Plant hormone manipulation impacts salt spray tolerance, which preempts herbivory as a driver of local adaptation in the yellow monkeyflower, Mimulus guttatus

A major challenge in evolutionary biology is identifying the selective agents and phenotypes underlying local adaptation. Local adaptation along environmental gradients may be driven by trade-offs in allocation to reproduction, growth, and herbivore resistance. To identify environmental agents of selection and their phenotypic targets, we performed a manipulative field reciprocal transplant experiment with coastal perennial and inland annual ecotypes of the common yellow monkeyflower (Mimulus guttatus). We manipulated herbivory with exclosures built in the field and exogenously manipulated gibberellin and jasmonic acid to shift allocation of plant resources among growth, reproduction, and herbivore resistance. Our hormone treatments influenced the timing of allocation to reproduction and allocation to phytochemical defense, but this shift was small relative to ecotype differences in allocation. Herbivore exclosures reduced herbivory and increased fitness of plants at the coastal site. However, this reduction in herbivory did not decrease the homesite advantage of coastal perennials. Unexpectedly, we found that the application of exogenous gibberellin increased mortality due to salt spray at the coastal site for both ecotypes. Our results suggest that divergence in salt spray tolerance, potentially mediated by ecotype differences in gibberellin synthesis or bioactivity, is a strong driver of local adaptation and preempts any impacts of herbivory in coastal habitats that experience salt spray.