MOLECULAR-GENETICS OF GROWTH AND DEVELOPMENT IN POPULUS .4. MAPPING QTLS WITH LARGE EFFECTS ON GROWTH, FORM, AND PHENOLOGY TRAITS IN A FOREST TREE

We have mapped quantitative trait loci (QTLs) for commercially importa nt traits (stem growth and form) and an adaptive trait (spring leaf fl ush) in a Populus F-2 generation derived from a cross between interspe cific F-1 hybrids (P. trichocarpa X P. deltoides). Phenotypic data wer e collected over a 2-year period from a replicated clonal trial contai ning ramets of the parental, F-1, and F-2 trees. Contrary to the assum ptions of simple polygenic models of quantitative trait inheritance, 1 -5 QTLs of large effect are responsible for a large portion of the gen etic variance in each of the traits measured. For example, 44.7% of th e genetic variance in stem Volume after 2 years of growth is controlle d by just two QTLs. QTLs governing stem basal area were found clustere d with QTLs for sylleptic branch leaf area, sharing similar chromosoma l position and mode of action and suggesting a pleiotropic effect of Q TLs ultimately responsible for stem diameter growth.