Genetic causes of heterosis in juvenile aspen:a quantitative comparison across intra- and inter-specific hybrids.

The genetic causes of heterosis in tree growth were investigated by a comparative genetic analysis of intra- and inter-specific crosses derived from Populus tremuloides and P. tremula. A new analytical method was developed to estimate the effective number of loci affecting a quantitative trait and the magnitudes of their additive and dominant effects across loci. The method combines the assumption of multiple alleles, as frequently found in outcrossing species, and the family structure analysis at different hierarchical levels. During the first 3 years of growth, interspecific hybrids displayed strong heterosis in stem growth, especially volume index, over intraspecific hybrids. By a series of joint analyses on the combining ability and the genetic component, we found that F1 heterosis might be due to overdominant interaction between two alleles, one from the P. tremuloides parent and the other from the P. tremula parent, at the same loci. This inference was derived from the finding that heterozygotes, newly formed through species combination, showed much greater growth than the heterozygotes from intraspecifc crosses at a reference locus. Heterosis in aspen growth appeared to be under multi-genic control, with a slightly larger number of loci for stem diameter and volume (9-10) than for height (6-8). For traits with non-significant heterosis, such as stem allometry and internode number and length, the number of underlying loci seemed to be much fewer (3-4). While additive effects appeared to influence seedling traits collectively across loci, a few major dominant loci had much larger effects on stem growth.