Inconsistent Clines and Shifting Thresholds: The Genetics of Seasonal Adaptation in Annual Mimulus guttatus

Diverse taxa have adapted to use day length as an environmental cue for promoting the onset of reproduction because it reliably indicates the local seasonal progression. Indeed, many plants must be exposed to days longer than a critical minimum photoperiod before they will flower.  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 critical photoperiod for floral initiation.  We have investigated the geographic diversity of critical photoperiod, flowering time, and their genetic basis in annual populations of the common monkeyflower Mimulus guttatus, an obligate long-day plant.  Although the minimum photoperiod necessary for flowering is strongly correlated with elevation and growth season start date, flowering time in inductive conditions is correlated with season duration and its relation to elevation changes with latitude. That these two components of an integrated seasonal phenology can separately evolve not only indicates that different agents of selection drive their evolution, but also predicts that unique genetic architectures govern divergence of each trait. Genetic mapping by multiplex shotgun genotyping in four high x low elevation crosses confirms this prediction: critical photoperiod loci do not co-localize with flowering time loci. Surprisingly, the loci contributing to clinal divergence in critical photoperiod are nearly all transect-specific, in contrast to many recent demonstrations of convergent evolution at the genetic level and contradicting one set of predictions about how plasticity evolves.