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

Abstract

Speciation is typically accompanied by the formation of isolation barriers between lineages. Commonly, reproductive barriers are separated into pre‐ and post‐zygotic mechanisms that can evolve with different speed. In this study, we measured the strength of different reproductive barriers in two closely related, sympatric orchids of the Ophrys insectifera group, namely Ophrys insectifera and Ophrys aymoninii to infer possible mechanisms of speciation. We quantified pre‐ and post‐pollination barriers through observation of pollen flow, by performing artificial inter‐ and intraspecific crosses and analyzing scent bouquets. Additionally, we investigated differences in mycorrhizal fungi as a potential extrinsic factor of post‐zygotic isolation. Our results show that floral isolation mediated by the attraction of different pollinators acts apparently as the sole reproductive barrier between the two orchid species, with later‐acting intrinsic barriers seemingly absent. Also, the two orchids share most of their fungal mycorrhizal partners in sympatry, suggesting little or no importance of mycorrhizal symbiosis in reproductive isolation. Key traits underlying floral isolation were two alkenes and wax ester, present predominantly in the floral scent of O. aymoninii. These compounds, when applied to flowers of O. insectifera, triggered attraction and a copulation attempt of the bee pollinator of O. aymoninii and thus led to the (partial) breakdown of floral isolation. Based on our results, we suggest that adaptation to different pollinators, mediated by floral scent, underlies species isolation in this plant group. Pollinator switches may be promoted by low pollination success of individuals in dense patches of plants, an assumption that we also confirmed in our study.

Why are there so many bee‐orchid species? Adaptive radiation by intra‐specific competition for mnesic pollinators

“…McCormick & Jacquemyn, 2014; Jacquemyn et al ., 2015; Rasmussen et al ., 2015). Some phylogenetically close species are known to share mycorrhizal partners in sympatry, suggesting that mycorrhizal symbiosis has not played a role in reproductive isolation (Gervasi et al ., 2017), as in other orchid genera (Schatz et al ., 2010). The generation time from a seed to a reproductive plant is still not known precisely, but, according to observations of seedling development, may take between two and several years (Fabre, 1852; Hutchings, 2010; Jacquemyn & Hutchings, 2015).…”

“…Claessens & Kleynen, 2011), although variable in space and time (Table 1). Three studies report that this crucial fitness parameter depends significantly on Ophrys population spatial structure (Vandewoestijne et al ., 2009; Gervasi et al ., 2017; Borràs & Cursach, 2018). Considering Ophrys species as separately evolving metapopulation lineages has the immense merit of putting the evolutionary dynamics associated with pollination by sexual swindling into a spatio‐temporal context.…”

“…Synthetic, analogous blends of these compounds (C21–C29 n‐alkanes and n‐alkenes) applied to female dummies triggered copulation attempts by male pollinators (Schiestl et al ., 1999, 2000). For all Ophrys species investigated to date, copulation attempts of pollinators can only be elicited by a blend of organic compounds that is similar in composition and quantity to the female sexual pheromones of the pollinator (Schiestl et al ., 1999, 2000; Ayasse et al ., 2000, 2003; Schiestl & Ayasse, 2002; Mant, Peakall, & Schiestl, 2005; Stökl et al ., 2005, 2007, 2008, 2009; Ayasse, 2006; Vereecken & Schiestl, 2008; Gögler et al ., 2009, 2011; Cuervo et al ., 2017; Gervasi et al ., 2017).…”

“…Secondly, the analyses of the composition of pheromone analogues within the Ophrys insectifera clade, which is the most basal clade according to most molecular phylogenetic hypotheses [Fig. 1; but see Bateman, Sramkó, & Paun, 2018 and Piñeiro Fernandes et al ., 2019], using DNA sequence data showed the simultaneous emission of both esters and non‐polar hydrocarbons (Joffard, Buatois, & Schatz, 2016; Gervasi et al ., 2017). Moreover, among the three species in this clade, O. aymoninii , which is the only one to be pollinated by a species of Andrena ( A. combinata ), is also the only one to emit esters (Joffard, Buatois, & Schatz, 2016).…”

Comparative transcriptomics provides insight into the molecular basis of species diversification of section Trigonopedia (Cypripedium) on the Qinghai-Tibetan Plateau

“…Floral scents function as olfactory cues for pollinator attraction and play a vital role in reproductive isolation. Studies of sexually deceptive orchids have indicated that the interaction between floral scent and pollinators promotes speciation, e.g., Chiloglottis 46 and Ophrys 3 , 47 , 48 . This phenomenon has also been discovered in other non-deceptive pollination groups, e.g., Silene 49 , Petunia 50 , and Mimulus 51 .…”