We report molecular genetic linkage maps for an interspecific hybrid population of Populus, a model system in forest-tree biology. The hybrids were produced by crosses between P. deltoides (mother) and P. euramericana (father), which is a natural hybrid of P. deltoides (grandmother) and P. nigra (grandfather). Linkage analysis from 93 of the 450 backcross progeny grown in the field for 15 years was performed using random amplified polymorphic DNAs (RAPDs), amplified fragment length polymorphisms (AFLPs), and inter-simple sequence repeats (ISSRs). Of a total of 839 polymorphic markers identified, 560 (67%) were testcross markers heterozygous in one parent but null in the other (segregating 1:1), 206 (25%) were intercross dominant markers heterozygous in both parents (segregating 3:1), and the remaining 73 (9%) were 19 non-parental RAPD markers (segregating 1:1) and 54 codominant AFLP markers (segregating 1:1:1:1). A mixed set of the testcross markers, non-parental RAPD markers, and codominant AFLP markers was used to construct two linkage maps, one based on the P. deltoides (D) genome and the other based on P. euramericana (E). The two maps showed nearly complete coverage of the genome, spanning 3801 and 3452 cM, respectively. The availability of non-parental RAPD and codominant AFLP markers as orthologous genes allowed for a direct comparison of the rate of meiotic recombination between the two different parental species. Generally, the rate of meiotic recombination was greater for males than females in our interspecific poplar hybrids. The confounded effect of sexes and species causes the mean recombination distance of orthologous markers to be 11% longer for the father (P. euramericana; interspecific hybrid) than for the mother (P. deltoides; pure species). The linkage maps constructed and the interspecific poplar hybrid population in which clonal replicates for individual genotypes are available present a comprehensive foundation for future genomic studies and quantitative trait locus (QTL) identification.

We have constructed nearly complete linkage maps of Pinus sylvestris (L.) using AFLP markers basedon a two-way pseudo-testcross strategy in a full-sib family founded in an advanced breeding program. With 39 primer combinations, a total of 737 markers (320 from the mother and 417 from the father) segregated in a 1:1 ratio, corresponding to DNA polymorphism: heterozygous in one parent and null in the other. In the maternal parent, 188 framework markers were mapped in 12 linkage groups, equivalent to the Pinus haploid chromosome number, with a total coverage of 1,695.5cM. In the paternal parent, 245 framework markers established a map with 15 linkage groups, spanning a genome length of 1,718.5 cM. The estimated total map length was LF

Variation in the distribution of biomass among leaves, stems, and roots in forest trees is often reported to be influenced by environment, nutrition, ontogeny or stage of development, and genetics. However, although patterns of whole-tree distribution of dry mass are known to vary in response to soil water and nitrogen availability, and as a function of developmental stage, evidence that the above- and below-ground distribution of biomass varies as a function of genetics is scarce. Therefore, to better assess the genetic control of biomass distribution in trees, phenotypic variation in growth and patterns of biomass distribution to stems, branches, leaves, coarse roots, and fine roots was examined in two advanced-generation hybrid poplar (Populus trichocarpa x P. deltoides) families grown under field conditions during 2000-2002 in the Pacific Northwest. Family 331 was an inbred F2 (TD x TD) pedigree, whereas Family 13 was an outbred backcross BC1 (TD x D) pedigree. Fractional distribution of total biomass (i.e., mass fraction) to shoots and roots averaged 0.62

We have constructed RAPD-based linkage maps for an interspecific cross between two species of the genus Populus (P. adenopoda and P. alba), based on a double pseudo-test-cross strategy. Of a total of 360 polymorphic fragments scored, 290 showed a test-cross configuration, corresponding to DNA polymorphisms heterozygous in one parent and null in the other. In the female parent, P. adenopoda, 82 markers were grouped in 19 different linkage groups (553cM), whereas in the male parent P. alba, 197 markers established a much more complete framework map with an observed genome length of 2300cM covering 87% of the total P. alba genome. The larger number of test-cross markers detected for the P. alba parent than for the P. adenopoda parent might be due to a higher level of heterozygosity in the former than in the latter. In this study, we detected only a small percentage (2%) of the intercross dominant markers heterozygous in both parents and segregating 3:1 in the progeny. The further focus in this mapping study should be on the identification of more intercross markers, to align the two parent-specific maps into a consensus map for mapping important genes causing species differentiation during long evolutionary divergences.

Growth trajectories are a biological process important to plant and animal breeding, and to evolutionary genetic studies. In this article, we report the detection of quantitative trait loci (QTLs) responsible for growth trajectories in poplars that are used as a model system for the study of forest biology. These QTLs were localized on a genetic linkage map of polymorphic markers using a statistical mapping method incorporating growth-curve models. The effects of the QTLs on growth are described as a function of age, so that age-specific changes in QTL effects can be readily projected throughout the entire growth process. The QTLs identified display increased effects on growth when trees age, yet the timing of QTL activation is earlier for stem height than diameter, which is consistent with the ecological viewpoint of canopy competition. The implications of the results for breeding and silviculture are discussed.