Phenotypic plasticity is broadly adaptive across an elevation gradient in the Cutleaf Monkeyflower

Phenotypic plasticity is a key mechanism by which organisms cope with environmental heterogeneity, but its evolutionary consequences depend on how plastic responses align with the broader adaptive landscape. We tested whether plasticity in leaf shape is associated with differential fitness across an elevational gradient in Mimulus laciniatus, an annual wildflower endemic to montane California. Using a reciprocal transplant experiment and recombinant inbred lines (RILs) previously phenotyped for plasticity in controlled conditions, we measured variation in survival and fecundity in native low- and high-elevation habitats. Based on previous work, we expected to find selection against leaf shape plasticity at low elevations and selection for leaf shape plasticity in the high-elevation direction (increased leaf lobing under long-day conditions) at high elevations. However, plastic RILs outperformed nonplastic genotypes at all elevations. We found that RIL genotypes exhibiting high-elevation plasticity were most likely to set seed at high elevations, but low-elevation plasticity (increased leaf lobing under short-day conditions) was associated with greater seed number (fecundity) across elevations. This pattern suggests that plasticity is broadly beneficial across M. laciniatus's geographic range and that the direction of plasticity is subject to locally varying selection.