Quantitative genetic variation of leaf size and shape in a mixed diploid and triploid population of Populus

In the interspecific cross of Populus trichocarpa x P. deltoides, unexpecte d simultaneous occurrence of diploid hybrids and triploid hybrids (with two alleles from the female parent and one from the male parent at each locus) led us to examine the evolutionary genetic significance of this phenomenon . As expected, leaf size and shape of the triploid progeny are closer to th e female P. trichocarpa than male P. deltoides parent. Although the pure tr iploid progeny population did not have higher genetic variance in leaf trai ts than the pure diploid population, the former appears to hide much non-ad ditive genetic variance and display strong genetic control over the phenoty pic plasticity of leaf traits. It is suggested that the cryptic non-additiv e variance, especially epistasis, can be released when a population is dist urbed by changes in the environment. A mixed diploid and triploid progeny p opulation combines phenotypic and genetic characteristics of both pure hybr ids and is considered to be of adaptive significance for poplars to survive and evolve in a fluctuating environment. The significant effect due to gen eral and specific combining ability differences at the population level sug gests that the population divergence of these two species is under additive and non-additive genetic control.