Hybridization and conservation of Mediterranean orchids: Should we protect the orchid hybrids or the orchid hybrid zones? Original Research Article

Natural hybridization between plant species often occurs in disturbed habitats and it is generally considered a threat for rare and endangered species. A different situation occurs in Mediterranean food deceptive orchids, where hybridization is a common phenomenon, as a natural consequence of their unspecific pollination system. Here, we present molecular and ecological evidence from a hybrid zone between Orchis mascula and O. pauciflora in order to address the consequences of hybridization on local orchid evolution and to assess conservation priorities for hybrids and hybrid zones in orchid conservation programs. We find that, although hybrids among the two target taxa are formed relatively frequently, hybrid zones often consist of F1 individuals, whereas backcrosses and later generation hybrids appear to be either rare or absent, presumably as a consequence of post-zygotic reproductive barriers acting in these food-deceptive orchids. Experimental evidence further indicates that hybrids are less fit than the parental species in attracting pollinators and that parental taxa exhibit higher or similar fitness in sympatry compared to allopatric populations. Since introgression is not frequent as indicated by molecular analyses, fitness and phenotypic trait differences observed among sympatric and allopatric populations of the parental taxa may be a consequence of pollinator-mediated selection. Our experimental data confirm that hybridization is a natural phenomenon in food-deceptive orchids and that it does not pose a threat to their survival. Furthermore, sympatric zones provide the stage for evolutionary processes in orchids, and this peculiarity should be taken into account when devising orchid conservation strategies.