Barriers to the production of interspecific hybrids in Eucalyptus

A detailed knowledge of crossability patterns is essential for the effective use of a hybrid breeding strategy as reproductive barriers may preclude many otherwise desirable species combinations from direct commercial exploitation. The major eucalypt subgenera are reproductively isolated and, despite natural bybridisation being relatively common, there is evidence for post-mating barriers of varying strength within subgenera. Many hybrid combinations result in high levels of deleterious abnormalities which may be expressed at various stages of the life cycle. Even in the case of hybridisation amongst the closely related species Eucalyptus globulus, E. nitens and E. bicostata, the level of deleterious abnormalities was significantly greater than outcross and inbred controls. Abnormalities in hybrid families were clearly evident in the nursery whereas the deleterious effects of inbreeding only became significant after 1 years' plantation growth. An optimum level of divergence associated with inter-provenance crossing was apparent for cross success and early growth. The expression of deleterious abnormalities in specific hybrid families was not predictable on the basis of the intraspecific performance of the parents and appears to be due to a different genetic mechanism to that resulting in inbreeding depression. The implications of such findings for hybrid breeding are addressed. In the case of the temperate species examined. the successful use of hybrids will depend on the development of efficient methods of clonal propagation as the operational production of F_l hybrid seed appears prohibitive.