No abstract

Microsatellites or simple sequence repeats (SSRs) are highly polymorphic, codominant markers that have great value for the construction of genetic maps, comparative mapping, population genetic surveys, and paternity analyses. Here, we report the development and testing of a set of SSR markers derived from shotgun sequencing from Populus trichocarpa Torr. & A. Gray, a nonenriched genomic DNA library, and bacterial artificial chromosomes. Approximately 23% of the 1536 genomic clones and 48% of the 768 bacterial artificial chromosome subclones contained an SSR. Of the sequences containing an SSR, 72.4% contained a dinucleotide, 19.5% a trinucleotide, and 8.1% a tetranucleotide repeat unit; 26.6% of the sequences contained multiple SSR motifs in a complex or compound repeat structures. A survey of the genome sequence database revealed very similar proportional distribution, indicating that our limited rapid, shallow sequencing effort is representative of genome-wide patterns. In total, 492 primer pairs were designed and these yielded 77 markers that were mapped in an F2 pedigree, including 26 that were sufficiently informative to be included in a Populus framework map. SSRs with GC-rich motifs mapped at a significantly higher frequency than expected, although AT-rich SSRs accounted for the majority of mapped markers due to their higher representation in the genome. SSR markers developed from P. trichocarpa showed high utility throughout the genus, with amplification rates in excess of 70% for all Populus species tested. Finally, at least 30% of the markers amplified in several willow species, suggesting that some of these SSRs will be transferable across genera.

No abstract

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.

Chemical wood property traits were analyzed for the presence of quantitative trait loci (QTLs) in a three-generation outbred pedigree of loblolly pine ( Pinus taeda L.). These traits were assayed using pyrolysis molecular beam mass spectrometry and include mass spectrum peak intensities associated with carbohydrates, alpha-cellulose and hemicellulose sugars, and lignin. Models for projection to latent structures (PLS) were used to also estimate the chemical composition of cell walls (i.e., alpha-cellulose, galactan and lignin) from mass spectrum data using multivariate regression. Both earlywood and latewood fractions from the fifth annual ring were analyzed for each trait. An interval mapping approach designed for an outbred pedigree was used to estimate the number of QTLs, the magnitude of QTL effects, and their genomic position. Eight unique QTLs influencing cell wall chemistry were detected from multiple peak intensities and/or PLS estimates using the one- and two-QTL models. Significant differences in chemical contents were observed among the populations from North Carolina vs Oklahoma, and results from QTLxenvironment analyses suggest that QTLs interact with environmental location. QTLs should be verified in larger experiments and in different genetic and environmental backgrounds. QTL mapping will help towards eventually identifying genes having a major effect on chemical wood properties.