Patterns of Reproductive Isolation in Mediterranean Deceptive Orchids

Abstract

The evolution of reproductive isolation is of central interest in evolutionary biology. In plants, this is typically achieved by a combi-nation of pre-and postpollination mechanisms that prevent, or limit, the amount of interspecific gene flow. Here, we investigated and compared two ecologically defined groups of Mediterranean orchids that differ in pollination biology and pollinator specificity: sexually deceptive orchids versus food-deceptive orchids. We used experimental crosses to assess the strength of postmating prezygotic, and postzygotic reproductive isolation, and a phylogenetic framework to determine their relative rates of evolution. We found quantitative and qualitative differences between the two groups. Food-deceptive orchids have weak premating isolation but strong postmating isolation, whereas the converse situation characterizes sexually deceptive orchids. Only postzygotic reproductive isolation among food-deceptive orchids was found to evolve in a clock-like manner. Comparison of evolutionary rates, within a common interval of genetic distance, showed that the contribution of postmating barriers was more relevant in the food-deceptive species than in the sexually deceptive species. Asymmetry in prezygotic isolation was found among food-deceptive species. Our results indicate that postmating barriers are most important for reproductive isolation in food-deceptive orchids, whereas premating barriers are most important in sexually deceptive orchids. The different rate of evolution of reproductive isolation and the relative strength of pre-and postmating barriers may have implication for speciation processes in the two orchid groups.

Floral scent and species divergence in a pair of sexually deceptive orchids

“…Experimental investigations on the evolution and nature of reproductive isolation barriers can provide insights into the process of diversification (Coyne & Orr, 1998, 2004; Moyle et al., 2004; Schemske, 2010; Scopece et al., 2007, 2008; Widmer et al., 2009). In our study, floral isolation mediated by floral scent appears to be the only significant barrier to gene flow between two recently diverged orchid species.…”

“…In our study, floral isolation mediated by floral scent appears to be the only significant barrier to gene flow between two recently diverged orchid species. Although the predominant importance of ethological floral isolation in sexual mimics has also been shown in other species (Scopece et al., 2007; Sedeek et al., 2014; Whitehead & Peakall, 2014; Xu et al., 2011), our study adds information on the traits underlying floral isolation and shows a negative association between fruit set and plant density, a situation that may favor a pollinator switch. In addition, it considers mycorrhizal fungi as a factor for species isolation, which has rarely been done in orchids and never before in the genus Ophrys .…”

“…The distance between each plot in a transect was 20 m. The plants were examined every 5 days for pollinia removal as well as deposition of stained massulae (pollen packages) on stigmata. Floral isolation (RI floral ) was calculated as 1 − (total number of interspecific pollination events/total number of intraspecific pollination events) (Scopece, Musacchio, Widmer, & Cozzolino, 2007). This value can vary between 0 (no floral isolation) and 1 (total floral isolation).…”

“…RI fruiting was calculated as 1 − (mean number of fruits in interspecific crosses/mean number of fruits in intraspecific crosses). In cases where interspecific crosses performed better than the intraspecific crosses (resulting in a negative value for RI), the reproductive isolation value was set to zero (Scopece et al., 2007). Finally, fruits were collected when they were ripe and dried in silica gel (Fluka).…”

“…(2007). In cases where interspecific crosses performed better than the intraspecific crosses, the reproductive isolation value was set to zero (Scopece et al., 2007). …”