Some of the most informative adaptive solutions plants present include many remarkable instances of adaptation to extreme edaphic stress. Therefore I am leveraging genome scanning approaches to understand the solutions evolution offers in some of the many independently-evolved lineages of metal, drought, and serpentine-tolerant plants. Initial results clearly indicate broadly orchestrated, polygenic responses to external selective pressures. These concerted changes indicate that even in the case of an external pressure, well-orchestrated internal adaptations must be marshaled in harmony with one another, calling for a better understanding of internal adaptation (systems-based analysis of compensatory changes in the genome). In addition, these initial results are pointing to striking instances of repeated evolution and specific mechanisms underlying preadaptation to colonization of challenging environments following genome duplication.
Serpentine soils present a multidimensional hazard to plant life. Not only do they offer marginal levels of essential nutrients such as calcium, phosphorous, potassium, and nitrogen, but they are also usually very porous, with a high propensity toward drought. Low calcium to magnesium ratios are a defining feature of serpentine environments and result in very low calcium uptake. These insults are typically compounded by the presence of phytotoxic levels of heavy metals such as nickel, cadmium, and zinc, which leads to stunting and chlorosis, along with antagonistic effects on iron uptake. As a result, serpentine environments are characterized by minimal ecosystem productivity and high rates of endemism. Evolution has nevertheless produced populations that tolerate these multiple stresses, which by adapting to this ‘serpentine syndrome,’ suggest possible solutions to multiple challenges in crop improvement.
In contrast to our recent work on adaptation to genome duplication in the same A. arenosa system, we observe a relatively diverse array of genes implicated in serpentine adaptation, from strong sweeps in dehydration tolerance coding loci (ERD4 below), to loci involved in sulfur transport (SULTR1;1 below), heavy metal transport, and root growth. We see clear selective sweeps in many categories consistent with adaptation to ‘serpentine syndrome’: dehydration tolerance, ion transport (Ca, Mg, and K transport-related genes), stress, and root branching and growth.