V. Storme

Identification of AFLP molecular markers for resistance against Melampsora larici-populina in Populus

We have identified AFLP markers tightly linked to the locus conferring resistance to the leaf rust Melampsora larici-populina in Populus. The study was carried out using a hybrid progeny derived from an inter-specific, controlled cross between a resistant Populus deltoides female and a susceptible P. nigra male. The segregation ratio of resistant to susceptible plants suggested that a single, dominant locus defined this resistance. This locus, which we have designated Melampsora resistance (Mer), confers resistance against E1, E2, and E3, three different races of Melampsora larici-populina. In order to identify molecular markers linked to the Mer locus we decided to combine two different techniques: (1) the high-density marker technology, AFLP, which allows the analysis of thousands of markers in a relatively short time, and (2) the Bulked Segregant Analysis (BSA), a method which facilitates the identification of markers that are tightly linked to the locus of interest. We analyzed approximately 11,500 selectively amplified DNA fragments using 144 primer combinations and identified three markers tightly linked to the Mer locus. The markers can be useful in current breeding programs and are the basis for future cloning of the resistance gene.

Intraspecific and interspecific genetic and phylogenetic relationships in the genus Populus based on AFLP markers

Although Populus has become the model genus for molecular genetics and genomics research on forest trees, genetic and phylogenetic relationships within this genus have not yet been comprehensively studied at the molecular level. By using 151 AFLP® (AFLP® is a registered trademark of Keygene) markers, 178 accessions belonging to 25 poplar species and three interspecific hybrids were analyzed, using three accessions belonging to two willow species as outgroups. The genetic and phylogenetic relationships were generally consistent with the known taxonomy, although notable exceptions were observed. A dendrogram as well as a single most parsimonious tree, ordered the Populus sections from the oldest Leuce to the latest Aigeiros, a pattern consistent with their known evolutionary relationships. A close relationship between Populus deltoides of the Aigeiros section and species of the Tacamahaca section was observed and, with the exception of Populus wilsonii, between the species of the Leucoides, Tacamahaca, and Aigeiros sections. Populus nigra was clearly separated from its consectional P. deltoides, and should be classified separately from P. deltoides. The AFLP profiles pointed out to the lack of divergence between some species and revealed that some accessions corresponded with interspecific hybrids. This molecular study provides useful information about genetic relationships among several Populus species and, together with morphological descriptions and crossability, it may help review and update systematic classification within the Populus genus.

Fine Mapping and Identification of Nucleotide Binding Site/Leucine-Rich Repeat Sequences at the MER Locus in Populus deltoides 'S9-2'

ABSTRACT Melampsora larici-populina is the most damaging leaf pathogen for poplar in Europe. Previous genetic analyses have revealed both qualitative and quantitative resistance to this fungus. As a starting point for positional cloning of the gene or genes conferring qualitative resistance to M. larici-populina races E1, E2, and E3, a local genetic map of the Melampsora resistance (MER) locus was constructed based on amplified fragment length polymorphism (AFLP) markers. Eleven AFLP markers were identified by bulked segregant analysis. These markers were used to identify 17 recombinants at the MER locus, from a total of 512 progenies derived from three interspecific crosses involving the same resistant female parent, Populus deltoides S9-2. The local genetic map covered a 3.4-centimorgan interval encompassing the target locus. Sequence analysis of these AFLP markers revealed similarities to the nucleotide binding site/leucine-rich repeat class of disease resistance genes.

Genomic regions involved in productivity of two interspecific poplar families in Europe. 1. Stem height, circumference and volume

Interspecific hybrids of Populus species are known for their superior growth. In this study, we examined the effect of the genetic background and contrasting environmental conditions on growth and searched for quantitative trait loci (QTL) affecting growth traits. To this end, two hybrid poplar families resulting from controlled crosses, Populus deltoides ‘S9-2’u2009×u2009P. nigra ‘Ghoy’ (Du2009×u2009N, 180 F1) and P. deltoides ‘S9-2’u2009×u2009P. trichocarpa ‘V24’ (Du2009×u2009T, 182 F1), were grown at two contrasting sites, Northern Italy and Central France. At the end of the second growing season, tree dimensions (stem height, circumference, and volume) were assessed. The performances of both families significantly differed within and between sites. Tree volume was significantly larger at the Italian site as compared to the French site. Genotype by environment interactions were significant but low for both families and for all growth traits. Tight correlations among the individual growth traits indicated that there may be a common genetic mechanism with pleiotropic effects on these growth traits. In line with previous studies, linkage groups I, VII, IX, X, XVI, XVII, and XIX appeared to have genomic regions with the largest effects on growth traits. This study revealed that (1) both families have high potential for selection of superior poplar hybrids due to the pronounced heterosis (hybrid vigor) and the large genetic variability in terms of growth and (2) the choice of site is crucial for poplar cultivation.

Genomic regions involved in productivity of two interspecific poplar families in Europe. 2. Biomass production and its relationships with tree architecture and phenology

Short-rotation coppice of hybrid poplar is a promising renewable feedstock for biofuel production. Breeding for high biomass in short-rotation coppice has started only recently. Two hybrid poplar families were grown at two sites in Europe and phenotyped for a variety of biomass-related traits (1) to examine the extent of phenotypic and genetic variation in biomass production, ramification, resprouting, and phenology, (2) to search for genomic regions involved in productivity, and (3) to determine the effect of the environment on the expression of these traits. The performance of both families differed within and among sites. A pronounced heterosis was observed in most cases. Moderate to high heritability values were found. Seventeen quantitative trait loci (QTL) for biomass production, 13 for ramification, ten for resprouting, 21 for bud burst, and ten for bud set were identified. Genetic correlations and QTL colocation showed that high wood production was associated with high allocation of wood into branches and with high production of resprouts after coppicing. Correlations and QTL colocation between biomass production and phenology traits were weak. Our study provides valuable information on genomic regions involved in biomass production, ramification, and phenology and on phenotypic and genetic relationships among these three trait categories.

THE USE OF BULKED SEGREGANT ANALYSIS TO IDENTIFY AFLP"" MOLECULAR MARKERS CLOSELY LINKED TO MELAMPSORA LARICI-POPULINA RESISTANCE IN POPULUS

We have identified three AFLP markers tightly linked to the locus conferring resistance to the leaf rust Melampsora larici-populina in Populus by Bulked Segregant Analysis. The study was carried out using a hybrid progeny derived from an inter-specific, controlled cross between a resistant Populus deltoides female and a susceptible P. nigra male. The segregation ratio of resistant to susceptible plants suggested that a single, dominant locus (Mer) defined the resistance to Melampsora.