Genetics of drought adaptation in Arabidopsis thaliana:: I.: Pleiotropy contributes to genetic correlations among ecological traits

We examined patterns of genetic variance and covariance in two traits (i) carbon stable isotope ratio delta(13)C (dehydration avoidance) and (ii) time to flowering (drought escape), both of which are putative adaptations to local water availability. Greenhouse screening of 39 genotypes of Arabidopsis thaliana native to habitats spanning a wide range of climatic conditions, revealed a highly significant positive genetic correlation between delta(13)C and flowering time. Studies in a range of C-3 annuals have also reported large positive correlations, suggesting the presence of a genetically based trade-off between mechanisms of dehydration avoidance (delta(13)C) and drought escape (early flowering). We examined the contribution of pleiotropy by using a combination of mutant and near-isogenic lines to test for positive mutational covariance between delta(13)C and flowering time. Ecophysiological mutants generally showed variation in delta(13) C but not flowering time. However, flowering time mutants generally demonstrated pleiotropic effects consistent with natural variation. Mutations that caused later flowering also typically resulted in less negative delta(13)C and thus probably higher water use efficiency. We found strong evidence for pleiotropy using near-isogenic lines of FRIGIDA and FLOWERING LOCUS C, cloned loci known to be responsible for natural variation in flowering time. These data suggest the correlated evolution of delta(13)C and flowering time is explained in part by the fixation of pleiotropic alleles that alter both delta(13)C and time to flowering.