Elucidating relationships of adaptation to extreme soil and climate with RAD sequencing
Understanding how plants have adapted to existing environments is essential to successfully predicting the degree to which species can adapt to changing conditions. Climate and soil conditions are obviously important factors in determining the distributions of species. Previous studies suggest that evolution of plant tolerances are crucial to existence in a range of climate and soil conditions. We examine the genetic basis of adaptation to extreme soil and climate in plants that inhabit an ultramafic soil type, serpentine, that in Spain is found primarily in the vicinity of Sierra Bermeja, in southwest Spain. This area is recognized in Spain as a center of endemic biodiversity.We investigate how variation in soil and climate have acted to influence the morphology of plants and their genetic makeup, by using a technique, Restriction Site Associated sequencing (RADseq), that samples variation broadly across the plant genome. The resulting data are single nucleotide polymorphisms (SNPs) in specific genomic regions (loci). We study three plant species (Lavandula stoechas, Halimium atriplicifolium, and Phlomis purpurea) that occur widely in Spain and both on and off serpentine soil, an extreme soil type characterized by moisture scarcity, and high levels of heavy metals, such as nickel, zinc, copper. We investigate whether populations affected by these environments should share a set of candidate loci because of similar selection due to aridity. We examine the possibility that populations from serpentine and non-serpentine soils may donate adaptive alleles to populations in other environments through gene flow of adaptive alleles.A second focus is the evolutionary history reflected by populations that inhabit serpentine soil and the implications of implication of evolutionary history for conservation. Serpentine populations in the study area represent one or more evolutionary lines that colonized these soils. If acquiring adaptations to enable life on serpentine is infrequent, then the serpentine populations likely represent a single evolutionary lineage with a common ancestor that also lived on serpentine. Such shared history would suggest the lineages unique contribution to intraspecific diversity and would interest conservationists. Alternatively, if multiple serpentine lineages exist, then conservation efforts should take this diversity into account.Our third focus addresses how adaptation occurs as the environment exerts selection on intraspecific genetic variation. If a portion of populations becomes subject to environmental selection, say because of climate change or dispersal to an area of unique soil, a favorable allele at a variable locus may increase greatly in frequency, due to positive selection. When other, non-selected populations in other environments also have the allele, this is evidence that the variation existed before the selection was imposed, and would suggest variation existed before the selection arose, thus constituting a soft selective sweep. Alternatively, a lineage may evolve to inhabit serpentine, then diversify to inhabit areas of varying strength of serpentine character. A gene variant may appear only and exclusively in related populations that inhabit the areas of strongest serpentine character, suggesting the mutation arose after the lineage colonized serpentine soil, and that a hard selective sweep had occurred. Areas of the genome potentially exhibiting either type of selective history would form a focus for subsequent studies.