Genetic parameters and improvement strategies for the Pinus elliottii var. elliottii x Pinus caribaea var. hondurensis hybrid in Queensland, Australia

Conventional and novel quantitative genetic methods were used to examine the genetic basis of variation in wood and growth properties in a progeny test of Pinus elliottii var.elliottii(PEE),Pinus caribaea var. hondurensis(PCH) and their F1 and F2 hybrids(PEExPCH). Progeny originated from four NCII mating designs:6x6-parent designs in PEE and PCH,a 12xl2-parent design in the F1 hybrid using the same set of PEE and PCH parents, and an unrelated 6x6-parent design in the F2 hybrid. Wood density and tree basal area at each tree age were measured using x-ray densitometry of radial samples taken at breast height from an 11-year-old trial of the four taxa planted at two sites in South-East Queensland, Australia. Spiral grain angle was measured on a ring-by-ring basis in the F1 hybrid.Based on the results,comparisons were made among candidate hybrid breeding strategies for the genetic improvement of PEExPCH. The PEE population was characterised by a relatively low stem volume and high wood density, and PCH and PEExPCH by the opposite.Economic pressures are leading Queensland plantation managers of PEExPCH to seek to reduce rotation lengths,but this is dependent on maintaining wood quality comparable to current mature plantations. This study found that low wood density and moderate spiral grain angles in the inner growth rings of the F1 hybrid will require improvement in order to meet this quality criterion. Wood density and spiral grain were more strongly heritable than stem volume in PEE, PCH and the F1 hybrid.Economically significant improvements in all three traits could be achieved by family selection, though strong positive genetic correlations between wood density and measures of density variability suggested that the economic significance of the latter should be investigated. Concurrent improvement of wood density and stem volume in PCH is complicated by a strong adverse genetic correlation between these traits (rA=-0.84). A comparison of genetic gains indicated that selection of parents for pure species progeny performance at tree age 12 was nearly as efficient as direct selection of parents for F1 hybrid progeny performance at age 12,for both stem volume and wood density.Given its shorter breeding cycle interval, pure species selection is likely to be an efficient alternative breeding strategy to direct selection in PEExPCH F1 hybrids, for the purpose of improving wood density.However, the pure-hybrid genetic correlation was only moderate in stem volume in PEE, casting doubt on the value of pure species selection for stem volume, despite the high indirect selection efficiency.Under any type of breeding strategy,density and spiral grain at age 12 could be efficiently improved by selection on field screening measurements using the Pilodyn and Bubble Protractor, respectively, and on direct single-ring measurements of these traits from age 4 onwards; tree basal area could be selected from age 6 onwards in the F1 hybrid, and age 4 onwards in PEE and PCH. The genetic basis of F1 hybrid heterosis in stem volume and wood density was investigated using the quantitative genetic model of Li and Wu(1996). Overdominance or pseudo­ overdominance did not contribute to better-parent heterosis in stem volume or to negative mid­ parent heterosis in wood density. Alleles contributed by PEE conferred increased wood density,and alleles contributed by PCH conferred increased stem volume, providing evidence for the genetic basis of "complementarity" in the PEExPCH hybrid.The primary importance of additive gene action in both traits in the F1 hybrid is consistent with results from the genetic gain analysis,but further suggests that direct forward selection in a composite hybrid population may be an efficient alternative to pure species selection, for PEExPCH hybrid improvement;the results hence do not support Shelbourne's hypothesis that interspecific tree hybrids are "genetic dead ends".This study corroborates scattered evidence from previous studies that advanced generation hybridisation may provide a powerful future tool for tree breeders in "complementary" interspecific hybrid tree taxa