Hybridizing ability and heterosis between Eucalyptus urophylla and E. tereticornis for growth and wood density over two environments

Interspecific hybrids of Eucalyptus urophylla × E. tereticornis in a factorial-mating design were used to analyze general hybridizing ability (GHA), specific hybridizing ability (SHA) and heterosis for height (H; 0.5, 1.5, 2.5, 4 and 7.5 years in age), diameter at breast height (D; ages 1.5, 2.5, 4 and 7.5) and wood density (WD; age 7.5) across two environments. The GHA variances were significant for all traits, and those of the SHA were also significant for most of the traits but with less magnitude, indicating the greater importance of additive gene effects in explaining the phenotypic variation among hybrids. The narrow-sense heritability (h²) and dominance (d²) estimates with hybrid growth and wood density ranged between 0.02±0.05 (d² in D_7.5) and 0.23±0.10 (d² in H_1.5) over the different ages, suggesting weak additive and dominant effects on these traits. Additive genetic correlations of growth with wood density were weak at age 7.5. Female general combining ability (GCA) based on maternal open-pollinated families was not necessarily a good indicator of GHA as their correlations were 0.48 (P=0.17), 0.65 (P=0.04) and -0.56 (P=0.10) for H_7.5, D_7.5 and WD_7.5, respectively. A great proportion of hybrids showed positive female-parent heterosis (FPH) at age 7.5, with the highest relative FPH of 47.2% in D_7.5. This study demonstrates the heterosis between inter-sectional species and could have implications for E. urophylla × E. tereticornis hybrid breeding.