Review. Specificity in pollination and consequences for postmating reproductive isolation in deceptive Mediterranean orchids

Abstract

The type of reproductive isolation prevalent in the initial stages of species divergence can affect the nature and rate of emergence of additional reproductive barriers that subsequently strengthen isolation between species. Different groups of Mediterranean deceptive orchids are characterized by different levels of pollinator specificity. Whereas food-deceptive orchid species show weak pollinator specificity, the sexually deceptive Ophrys species display a more specialized pollination strategy. Comparative analyses reveal that orchids with high pollinator specificity mostly rely on premating reproductive barriers and have very little postmating isolation. In this group, a shift to a novel pollinator achieved by modifying the odour bouquet may represent the main isolation mechanism involved in speciation. By contrast, orchids with weak premating isolation, such as generalized food-deceptive orchids, show strong evidence for intrinsic postmating reproductive barriers, particularly for late-acting postzygotic barriers such as hybrid sterility. In such species, chromosomal differences may have played a key role in species isolation, although strong postmating-prezygotic isolation has also evolved in these orchids. Molecular analyses of hybrid zones indicate that the types and strength of reproductive barriers in deceptive orchids with contrasting premating isolation mechanisms directly affect the rate and evolutionary consequences of hybridization and the nature of species differentiation.

Figure 1

Comparison of the strength of postmating–prezygotic isolation (pollen–ovule interaction) for food-deceptive and sexually deceptive orchids measured by crossing species pairs separated by a genetic distance of 0.008–0.032 (data from Scopece et al. 2007). Points and error bars represent means and standard errors.

Figure 2

Relationship between percentage of food-deceptive species pairs displaying total reproductive isolation for three components of postzygotic isolation and three intervals of genetic distance (from Scopece et al. 2008). White bars, embryo mortality; grey bars, hybrid inviability; black bars, hybrid sterility.

Figure 3

Maximum-likelihood estimates of hybrid indices (calculated with the software Hindex; Buerkle 2005) for individuals in hybrid zones of (a) sexually deceptive Ophrys bilunulata×O. lupercalis (from Stökl et al. submitted) and (b) food-deceptive A. morio×A. papilionacea (from Moccia et al. 2007). Low- and high-molecular hybrid scores are indicative of parental population. Individuals within populations are ordered by increasing hybrid indices (±s.d.).