Genetic control of macro- and micro-environmental sensitivities in Populus

Understanding the genetic mechanisms for the phenotypic plasticity and developmental instability of a quantitative trait has important implications for breeding and evolution. Two clonally replicated plantations of two 3-generation inbred pedigrees derived from the highly divergent species Populus trichocarpa and P. deltoides were used to examine the genetic control of macro- and micro-environmental sensitivities and their genetic relationships with the trait mean across two contrasting environments. For all stem-growth traits studied, the trait mean had a higher broad-sense heritability (H²) level than macroenvironmental sensitivity, both with much higher H² values than microenvironmental sensitivity. Genetic correlation analyses indicated that the trait mean was more or less independent of macro- or micro-environmental sensitivity in stem height. Thus, for this trait, the genetic difference in response to the two environments might be mainly due to epistasis between some regulatory loci for plasticity and loci for trait mean. However, for basal area and volume index, pleiotropic loci might be more important for their genetic differences between the two environments. No evidence was found to support Lerner's (1954) homeostasis theory in which macro- or micro-environmental sensitivity is the inverse function of heterozygosity.