Conventional genetic approaches in trees are limited by the large size, long generation interval, and outcrossing mating system of most species. The need for a tree model reflects the importance of many traits that are not shared by an herbaceous annual plant such as Arabidopsis. Important among these are wood formation, longevity, seasonal growth, and hardiness. The genus Populus consists of 30 to 40 species, 4 of which have significant commercial importance. Selection and hybridization programs in poplars began in North America in the 1960s, and the most commonly exploited crosses have involved P. trichocarpa, P. deltoides, P. nigra, P. grandidentata, P. alba, P. tremuloides, and P. tremula.
Because the genomic resources of P. trichocarpa were the most developed at the time that genome sequencing was proposed, this species became the accepted tree model. It was chosen as the first tree for genome sequencing largely because of its modest genome size (0.6 pg)—about 40 times smaller than that of pine, the most important of all forestry species. It also has a number of other advantages over potential alternative tree species specifically related to its rapid juvenile growth, which allows for phenotypic assessments to be made relatively quickly; its wellestablished transformation and regeneration protocols; and the pre-existence of a body of genetic mapping, which includes placement and tagging of a number of quantitative trait loci (QTL). The final draft sequence was scheduled for release in early 2005, but is still awaited at the time of writing.
Because the genomic resources of P. trichocarpa were the most developed at the time that genome sequencing was proposed, this species became the accepted tree model. It was chosen as the first tree for genome sequencing largely because of its modest genome size (0.6 pg)—about 40 times smaller than that of pine, the most important of all forestry species. It also has a number of other advantages over potential alternative tree species specifically related to its rapid juvenile growth, which allows for phenotypic assessments to be made relatively quickly; its wellestablished transformation and regeneration protocols; and the pre-existence of a body of genetic mapping, which includes placement and tagging of a number of quantitative trait loci (QTL). The final draft sequence was scheduled for release in early 2005, but is still awaited at the time of writing.