Soil moisture effects on the evolution of flowering time in Mimulus

Background/Question/Methods The timing of life history events in plants is often linked to predictable environmental variation.  Selection for a shift in life history strategy may be imposed by a novel seasonal pattern via, for example, colonization near range margins or broader regional climatic changes.  If genetic variation for the life history trait exists, adaptive evolution may result.  Our understanding of the potential evolutionary responses of plant phenology or life history to shifts in climatic regimes, however, is relatively limited.  In Mediterranean climates, seasonal drought is a predictable feature of the climate that has shaped the life histories of plants, and fine-scale local variation in drought timing can explain variation in species distributions.  Drought variability and severity, moreover, are predicted to increase in the coming decades, at least in California.  The fragmented and patchy landscape may limit opportunities for migration, which highlights the importance of adaptive evolutionary change for population persistence.  I conducted greenhouse experiments to examine how the timing of reproduction responded to experimental manipulation of water availability in two closely related species of herbaceous monkeyflowers (genus Mimulus), one of which is considered to be better adapted to early-drying soils.  The two species also have contrasting mating systems, an evolutionarily labile life history trait that is associated with flowering time in the genus Mimulus.  In addition, I estimated the magnitude of genetic variation for flowering time in M. guttatus, using a paternal half-sib breeding design.

Results/Conclusions

Both species flowered earlier in dryer soil, and earlier flowering was advantageous for both species in both environments, as measured by flower production.  In M. guttatus, I found substantial narrow-sense heritability (about 50%) for flowering time under both wet and dry conditions.  In sum, these results suggest that flowering time may retain the ability to evolve adaptively in response to drought in these species.  Evolutionary changes in flowering time occur in concert with shifts in mating system, suggesting that response to drought may play a role in mating system evolution in Mimulus.