A low resolution epistasis mapping approach to identify chromosome arm interactions in allohexaploid wheat

Abstract

1Epistasis is an important contributor to genetic variance, even in inbred populations where it is present as additive by additive interactions. Testing for epistasis presents a multiple testing problem as the search space for modest numbers of markers is large. Additionally, single markers do not necessarily track functional units of interacting chromatin as well as haplotype based methods do. To harness the power of multiple markers while drastically minimizing the number of tests conducted, we present a low resolution test for epistatic interactions across whole chromosome arms. Two additive genetic covariance matrices are constructed from markers on two different chromosome arms. The Hadamard product of these additive covariance matrices is then used to produce the additive by additive epistasis covariance matrix between the two chromosome arms. The covariance matrices are subsequently used to estimate an epistatic interaction variance parameter in a mixed model framework, while correcting for background additive and epistatic effects. We find significant epistatic interactions for 2% of interactions tested for four agronomic traits in a population of winter wheat. Interactions across homeologous chromosome arms were identified, but were less abundant than other interaction chromosome arm pairs. Of these, homeologous chromosome arm pair 4BL and 4DL showed a strong relationship between the product of their additive effects and the interaction effect that may be indicative of functional redundancy. Several chromosome arms were involved in many interactions across the genome, suggesting that they may contain important large effect regulatory factors. The differential patterns of epistasis across different traits suggests that detection of epistatic interactions is robust when correcting for background additive and epistatic effects in the population. The low resolution epistasis mapping method presented here identifies important epistatic interactions with a limited number of statistical tests at the cost of relatively lower precision.