Genetic Variation of Morphological Traits and Transpiration in an Apple Core Collection under Well-Watered Conditions: Towards the Identification of Morphotypes with High Water Use Efficiency

Water use efficiency (WUE) is a quantitative measurement which improvement is a major issue in the context of global warming and restrictions in water availability for agriculture. In this study, we aimed at studying the variation and genetic control of WUE and the respective role of its components (plant biomass and transpiration) in a perennial fruit crop. We explored an INRA apple core collection grown in a phenotyping platform to screen one-year-old scions for their accumulated biomass, transpiration and WUE under optimal growing conditions. Plant biomass was decompose into morphological components related to either growth or organ expansion. For each trait, nine mixed models were evaluated to account for the genetic effect and spatial heterogeneity inside the platform. The Best Linear Unbiased Predictors of genetic values were estimated after model selection. Mean broad-sense heritabilities were calculated from variance estimates. Heritability values indicated that biomass (0.76) and WUE (0.73) were under genetic control. This genetic control was lower in plant transpiration with an heritability of 0.54. Across the collection, biomass accounted for 70% of the WUE variability. A Hierarchical Ascendant Classification of the core collection indicated the existence of six groups of genotypes with contrasting morphology and WUE. Differences between morphotypes were interpreted as resulting from differences in the main processes responsible for plant growth: cell division leading to the generation of new organs and cell elongation leading to organ dimension. Although further studies will be necessary on mature trees with more complex architecture and multiple sinks such as fruits, this study is a first step for improving apple plant material for the use of water.     

Relationship between inbreeding depression and inbreeding coefficient in maritime pine (Pinus pinaster)

The relationship between inbreeding depression and inbreeding coefficient (F) for several important traits was investigated in an 11-year trial of maritime pine (Pinus pinaster). Five levels of inbreeding (F=0; 0.125; 0.25; 0.5; 0.75) were obtained in a mating design involving ten plus-trees, or their progenies, as parents (total of 51 families). For F=0.75, the mean inbreeding depressions were 27% for height, 37% for circumference at breast height (63% for bole volume), 23% for basal straightness (better straightness of the inbred trees), and 89% for female fertility (number of cones). Large differences were observed among inbred families for the same level of inbreeding. The evolution of depression with F was more or less linear, depending on the traits. Significant differences among F-levels appeared very early for height (from 5-years of age). Inbreeding depression was much more expressed during unfavorable years than during favorable years for yearly height growth. When compared with other Pinus species, maritime pine appears to be less affected by inbreeding, especially for the percentage of filled seeds and general vigor. A reduced genetic load in maritime pine may result from the evolutionary history of the species and its scattered distribution.     

Electron tomography of frozen-hydrated isolated triad junctions

Cryoelectron microscopy and tomography have been applied for the first time to isolated, frozen-hydrated skeletal muscle triad junctions (triads) and terminal cisternae (TC) vesicles derived from sarcoplasmic reticulum. Isolated triads were selected on the basis of their appearance as two spherical TC vesicles attached to opposite sides of a flattened vesicle derived from a transverse tubule (TT). Foot structures (ryanodine receptors) were resolved within the gap between the TC vesicles and TT vesicles, and some residual ordering of the receptors into arrays was apparent. Organized dense layers, apparently containing the calcium-binding protein calsequestrin, were found in the lumen of TC vesicles underlying the foot structures. The lamellar regions did not directly contact the sarcoplasmic reticulum membrane, thereby creating an approximately 5-nm-thick zone that potentially constitutes a subcompartment for achieving locally elevated [Ca(2+) ] in the immediate vicinity of the Ca(2+)-conducting ryanodine receptors. The lumen of the TT vesicles contained globular mass densities of unknown origin, some of which form cross-bridges that may be responsible for the flattened appearance of the transverse tubules when viewed in cross-section. The spatial relationships among the TT membrane, ryanodine receptors, and calsequestrin-containing assemblage are revealed under conditions that do not use dehydration, heavy-metal staining, or chemical fixation, thus exemplifying the potential of cryoelectron microscopy and tomography to reveal structural detail of complex subcellular structures.     

