Phenotypic variation and quantitative trait locus identification for osmotic potential in an interspecific hybrid inbred F2 poplar pedigree grown in contrasting environments.

Elucidation of the mechanisms of dehydration tolerance in poplar (Populus sp.) trees will permit development of biochemical and molecular indicators to identify dehydration-tolerant genotypes during genetic selection. The objectives of this study were to characterize the degree of phenotypic variation in osmotic potential (a determinant of dehydration tolerance), determine the relationship between osmotic potential at full turgor and relative growth rate, and identify quantitative trait loci (QTL) for osmotic potential in an advanced-generation, interspecific poplar pedigree established in contrasting environments. A three-generation, sib-mated black cottonwood (Populus trichocarpa Torr. & Gray) and eastern cottonwood (P. deltoides Bartr.) segregating F(2) family (Family 331) was analyzed at a dry site east of the Cascade Mountain Range (Boardman, OR) and at a wet site west of the mountains (Clatskanie, OR). At the Boardman site, 2-year-old trees (59 clones) were either irrigated everyday (wet) or every other day (dry), whereas 3- and 4-year-old trees (58 clones) at the Clatskanie site were unirrigated. At the Boardman site, the typically narrow range of osmotic potentials exhibited by grandparents and parents was greatly expanded in the F(2) population, spanning from -1.38 to -2.35 MPa under wet conditions, with a similar range under dry conditions (-1.40 to -2.15 MPa). Clones that had osmotic potentials < or = -1.90 MPa generally displayed full maintenance of stem relative growth rates under dry conditions in contrast to clones with osmotic potentials that were < or = -1.60 MPa, in which stem relative growth rates were reduced by an average of 38% in the dry treatment relative to the wet treatment. Although osmotic adjustments of 0.13 to 0.36 MPa were observed in nine out of 59 clones, adjustment typically occurred from relatively high baseline osmotic potentials. The range in osmotic potential at the wetter Clatskanie site at age three was higher (-1.27 to -1.84 MPa) and was further expanded the following year (-1.14 to -1.94 MPa), which had a wetter spring than the previous year, followed by a typically dry July. Seven QTL for osmotic potential were identified that each explained > 7.5% of the variation in osmotic potential. Given that four clones (7%) had osmotic potentials of -2.00 MPa or less and that QTL for osmotic potential have been identified, we suggest that there are opportunities to extend the limit of dehydration tolerance in Populus.

Identification of quantitative trait loci influencing wood property traits in loblolly pine (Pinus taeda L.). III. QTL Verification and candidate gene mapping.

A long-term series of experiments to map QTL influencing wood property traits in loblolly pine has been completed. These experiments were designed to identify and subsequently verify QTL in multiple genetic backgrounds, environments, and growing seasons. Verification of QTL is necessary to substantiate a biological basis for observed marker-trait associations, to provide precise estimates of the magnitude of QTL effects, and to predict QTL expression at a given age or in a particular environment. Verification was based on the repeated detection of QTL among populations, as well as among multiple growing seasons for each population. Temporal stability of QTL was moderate, with approximately half being detected in multiple seasons. Fewer QTL were common to different populations, but the results are nonetheless encouraging for restricted applications of marker-assisted selection. QTL from larger populations accounted for less phenotypic variation than QTL detected in smaller populations, emphasizing the need for experiments employing much larger families. Additionally, 18 candidate genes related to lignin biosynthesis and cell wall structure were mapped genetically. Several candidate genes colocated with wood property QTL; however, these relationships must be verified in future experiments.

Genome anchored QTLs for biomass productivity in hybrid Populus grown under contrasting environments.

Traits related to biomass production were analyzed for the presence of quantitative trait loci (QTLs) in a Populus trichocarpa × P. deltoides F(2) population. A genetic linkage map composed of 841 SSR, AFLP, and RAPD markers and phenotypic data from 310 progeny were used to identify genomic regions harboring biomass QTLs. Twelve intervals were identified, of which BM-1, BM-2, and BM-7 were identified in all three years for both height and diameter. One putative QTL, BM-7, and one suggestive QTL exhibited significant evidence of over-dominance in all three years for both traits. Conversely, QTLs BM-4 and BM-6 exhibited evidence of under-dominance in both environments for height and diameter. Seven of the nine QTLs were successfully anchored, and QTL peak positions were estimated for each one on the P. trichocarpa genome assembly using flanking SSR markers with known physical positions. Of the 3,031 genes located in genome-anchored QTL intervals, 1,892 had PFAM annotations. Of these, 1,313, representing 255 unique annotations, had at least one duplicate copy in a QTL interval identified on a separate scaffold. This observation suggests that some QTLs identified in this study may have shared the same ancestral sequence prior to the salicoid genome duplication in Populus.

INTRASPECIFIC CHLOROPLAST DNA VARIATION AND BIOGEOGRAPHY OF NORTH AMERICAN LIRIODENDRON L. (MAGNOLIACEAE).

Restriction site variation in chloroplast DNA (cpDNA) was surveyed to analyze population dynamics in Liriodendron tulipifera L., a woody angiosperm found in eastern North America. Two cpDNA haplotypes, differing by the presence or absence of five restriction site changes (nucleotide sequence divergence estimated as approximately 0.15%) are geographically structured; 61 widespread populations possess the "northern" haplotype and three isolated populations of central Florida possess the "southern" haplotype. This geographic break in cpDNA distribution corresponds to patterns of geographic distribution revealed by a previous survey of allozyme variation, with the exception that analyses of allozyme data further divided the populations containing the northern cpDNA haplotype into two groups, a widespread upland group and a coastal intermediate group. Analyses of these two independent data sets together support the hypothesis that L. tulipifera survived the glacial advances of the Pleistocene in two distinct refugia, possibly as different taxa, and the intermediate coastal group was putatively formed from recent hybridizations between these entities.