Genetic Mapping Reveals an Anthocyanin Biosynthesis Pathway Gene Potentially Influencing Evolutionary Divergence between Two Subspecies of Scarlet Gilia (Ipomopsis aggregata)

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Immense floral trait variation has likely arisen as an adaptation to attract pollinators. Different pollinator syndromes—suites of floral traits that attract specific pollinator functional groups—are repeatedly observed across closely related taxa or divergent populations. The observation of these trait syndromes suggests that pollinators use floral cues to signal the underlying nectar reward, and that complex trait combinations may persist and evolve through genetic correlations. Here, we explore pollinator preferences and the genetic architecture of floral divergence using an extensive genetic mapping study in the hybrid zone of two Ipomopsis aggregata subspecies that exhibit a hummingbird and a hawkmoth pollinator syndrome. We found that natural selection acts on several floral traits, and that hummingbirds and hawkmoths exhibited flower color preferences as predicted by their respective pollinator syndromes. Our quantitative trait loci (QTL) analyses revealed 46 loci affecting floral features, many of which colocalize across the genome. Two of these QTL have large effects explaining >15% of the phenotypic variance. The strongest QTL was associated with flower color and localized to a SNP in the anthocyanin biosynthesis pathway gene, dihydroflavonol-4-reductase (DFR). Further analysis revealed strong associations between DFR SNP variants, gene expression, and flower color across populations from the hybrid zone. Hence, DFR may be a target of pollinator-mediated selection in the hybrid zone of these two subspecies. Together, our findings suggest that hummingbirds and hawkmoths exhibit contrasting flower color preferences, which may drive the divergence of several floral traits through correlated trait evolution.

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