Adaptive differentiation on serpentine soil in diploid versus autotetraploid populations of Biscutella laevigata (Brassicaceae)

Abstract

Serpentine soils exhibit extreme properties (e.g. high magnesium content) influencing plant growth and survival, and have been repeatedly documented to promote adaptive edaphic differentiation in plants. Individuals from four pairs of nearby diploid and autotetraploid populations of Biscutella laevigata sampled on serpentine versus non‐serpentine soils in a factorial design are used to assess the genetic and phenotypic changes associated with edaphic origin and ploidy level. Individual samples from natural populations were subjected to soil elemental analysis and genotyping using restriction site‐associated DNA sequences (RAD‐seq) to link genetic variation with contrasting soils and ploidy levels. In diploids, genetic variation was consistent with demographic contraction and a pattern of isolation by environment with respect to the ratio of calcium/magnesium concentrations, whereas tetraploids presented evidence of expansion with limited edaphic differentiation. The genetic basis of tolerance and adaptation to serpentine was further assessed experimentally on seed‐grown individuals from all populations subjected to high (serpentine‐like) versus low (control) concentrations of magnesium in hydropony. Fitness‐related phenotypic traits under experimental cultivation were consistent with adaptive differentiation among diploid ecotypes but not among the tetraploids that similarly grow in both habitats and consistently present higher investment in roots. Further work comparing experimentally resynthesized polyploids to natural diploids and polyploids would help to tease the role of whole genome duplication apart from the effects of post‐polyploidy evolution.