The Genomics of Shifting Thresholds: Flowering Time, Photoperiod Cues, and Adaptation to Elevation in Mimulus

Since day length is often a reliable indicator of seasonal progression, it frequently serves as a crucial environmental signal that diverse organisms sense to cue the onset of reproduction.  Indeed, many plants will not flower unless exposed to days longer than a particular threshold day length.  For crops or wild species that grow over broad geographic ranges, the timing of the growth season often shifts with latitude or elevation, resulting in spatially varying selective regimes that can maintain variation in flowering time as well as the threshold day length for floral initation.  Here, we report a diversity survey in the common monkeyflower Mimulus guttatus, an obligate long-day plant.  Our data suggests evolutionary changes in photoperiod threshold confer local adaptation to such geographically variable selection on flowering time.  The number of hours of light per day required for floral initiation was strongly correlated with elevation and co-varying climatic factors. Further, we explore the genomic basis of multiple parallel shifts in photoperiod threshold by QTL mapping in seven F2 mapping populations with multiplexed-shotgun genotyping.  Finally, we ask whether genetic correlations caused by pleiotropy or close linkage exist between flowering time under inductive conditions and the photoperiod threshold sufficient for induction.