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Abstract
Understanding how reproductive isolation is maintained in diverged species is integral to understanding species preservation and the evolutionary dynamics of natural sympatric systems. Are partially reproductively isolated species which hybridize at some level destined to merge into a single species, or can isolation be maintained despite the homogenizing effects of geneflow? Ecological isolation, based in divergent selection to variable ecological conditions in partially isolated taxa, has been implicated as an important factor maintaining species boundaries in the face of hybridization. Here we aim to understand the phenotypic and genetic basis of important divergent drought response traits in sympatric sister species, Mimulus guttatus and M. nasutus, which may play and important role in maintaining species boundaries when the species exist in sympatry. Under water-limited conditions we have identified substantial differentiation in survival and seedset between sympatric M. guttatus and M. nasutus in the greenhouse. Consistent with observed microhabitat differences in the field, M. guttatus was almost completely unable to produce seeds under drought conditions. While M. nasutus was able to decrease flower and seed maturation time under drought, M. guttatus was unable to do so. This difference may be indicative of differential evolution of plasticity to a water limiting environment in Mimulus, which may contribute to immigrant inviability and maladaptation in hybrids. In a custom-made mapping population of admixed individuals made from a M. nasutus and three M. guttatus parental lines, we investigate the phenotypic and genetic basis of this divergent drought response in sympatric Mimulus which holds considerable promise for identifying genomic variation responsible for reproductive isolation in this system.