Major QTL controls adaptation to serpentine soils in Mimulus guttatus

Spatially varying selection is a critical driver of adaptive differentiation. Yet, there are few examples where the fitness effects of naturally segregating variants that contribute to local adaptation have been measured in the field. Plant adaptation to harsh soil habitats provides an ideal study system for investigating the genetic basis of local adaptation. The work presented here identifies a major locus underlying adaptation to serpentine soils in Mimulus guttatus and estimates the strength of selection on this locus in native field sites. Reciprocal transplant and common garden studies show that serpentine and non-serpentine populations of M. guttatus differ in their ability to survive on serpentine soils. We directly mapped these field survival differences by performing a bulk segregant analysis with F2 survivors from a field transplant study and identify a single QTL where individuals that are homozygous for the non-serpentine allele do not survive on serpentine soils. Genotyping the survivors from an independent mapping population reveals that this same QTL controls serpentine tolerance in a second, geographically distant population. Finally, we show that this QTL controls tolerance to soil properties, as opposed to some other aspect of the field sites that may differ, by performing a lab-based common-garden experiment in native serpentine soils that replicates the survival differences observed in the field. These results indicate that despite the myriad chemical and physical challenges that plants face in serpentine habitats, adaptation to these soils in M. guttatus has a simple genetic basis.