Interspecific hybridization of Eucalyptus: key issues for breeders and geneticists

Eucalypt hybrids are significant in forestry, particularly in sub-tropic and tropical regions, where cost efficient, clonal propagation is the key to their exploitation. However, the outstanding success of selected hybrid clones has given a biased impression of the vigor of eucalypt hybrids and the strength of reproductive barriers in the genus. When full account is made of losses through the life cycle, a picture of high incompatibility and inviability often emerges. Hybrid inviability tends to increase with increasing taxonomic distance between parents, but there are exceptions. Hybrids also seem more susceptible to pests than their pure species. Intense selection may still result in elite hybrid clones, but such inviability and susceptibility adds a significant cost to their development. Breeders must carefully evaluate the costs of hybrid development and deployment compared with pure species options. A key to hybrid selection is the rapid development and testing of large populations and application of high selection intensities. However, eventually this approach must be linked with more formal breeding strategies. As most traits are intermediate in F1 hybrids there is increasing interest in advanced generation hybrids to provide desirable trait combinations. In such cases, there is a clear role for marker assisted selection to speed introgression.