Variation in the incompatibility reactions in tristylous Oxalis pes-caprae: large-scale screening in South African native and Mediterranean basin invasive populations

Establishment and spread of organisms after long-distance dispersal often involve changes to the mating systems. Here, we tested for increased compatibility under a mate limitation scenario after long-distance dispersal of the tristylous Oxalis pes-caprae, a South African geophyte invasive in regions with Mediterranean climate. We investigated variation in the expression of the trimorphic incompatibility system in plants from 16 native populations covering the entire distribution range in South Africa, and 18 invasive populations from the western Mediterranean basin by performing controlled hand-pollinations. After legitimate pollinations, native plants had higher fruit and seed set than invasives, while after illegitimate pollinations (self- and intramorph pollinations), invasives were more successful than natives. We detected morph-specific differences in the weakening of the incompatibility system and within-population variability in the expression of the incompatibility reactions for invasive plants. Additionally, the offspring obtained after illegitimate pollinations was able to germinate, survive and flower under glasshouse conditions. Our results provide experimental evidence for differences in the expression of the incompatibility system between native and invasive populations of O. pes-caprae after introduction and under strong compatible mate's limitation. The weakening in the incompatibility system reported here for the L- and M-morphs might possibly constitute an additional strategy for reproductive assurance in the invaded range of the Mediterranean basin.