The evolutionary basis of reproductive isolation in Mediterranean orchids

The extraordinary floral diversity in orchids reflects the importance of orchid-pollinator associations in orchid evolution. and pollination biology is regarded as a driving force in orchid diversification and speciation. As a consequence, a prominent role has been attributed to the specificity of orchid-pollinator associations and to pollinator mediated pre-mating barriers for maintaining reproductive isolation among orchid species. Changes in floral morphology, coloration or odor may lead to the attraction of different pollinators and to the occupation of different ethological niches. Although this concept is appealing, it does not find strong support in Mediterranean orchids, subtribe Orchidinae, where phylogenetic studies have revealed that speciation is not strictly associated with shifts in pollination biology and where numerous literature reports described the wide occurrence of hybridization. Most Mediterranean orchids are food-deceptive, by generally mimicking rewarding plants, and the low specificity of this pollination strategy is the main cause of the observed hybridization. Genetic analyses of hybrid zones have revealed that most hybrid individuals were first generation hybrids. These findings, together with the absence of polyploid chromosome numbers, suggest that hybridization is of limited importance as basis for speciation in Mediterranean orchids. However, strong karyotype differences between closely related species that share pollinators, suggest that chromosomal changes play a prominent role in reducing hybrid fitness and maintaining reproductive isolation among sympatric Mediterranean orchid species.