Adaptation to warmer climates by parallel functional evolution of CBF genes in Arabidopsis thaliana.

The evolutionary processes and genetics underlying local adaptation at a specieswide level are largely unknown. Recent work has indicated that a frameshift mutation in a member of a family of transcription factors, C-repeat binding factors or CBFs, underlies local adaptation and freezing tolerance divergence between two European populations of Arabidopsis thaliana. To ask whether the specieswide evolution of CBF genes in Arabidopsis is consistent with local adaptation, we surveyed CBF variation from 477 wild accessions collected across the species' range. We found that CBF sequence variation is strongly associated with winter temperature variables. Looking specifically at the minimum temperature experienced during the coldest month, we found that Arabidopsis from warmer climates exhibit a significant excess of nonsynonymous polymorphisms in CBF genes and revealed a CBF haplotype network whose structure points to multiple independent transitions to warmer climates. We also identified a number of newly described mutations of significant functional effect in CBF genes, similar to the frameshift mutation previously indicated to be locally adaptive in Italy, and find that they are significantly associated with warm winters. Lastly, we uncover relationships between climate and the position of significant functional effect mutations between and within CBF paralogs, suggesting variation in adaptive function of different mutations. Cumulatively, these findings support the hypothesis that disruption of CBF gene function is adaptive in warmer climates, and illustrate how parallel evolution in a transcription factor can underlie adaptation to climate.

Mating system and environmental variation drive patterns of adaptation in Boechera spatifolia (Brassicaceae).

Determining the relative contribution of population genetic processes to the distribution of natural variation is a major goal of evolutionary biology. Here, we take advantage of variation in mating system to test the hypothesis that local adaptation is constrained by asexual reproduction. We explored patterns of variation in ecological traits and genome-wide molecular markers in Boechera spatifolia (Brassicaceae), a species that contains both apomictic (asexual) and sexual individuals. Using a combination of quantitative genetics, neutral genetic (SSR) and genome-wide single nucleotide polymorphism, we assessed the hypothesis that asexual lineages should have reduced signatures of adaptation relative to sexual conspecifics. All three measures (traits, SSRs, SNPs) demonstrated that apomicts are genetically distinct from sexuals, regardless of population location. Additionally, phylogenetic clustering revealed that the apomictic group shared a single common ancestor. Across the landscape, sexual genome-wide SNP variation was strongly associated with latitude (r(2)  > 0.9), indicating that sexual populations have differentiated across an environmental gradient. Furthermore, flowering time and growth rate, as assessed in a common garden, strongly covary with the elevation and latitude of the source population. Despite a wide geographic distribution that largely overlaps with sexual populations, there was little evidence for differentiation in molecular markers or quantitative characters among apomictic populations. Combined, these data indicated that, in contrast to asexual populations, sexual populations show evidence of local adaptation.

Identification of Polymorphisms Associated with Drought Adaptation QTL in Brassica napus by Resequencing.

Brassica napus is a globally important oilseed for which little is known about the genetics of drought adaptation. We previously mapped twelve quantitative trait loci (QTL) underlying drought-related traits in a biparental mapping population created from a cross between winter and spring B. napus cultivars. Here we resequence the genomes of the mapping population parents to identify genetic diversity across the genome and within QTL regions. We sequenced each parental cultivar on the Illumina HiSeq platform to a minimum depth of 23 × and performed a reference based assembly in order to describe the molecular variation differentiating them at the scale of the genome, QTL and gene. Genome-wide patterns of variation were characterized by an overall higher single nucleotide polymorphism (SNP) density in the A genome and a higher ratio of nonsynonymous to synonymous substitutions in the C genome. Nonsynonymous substitutions were used to categorize gene ontology terms differentiating the parent genomes along with a list of putative functional variants contained within each QTL. Marker assays were developed for several of the discovered polymorphisms within a pleiotropic QTL on chromosome A10. QTL analysis with the new, denser map showed the most associated marker to be that developed from an insertion/deletion polymorphism located in the candidate gene Bna.FLC.A10, and it was the only candidate within the QTL interval with observed polymorphism. Together, these results provide a glimpse of genome-wide variation differentiating annual and biennial B. napus ecotypes as well as a better understanding of the genetic basis of root and drought phenotypes.