Natural variation in circadian period correlates with diverse phenological measures in Boechera stricta

The circadian clock is a time-keeping mechanism that enables adaptive responses to temporal environmental changes. Circadian period exhibits significant segregating genetic variation within and among populations along an elevational cline, potentially resulting from variable selection across microsites on outputs of the circadian clock. Reproductive timing is an important clock output and selection may favor delayed phenology. We performed a three-year common garden field study with the short-lived perennial Boechera stricta to quantify life history differences among 20 populations from an elevational gradient and among 20 maternal families within one population. We measured days to bolting and later life history transitions such as days to flowering; in the same genotypes, we measured circadian period. The timing of all life history transitions varied among growing seasons, suggesting adaptive life history evolution to local climate conditions. Life history transitions after bolting were also associated with circadian period, such that lengthened period was associated with delayed life history transitions. Structural equation modeling indicated that indirect selection via days to flowering and fruit production favored lengthened clock period in the low elevation common garden site, suggesting that selection on phenology could explain the evolution of variable circadian periods observed among populations from differing environments.