Development of molecular markers linked to the 'Fiesta' linkage group 7 major QTL for fire blight resistance and their application for marker-assisted selection.

A fire blight resistance QTL explaining 34.3%-46.6% of the phenotypic variation was recently identified on linkage group 7 of apple cultivar 'Fiesta' (F7). However, markers flanking this QTL were AFLP and RAPD markers unsuitable for marker-assisted selection (MAS). Two RAPD markers bracketing the QTL have been transformed into SCAR (sequence-characterized amplified region) markers, and an SSR marker specific for the region was developed. Pedigree analysis of 'Fiesta' with these markers enabled tracking of the F7 QTL allele back to 'Cox's Orange Pippin'. Stability of the effect of this QTL allele in different backgrounds was analyzed by inoculating progeny plants of a cross between 'Milwa', a susceptible cultivar, and '1217', a moderately resistant cultivar, and a set of cultivars that carry or lack the allele conferring increased fire blight resistance. Progenies and cultivars that carried both markers were significantly more resistant than those that did not carry both markers, indicating high stability of the F7 QTL allele in different backgrounds. This stability and the availability of reproducible markers bracketing the QTL make this locus promising for use in MAS.     

A QTL detected in an interspecific pear population confers stable fire blight resistance across different environments and genetic backgrounds

Fire blight, caused by the bacterium Erwinia amylovora (Burrill) Winslow et al., is one of the most serious diseases of pear. The development of pear cultivars with a durable resistance is extremely important for effective control of fire blight and is a key objective of most pear breeding programs throughout the world. We phenotyped seedlings from the interspecific pear population PEAR3 (PremP003, P. × bretschneideri × P. communis) × ‘Moonglow’ (P. communis) for fire blight resistance at two different geographic locations, in France and New Zealand, respectively, employing two local E. amylovora isolates. Using a genetic map constructed with single nucleotide polymorphism (SNP) and microsatellite (SSR) markers previously developed for this segregating population, we detected a major quantitative trait locus (QTL) on linkage group (LG)2 of ‘Moonglow’ (R ² = 12.9–34.4 %), which was stable in both environments. We demonstrated that this QTL co-localizes with another major QTL for fire blight resistance previously detected in ‘Harrow Sweet’ and that the two favorable (i.e., resistant) alleles were not identical by descent. We also identified some smaller effect (R ² = 8.1–14.8 %) QTLs derived from the susceptible parent PEAR3. We propose SNP and SSR markers linked to the large effect QTL on LG2 as candidates for marker-assisted breeding for fire blight resistance in pear.     

Genotyping of pedigreed apple breeding material with a genome-covering set of SSRs: trueness-to-type of cultivars and their parentages

Apple cultivars and breeding lines that represent much of the diversity currently present in major European breeding programmes and are genetically related by their pedigree were examined for the trueness of their identity and parentage by consistency in marker scores using a genome-covering set of 80 microsatellite (SSR) markers and an ??identity-by-descent?? approach. One hundred and twenty-five individuals were validated for the trueness-to-type of both their parents and 49 were validated for one of their parents, their second being unknown (23 individuals) or not available in this study (26 individuals). In addition, 15 individuals for which we lacked one of or both the direct parents were validated by consistency with tested parents of earlier generations. Furthermore, the identity of 28 founder cultivars was validated, their marker scores being consistent with descending cultivars and breeding lines. Four of the eight triploids identified were clearly shown to have arisen from unreduced egg cells. The assumed pedigree of 15 further individuals was found to be incorrect; fully consistent pedigrees were suggested for three of the cultivars. The pedigrees of a further eight individuals were confirmed through inference from the molecular data.