Epistatic and environmental interactions for quantitative trait loci involved in maize evolution

To test for epistasis and allele-specific environmental responses among qua ntitative trait loci (QTL) involved in the evolution of maize from its ance stor (teosinte), teosinte alleles of two QTL previously shown to control mu ch of the morphological difference between these plants were introgressed i nto an isogenic maize background. Plants of each of the four two-locus homo zygous classes for the two QTL were grown in two environments. Three morpho logical traits and the level of mRNA accumulation for one QTL (teosinte bra nched1, tb1) were measured. tb1 has a large additive effect on morphology t hat was correlated with its message level. The second QTL had only negligib le effects on morphology when isolated in an isogenic background, but exhib ited a strong interaction effect on morphology in combination with tb1. Thi s interaction is also evident in tb1 message levels, suggesting that this s econd QTL may act as an upstream regulator of tb1. The combined effect of t he maize alleles at the two QTL makes tb1 message levels over fourfold high er. Plants homozygous for the teosinte allele at tb1 showed greater phenoty pic plasticity across environments than plants homozygous for the maize all ele. Our results support two hypotheses. First, maize plant architecture ma y have evolved by selection for a gene complex rather than the additive eff ects of individual loci alone. Secondly, selection during maize domesticati on for an allele of tb1 which lacks environmental plasticity may have led t o the fixation of a morphological form that can be induced in teosinte by e nvironmental conditions.