Genome-wide association mapping for growth rate at fluctuating and extreme temperatures

Constant temperatures and fluctuations of varying frequency affect fitness differently, which has led to suggestions of distinct genetic architectures and adaptation strategies between constant and fluctuating thermal environments. However, very little is known about the possible fitness trade-offs and genetic constraints underlying thermal adaptation and how they affect species ability to confront climatic changes. We addressed this gap in knowledge by integrating quantitative genetics and genome-wide association mapping in the filamentous fungus Neurospora crassa. Growth rates were measured for 434 strains under fast and slow frequency fluctuations, at high and low temperature range with respect to the species tolerance, and at constant mean and extreme temperatures of these ranges. We found strong genetic correlations between fast and slow frequency fluctuations, and between fluctuations and their mean temperatures, but not with the highest extreme temperature. Positive correlations were supported by high heritability values, pointing that in N. crassa there are no significant trade-offs or genetic constraints in adaptation when variance in temperature increases. Altogether, our results indicated clearly polygenic basis of thermal tolerance, with most of the variation in overall performance (83 %), and clearly less in hot-cold trade-off (8 %), or heat stress tolerance (4 %). Interestingly, GWAS discovered many SNPs associated with growth rate only at constant temperatures or at fast and slow fluctuations at high and low thermal range. However, the cellular functions of the associated genes were overlapping, and no opposite allelic effects were found between treatments. Hence, large-effect loci indicated no trade-offs, but a shared physiology across temperatures, probably owing to the general stress response or individuals overall fitness.