Pollen flow between flowers of the same morph in invasive populations of Oxalis pes-caprae L. in the western Mediterranean region

Oxalis pes-caprae is a tristylous South African geophyte that is invasive in regions with Mediterranean climate. Given the introduction of the short-styled morph only, vegetative reproduction was, until recently, the reproductive mode known for this invaded area. The detection of new floral morphs, fruit and seed production in natural populations of the invaded range, together with the reported weakening in the heteromorphic incompatibility system, raised the question on whether the short-styled flowers are able to exchange pollen among themselves, and if so, which mechanisms account for it. Thus, flower morphometric analysis and field experiments were conducted to assess pollinator visitation rates and pollen flow in three invasive populations. Flower morphological changes suggesting the evolution from tristyly towards semi-homostyly were observed. Moreover, O. pes-caprae flowers were visited by different functional groups of pollinators, mostly hymenopteran. Dye flow experiments revealed that the short-styled flowers were able to successfully exchange pollen among themselves in natural conditions. All of this, together with the weakening of the incompatibility system, constitutes further data that might help to explain the recently reported occurrence of sexual reproduction in this invaded range, which by itself bears important implications for the invasion process of O. pes-caprae.