Dominant markers have been commonly used in mapping quantitative trait loci (QTLs) in outcrossing species, in which not much prior genome information is available. But the dominant nature of these markers may lead to reduced QTL mapping precision and power. A new statistical method is proposed to incorporate growth laws into a QTL mapping framework, under which the use of the efficiency of dominant markers can be increased. This new method can be used to identify specific QTLs affecting differentiation in growth trajectories, and further estimate the timing of a QTL to turn on, or turn off, affecting growth during the entire ontogeny of a species. Using this method based on dominant markers we have successfully mapped a QTL for stem height growth trajectories to a linkage group in a forest tree. The implications of this method for the understanding of the genetic architecture of growth using dominant markers are discussed.

The logistic or S-shaped curve of growth is one of the few universal laws in biology. It is certain that there exist specific genes affecting growth curves, but, due to a lack of statistical models, it is unclear how these genes cause phenotypic differentiation in growth and developmental trajectories. In this paper we present a statistical model for detecting major genes responsible for growth trajectories. This model is incorporated with pervasive logistic growth curves under the maximum likelihood framework and, thus, is expected to improve over previous models in both parameter estimation and inference. The power of this model is demonstrated by an example using forest tree data, in which evidence of major genes affecting stem growth processes is successfully detected. The implications for this model and its extensions are discussed.

In an attempt to elucidate the molecular mechanisms of Melampsora rust resistance in Populus trichocarpa, we have mapped two resistance loci, MXC3 and MER, and intensively characterized the flanking genomic sequence for the MXC3 locus and the level of linkage disequilibrium (LD) in natural populations. • We used an interspecific backcross pedigree and a genetic map that was highly saturated with AFLP and SSR markers, and assembled shotgun-sequence data in the region containing markers linked to MXC3. • The two loci were mapped to different linkage groups. Linkage disequilibrium for MXC3 was confined to two closely linked regions spanning 34 and 16 kb, respectively. The MXC3 region also contained six disease-resistance candidate genes. • The MER and MXC3 loci are clearly distinct, and may have different mechanisms of resistance, as different classes of putative resistance genes were present near each locus. The suppressed recombination previously observed in the MXC3 region was possibly caused by extensive hemizygous rearrangements confined to the original parent tree. The relatively low observed LD may facilitate association studies using candidate genes for rust resistance, but will probably inhibit marker-aided selection.

A broad collection was made for 42 isolates of Marssonina brunnea affecting poplar trees from three different sections (Leuce, Aigeiros and Tacamahaca) within the same Populus genus in China. Genetic diversity among these isolates was analyzed for morphological traits, cultural features, pathogenicity, hyphal anastomosis and randomly amplified polymorphic DNA markers (RAPDs). No significant difference was found in conidial morphological features, such as size, shape and septum location. Yet, considerable differences occur in other characteristics, which leads to the classification of the 42 isolates into two distinct groups, M. brunnea f.sp. monogermtubi and M. brunnea f.sp. multigermtubi. Isolates of M. brunnea f.sp. monogermtubi, derived from section Leuce, germinate only one germ tube, grow fast, produce dark-reddish conidiosorus clusters on the PDA medium, and are highly pathogenic to Populus tomentosa of section Leuce. By contrast, isolates of M. brunnea f.sp. multigermtubi, derived from sections Aigeiros and Tacamahaca, germinate 1-5 germ tubes, grow slowly, produce yellow-greenish conidiosorus clusters on PDA medium, and are pathogenic to Populus

We report the most complete genetic map to have been constructed for the genus Populus. This map includes 544 markers mapped onto 19 linkage groups, equivalent to the Populus chromosome number, with all markers displaying internally consistent linkage patterns. We estimate the genome length to be between 2,300 and 2,500cM, based both on the observed number of crossovers in the maternal haplotypes, as well as the total observed map length. Genome coverage was estimated to be greater than 99.9% at 20cM per marker. We did not detect obvious recombination repression in the maternal tree (a hybrid of Populus trichocarpa Hooker