PROGRESS TOWARDS IDENTIFICATION AND VERIFICATION OF QTLs IN FOREST TREES

¹ Institute of Forest Genetics, USDA Forest Service, 2480 Carson Road, Placerville, CA 95667 USA

² Institute of Forest Genetics, USDA Forest Service, University of California, Davis, CA 95616 USA

Over the past several years, approximately 20 experiments have been designed to detect QTLs for economic traits in forest trees (Pinus, Pseudotsuga, Castanea, Eucalyptus and Populus). These experiments have primarily focused on components of yield (growth, adaptability and reproductive traits) and wood quality (wood density, wood chemistry and form traits). The genetic markers currently being used are RFLPs and RAPDs (and to a lesser extent, isozymes), however many labs are developing codominant, PCR-based marker systems from RFLP probes, RAPD primers and microsatellite DNAs. Mapping populations for forest trees have been constructed from both full-sib (two- and three-generation; outbred and selfed matings) and half-sib pedigrees, and where appropriate, interspecific hybrids have been used. The QTL model (outbred, selfed or pseudo-testcross) and statistical method of analysis (single- or multifactor) employed to detect QTLs is dependent on the experimental design of the study. Several general observations can be made from these experiments regarding the number and effect, distribution and stability of QTLs: 1) a relatively small number of QTLs have been detected for each trait, some of which explain a relatively high proportion of the phenotypic variance; 2) correlated traits often share a subset of QTLs; and 3) some QTLs are consistently expressed over time, whereas others appear to be expressed either early or late in development. Before a commitment can be made to marker-aided selection or breeding in forest trees, QTLs should be verified in validation experiments and in different genetic and environmental backgrounds.