The genomes of trees can now be mapped efficiently and thoroughly through a wide variety of DNA marker systems. DNA markers have been used to construct genetic linkage maps for poplar hybrids, eucalyptuses, loblolly pine, larch, white spruce, Douglas-fir, radiata pine, slash pine and longleaf pine. Complete genetic maps permit the identification of DNA markers linked to important traits such as single genetic loci controlling rust resistance and insect resistance. Quantitative trait loci (QTLs) controlling complex traits such as wood quality, growth and volume, stress resistance, phenology, form, and in vitro propagation can also be mapped. Once such traits are mapped, the associated DNA markers can be used in marker assisted selection (MAS) to accelerate breeding programs and to maximize genetic gain from seed orchards.
Once genetic linkage maps are constructed, efforts are made to associate linkage groups with the chromosomes that they represent. However, the cytological identification of chromosomes is hampered in conifers by poor structural differentiation between chromosomes. Physical genome mapping techniques such as fluorescence in situ hybridization (FISH), which has reinvigorated the field of cytogenetics, are now being used to identify conifer chromosomes in an approach known as molecular karyotyping. The sites of genes and structural elements of chromosomes can also be directly mapped onto chromosomes by FISH. Chromosome paints may soon be available for easy cytological identification and visualization of individual chromosomes cytologically.
Genome maps can reveal structural rearrangements in chromosomes which can be used to accelerate the introgression of traits from wide crosses, perhaps to the point of making introgression breeding a realistic component of tree improvement. Genome mapping also provides tools for the assessment of genetic variability, and for DNA fingerprinting, paternity analysis, seed certification, and early selection.
Key words: DNA markers, linkage, marker assisted selection, chromosomes, cytology.
Correspondence: John E. Carlson, University of British Columbia, Biotechnology Laboratory, 6174 University Blvd., Vancouver, B.C. V6T 1Z3, Canada
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