Using an Evolve + Resequencing experiment to estimate the strength of selection on candidate genes underlying local serpentine adaptation in Mimulus guttatus
Authors: Amelia Lawrence 1; Jessica Selby 2; John Willis 1
Affiliations: 1) Duke University, Durham, NC; 2) Bayer - Crop Science, St. Louis, MO
Keywords: Natural selection
Local adaptation occurs when spatially divergent selection at a locus is stronger than the rate of migration between habitats. Population genomic scans for loci contributing to local adaptation often identify tens or hundreds of candidate genes, but usually cannot estimate the strength of divergent selection at each locus and answer the key question: which loci are actually under the strongest selection, and therefore contribute the most to local adaptation? Serpentine outcrops are ideally suited for the study of local adaptation in plants because their patchy distribution results in a landscape with abrupt changes from “normal soils” to serpentine soils. Serpentine soils are unconducive to most plant life because of their characteristically low Ca:Mg ratio, a limited availability of macronutrients (K, P, N), and high levels of heavy metals (Ni, Zn). We have been studying parallel local serpentine adaptation of the wildflower Mimulus guttatus. Reciprocal transplant experiments in the field and greenhouse both reveal extreme local adaptation, where plants from non-serpentine populations are unable to survive and reproduce when transplanted onto serpentine soils. Population genomic scans of many pairs of adjacent serpentine and non-serpentine populations have identified dozens of candidate genes on all chromosomes that are under divergent selection. What are the relative fitness contributions of these genes to local adaptation? We marry a classic F2 mapping approach with an Evolve + Resequence experiment to estimate fitness effects of candidate loci. We cross a pair of well-characterized inbred lines, from adjacent serpentine and non-serpentine habitats, to form F2 progeny. We planted thousands of these hybrids on serpentine and control soils in the greenhouse and allowed them to die or survive and reproduce, using captive bumblebees to ensure outcrossing. By planting the seeds of survivors on their parental soils, we allow each population to adapt to its soil for several generations. We then use pool-sequencing of each population in each generation and apply likelihood test statistics to detect loci which responded to divergent selection and estimate marginal fitnesses of the alleles in each environment. Here I report the initial results from the first few generations.