Transitions in photoperiodic flowering are common and involve few loci in wild sunflowers (Helianthus; Asteraceae).

Premise of the study: Evolutionary changes in how flowering time responds to photoperiod cues have been instrumental in expanding the geographic range of agricultural production for many crop species. Locally adaptive natural variation in photoperiod response present in wild relatives of crop plants could be leveraged to further improve the present and future climatic ranges of cultivation or to increase region-specific yields. Previous work has demonstrated ample variability in photoperiod response among wild populations of the common sunflower, Helianthus annuus. Here, we characterize patterns of photoperiod response variation throughout the genus and examine the genetic architecture of intraspecific divergence. Methods: The requirement of short day lengths for floral induction was characterized for a phylogenetically dispersed sample of Helianthus species. In addition, flowering time was assessed under short days and long days for a population of F-3 individuals derived from crosses between day-neutral and short-day, wild H. annuus parents. Key results: An obligate requirement for short-day induced flowering has evolved repeatedly in Helianthus, and this character was correlated with geographic ranges restricted to the southern United States. Parental flowering times under long days were recovered in high proportion in the F-3 generation. Conclusions: Together, these findings (1) reveal that substantial variation in the nature of flowering time responses to photoperiod cues has arisen during the evolution of wild sunflowers and (2) suggest these transitions may be largely characterized by simple genetic architectures. Thus, introgression of wild alleles may be a tractable means of genetically tailoring sunflower cultivars for climate-specific production.