While we know that climate change can potentially cause rapid phenotypic evolution, our understanding of the genetic basis and degree of genetic parallelism of rapid evolutionary responses to climate change is limited. In this study, we combined the resurrection approach with an evolve and resequence design to examine genome-wide evolutionary changes following drought. We exposed genetically similar replicate populations of the annual plant Brassica rapa derived from a field population in southern California to four generations of experimental drought or watered conditions in a greenhouse. Genome-wide sequencing of ancestral and descendant population pools identified hundreds of SNPs that showed evidence of rapidly evolving in response to drought. Several of these were in stress response genes, and two were identified in a prior study of drought response in this species. However, almost all genetic changes were unique among experimental populations, indicating that the evolutionary changes were largely non-parallel, despite the fact that genetically similar replicates of the same founder population had experienced controlled and consistent selection regimes. This non-parallelism of evolution at the genetic level is potentially because of polygenetic adaptation allowing for multiple different genetic routes to similar phenotypic outcomes. Our findings help to elucidate the relationship between rapid phenotypic and genomic evolution and shed light on the degree of parallelism and predictability of genomic evolution to environmental change.

We combined experimental evolution with the resurrection approach to explore adaptation to experimental drought in Brassica rapa (field mustard).

We founded 24 experimental populations from lines which originated from a natural population and randomly assigned each experimental population to a watering regime. We assigned 8 populations for storage as ancestors, 8 to receive four generations of experimental drought, and 8 to receive four generations of experimental well-watered conditions. During these four generations of experimental evolution, drought regime populations received a draw-drown drought treatment aimed to mimic Mediterranean drought while well-watered regime populations were watered every other day. We collected data (flowering time, survival, and seed mass) on a subset of individuals each generation to explore trends in fitness and selection during experimental evolution.

After experimental evolution, we grew all experimental populations (ancestors included) under unstressed (well-watered) conditions and extracted DNA from bud tissue from individuals for all 8 drought populations, all 8 watered populations, and 2 ancestor replicates. Buds were pooled by population before DNA extraction and library prep with an Illumina DNA PCR-free library preparation kit. We then performed pooled sequencing on an Illumina NovaSeq 6000 to obtain our fastq files.

Our data submission includes Python and R scripts.