing counterparts. Additionally, in some cases the selfing individual may, in one fell swoop, purge its offspring of harmful recessive genes. However, when we look at the flowering plants around us, we see that selfing is not the predominant breeding system, and so it seems that such obligate selfers do not persist for long. The apparently short-lived nature of self-fertilising species is assumed to be due to their very lack of genetic diversity, which hampers their ability to respond when their environment changes.
The review authors point out that any re-evolution of outcrossing in selfing plants would be extremely rare, so that in order to reveal whether this occurs, and how often, we need to reconstruct evolutionary histories. They suggest the use of new phylogenetic methods and statistical modelling that have so far been lacking, or poorly executed. They also suggest that if selfing is so unsuccessful, then we might expect to find the fossil record littered with extinct selfing species, identifiable from their distinctive floral traits.
The report is freely available from the New Phytologist website.
Image: Fossil Flower by Slade Winstone / Wikipedia. [cc]by-sa[/cc]