Premise

Changes in climate can impose selection on populations and may lead to rapid evolution. One such climatic stress is drought, which plant populations may respond to with escape (rapid growth and early flowering) or avoidance (slow growth and efficient water use). However, it is unclear if drought escape would be a viable strategy for populations that already flower early from prior selection.

Methods

In an experimental evolution study, we subjected rapid-cycling Brassica rapa (RCBr), which was previously selected for early flowering, to four generations of experimental drought or watered conditions. We then grew ancestral and descendant populations concurrently under drought and watered conditions to assess evolution, plasticity, and adaptation.

Results

RCBr evolved under drought had earlier flowering and lower water-use efficiency than RCBr evolved under watered conditions, indicating evolutionary divergence. The drought descendants also had a trend of earlier flowering compared to ancestors, indicating evolution. Evolution of earlier flowering under drought followed the direction of selection and increased fitness, and was consistent with studies in natural and experimental populations of this species, suggesting adaptive evolution.

Conclusions

We found evidence for rapid adaptive evolution of drought escape in RCBr and little evidence for constraints on flowering, even though RCBr already flowers extremely early. Our results suggest that some populations may harbor sufficient genetic variation for evolution even after strong selection has occurred. Our study also illustrates the utility of combining artificial selection, experimental evolution, and the resurrection approach to study the evolution of functional traits.

We began this experiment with Wisconsin fast plant lines of rapid-cycling Brassica rapa (field mustard), which we grew and crossed to generate maternal lines of known maternity and paternity. We assigned one seed from each of 48 maternal lines to each of the 12 experimental populations (N=48 per population). We then assigned 4 populations for storage as ancestors, 4 to receive four generations of experimental drought, and 4 to receive four generations of experimental well-watered conditions. During these four generations of experimental evolution, drought populations received a drought treatment aimed to shorten the growing season while well-watered populations were watered every other day.
After experimental evolution, we grew all experimental populations (ancestors included) under unstressed (well-watered) conditions to reduce maternal and storage condition effects across the different regimes.

We then grew two seeds (siblings) from each individual grown in the refresher generation while applying a watering treatment with one sibling from each individual under each treatment in what was the test generation. This design allowed us to assess evolutionary responses (ancestor-descendant regime comparisons) and plasticity (watered-drought treatment comparisons) to experimental drought.
Detailed methods, statistical analysis, and results are discussed in the assocated manuscript published in American Journal of Botany.