One Locus to Rule Them All: Life History QTL in the Silverleaf Sunflower

The silverleaf sunflower, Helianthus argophyllus, is closely related to the domesticated sunflower and has been the donor of agronomically important genotypes for traits including downy mildew resistance and cytoplasmic male sterility. The silverleaf sunflower also exhibits remarkable variation in life history across its native range in coastal South Texas: plants are either short and early flowering with relatively large flowerheads, or tall and late flowering with relatively small flowerheads. Both life history syndromes can be observed in the central, coastal portion of the species range, while inland populations to the North or South are exclusively late flowering. Experimental crosses show that the early flowering syndrome is dominant over late flowering, and many quantitative traits are strongly genetically correlated with flowering time. We created two replicate F2 mapping populations and evaluated both populations in an agricultural setting in Northern California, measuring days to flower, height at flowering, specific leaf area, height at maturity, branch number, branching degree, basal stem circumference, and basal stem density. Both F2 populations exhibited transgressive values for all traits, but the distribution of traits remained bimodal and most traits were highly correlated with flowering time. We used a PstI–MspI genotyping-by-sequencing approach to construct a genetic map and identify quantitative trait loci for each trait. Strikingly, most traits show a dominant single QTL that appears to co-localize with previously identified QTl for flowering time in the sister species H. annuus and putatively contains three paralogs of FT.