Evolutionary contingency facilitates scent divergence in an emergent orchid population

Reproductive isolation can evolve rapidly even among populations with extensive genetic similarity, but the mechanisms underlying this divergence remain an open question in evolutionary biology. Here, we investigate the genetic basis of floral scent divergence, a key trait mediating differential pollinator attraction, in a group of nascent Gongora orchid lineages that occur in sympatry. We combined whole-genome resequencing and floral transcriptomics to characterize the transition from the monoterpene-emitting 'M chemotype' to the pollinator-isolated phenylpropanoid-emitting 'A chemotype' nested within it. While overall genetic differentiation was low, a few highly differentiated regions contained candidate genes in biosynthetic pathways. Of particular interest was a putative o-methyltransferase exhibiting evidence of a recent selective sweep in the A chemotype, with much higher expression in scent-emitting floral tissues of A than in M. Transcriptomic comparison with a more distantly related monoterpene-emitting lineage ('X chemotype') further revealed the coordinated and elevated expression of phenylpropanoid biosynthetic genes in M despite its absence of aromatic floral volatiles, including a putative key enzyme potentially involved in directing metabolic flux towards monolignol biosynthesis. Together, our results support a model in which pre-existing activity in a metabolic pathway may facilitate the evolution of a novel floral scent phenotype, potentially making transitions across the adaptive landscape of scent more accessible than others. We propose that such evolutionary contingency may help explain how divergent scent profiles, and consequently, reproductive isolation, can evolve rapidly via few genetic changes.