Inheritance of breeding system in Cakile (Brassicaceae) following hybridisation: implications for plant invasions.

Hybridisation is commonly assumed to aid invasions through adaptive introgression. In contrast, a recent theoretical model predicted that there can be non-adaptive demographic advantages from hybridisation and that the population consequences will depend on the breeding systems of the species and the extent to which subsequent generations are able to inter-breed and reproduce. We examined cross-fertilisation success and inheritance of breeding systems of two species in order to better assess the plausibility of the theoretical predictions.Reciprocal artificial crosses were made to produce F1, F2 and backcrosses between Cakile maritima (self-incompatible, SI) and Cakile edentula (self-compatible, SC) (Brassicaceae). Flowers were emasculated prior to anther dehiscence and pollen was introduced from donor plants to the recipient's stigma. Breeding system, pollen viability, pollen germination, pollen tube growth and reproductive output were then determined. The results were used to replace the assumptions made in the original population model and new simulations were made.The success rate with the SI species as the pollen recipient was lower than when it was the pollen donor, in quantitative agreement with the "SI x SC rule" of unilateral incompatibility. Similar outcomes were found in subsequent generations where fertile hybrids were produced but lower success rates were observed in crosses of SI pollen donors with SC pollen recipients. Much lower proportions of self-compatible hybrids were produced than expected from a single Mendelian allele. When incorporated into a population model, these results predicted an even faster rate of replacement of the self-compatible species by the self-incompatible species than previously reported.Our study of these two species provides even clearer support for the feasibility of the non-adaptive hybridisation hypothesis whereby the colonisation of a self-incompatible species can be assisted by transient hybridisation with a congener. It also provides novel insight into reproductive biology beyond the F1 generation.