Recent advances in molecular genetics of forest trees

The use of molecular markers has greatly enhanced our understanding of the genome structure of forest trees. Conifers, in particular, have a relativel y large genome, containing a very high proportion of repeated DNA, consisti ng of tandemly repetitive and dispersed repetitive DNA sequences. The natur e of highly conserved tandemly repetitive rRNA genes has been investigated in a number of tree species, and their sites mapped on specific chromosomes by fluorescent in situ hybridization (FISH). Different families of retrotr ansposons (IFG, and TPE1) have been isolated and characterized from the dis persed repetitive DNA of pines. Genome maps have been constructed in a numb er of forest tree genera: Pinus, Picea, Pseudotsuga, Cryptomeria, Taxus, Po pulus, and Eucalyptus. EST databases have been established from cDNA clones of pines and poplars. The structure and maternal or paternal modes of inhe ritance of organelle genomes have been investigated in forest trees. Compar ative mapping in conifers has shown that gene families are conserved across genera. Due to lack of polyploidy in conifers, the evolution of this group of trees may have occurred primarily by duplication and dispersal of genes , probably by retrotranspositions, to form complex gene families. The evolu tion of angiosperm tree species has presumably involved both gene duplicati on as well as genome duplication (polyploidy). Application of genetic engin eering has shown that genes from phylogenetically unrelated organisms can b e introduced and expressed in trees, thus offering prospects of genetic imp rovement of forest trees.