The Genetic Basis of Pollinator Adaptation in a Sexually Deceptive Orchid

<div><p>In plants, pollinator adaptation is considered to be a major driving force for floral diversification and speciation. However, the genetic basis of pollinator adaptation is poorly understood. The orchid genus <em>Ophrys</em> mimics its pollinators' mating signals and is pollinated by male insects during mating attempts. In many species of this genus, chemical mimicry of the pollinators' pheromones, especially of alkenes with different double-bond positions, plays a key role for specific pollinator attraction. Thus, different alkenes produced in different species are probably a consequence of pollinator adaptation. In this study, we identify genes that are likely involved in alkene biosynthesis, encoding stearoyl-acyl carrier protein (ACP) desaturases (SAD), in three closely related <em>Ophrys</em> species, <em>O. garganica</em>, <em>O. sphegodes</em>, and <em>O. exaltata</em>. Combining floral odor and gene expression analyses, two <em>SAD</em> homologs <em>(SAD1/2)</em> showed significant association with the production of (<em>Z</em>)-9- and (<em>Z</em>)-12-alkenes that were abundant in <em>O. garganica</em> and <em>O. sphegodes</em>, supporting previous biochemical data. In contrast, two other newly identified homologs <em>(SAD5/6)</em> were significantly associated with (<em>Z</em>)-7-alkenes that were highly abundant only in <em>O. exaltata</em>. Both molecular evolutionary analyses and pollinator preference tests suggest that the alkenes associated with <em>SAD1/2</em> and <em>SAD5/6</em> are under pollinator-mediated divergent selection among species. The expression patterns of these genes in F₁ hybrids indicate that species-specific expression differences in <em>SAD1/2</em> are likely due to <em>cis</em>-regulation, while changes in <em>SAD5/6</em> are likely due to <em>trans</em>-regulation. Taken together, we report a genetic mechanism for pollinator-mediated divergent selection that drives adaptive changes in floral alkene biosynthesis involved in reproductive isolation among <em>Ophrys</em> species.</p>