Mapping the molecular basis of growth form divergence in tropical alpine Arabidopsis thaliana

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In alpine environments steep gradients in temperature can occur over short geographical distances. This makes altitudinal clines effective systems for exposing the genetic and physiological basis for adaptations to allow survival in harsh environments. Several specific growth forms are enriched in high altitude environments, including low-stature and prostrate shrubs, cushion plants, tussock grasses and herbaceous rosette plants. Shifts to low-stature and more compact forms are strikingly common across species and have been hypothesized to result from an increased need for resistance to cold temperatures. Arabidopsis thaliana is a powerful ecological model because we can apply diverse genetic and molecular tools and knowledge developed in this system to identify the specific molecular and genetic bases for evolutionary change. We have collected over 1000 A. thaliana accessions from altitudes ranging from 2222 to 4432 m over eight mountain ranges in Ethiopia, including altitudinal transects across four of these. We find striking variation in growth form between high and lower altitude accessions, with accessions from the highest altitudes exhibiting one of two phenotypic extremes: either extreme dwarfism or a prostrate growth habit with plagiotropic growth. These phenotypic shifts are reminiscent of those classically associated with high altitude across diverse species. The main objective of this proposal is to identify the genetic variants responsible for growth form shifts and reconstruct the evolutionary history of these across four replicate clines in Ethiopian Afroalpine A. thaliana. The project has the following specific aims:Objective 1: To assemble 12 representative genomes of normal, dwarf and plagiotropic accessions, identify structural variants, and produce an Ethiopian pan-genome referenceObjective 2: To identify the genetic loci underlying growth form variation using genome wide association (GWAS) and recombinant populations (F2s) and assess evidence of adaptation based on reproductive fitnessObjective 3: To investigate evidence for adaptive evolution related to these growth form shifts using population genetic Analysis.