P. Cosson

Mapping With a Few Plants: Using Selective Mapping for Microsatellite Saturation of the Prunus Reference Map

The concept of selective (or bin) mapping is used here for the first time, using as an example the Prunus reference map constructed with an almond × peach F2 population. On the basis of this map, a set of six plants that jointly defined 65 possible different genotypes for the codominant markers mapped on it was selected. Sixty-three of these joint genotypes corresponded to a single chromosomal region (a bin) of the Prunus genome, and the two remaining corresponded to two bins each. The 67 bins defined by these six plants had a 7.8-cM average length and a maximum individual length of 24.7 cM. Using a unit of analysis composed of these six plants, their F1 hybrid parent, and one of the parents of the hybrid, we mapped 264 microsatellite (or simple-sequence repeat, SSR) markers from 401 different microsatellite primer pairs. Bin mapping proved to be a fast and economic strategy that could be used for further map saturation, the addition of valuable markers (such as those based on microsatellites or ESTs), and giving a wider scope to, and a more efficient use of, reference mapping populations.

Candidate genes and QTLs for sugar and organic acid content in peach [Prunus persica (L.) Batsch]

Abstract.The identification of genes involved in variation of peach fruit quality would assist breeders in creating new cultivars with improved fruit quality. Major genes and quantitative trait loci (QTLs) for physical and chemical components of fruit quality have already been detected, based on the peach [Prunus persica (L.) Batsch] cv. Ferjalou Jalousia® (low-acid peach) × cv. Fantasia (normally-acid nectarine) F2 intraspecific cross. Our aim was to associate these QTLs to structural genes using a candidate gene/QTL approach. Eighteen cDNAs encoding key proteins in soluble sugar and organic acid metabolic pathways as well as in cell expansion were isolated from peach fruit. A single-strand conformation polymorphism strategy based on specific cDNA-based primers was used to map the corresponding genes. Since no polymorphism could be detected in the Ferjalou Jalousia® × Fantasia population, gene mapping was performed on the almond [Prunus amygdalus (P. dulcis)] cv. Texas × peach cv. Earlygold F2 interspecific cross from which a saturated map was available. Twelve candidate genes were assigned to four linkage groups of the peach genome. In a second step, the previous QTL detection was enhanced by integrating anchor loci between the Ferjalou Jalousia® × Fantasia and Texas × Earlygold maps and data from a third year of trait assessment on the Ferjalou Jalousia® × Fantasia population. Comparative mapping allowed us to detect a candidate gene/QTL co-location. It involved a cDNA encoding a vacuolar H+-pyrophosphatase (PRUpe;Vp2) that energises solute accumulation, and QTLs for sucrose and soluble solid content. This preliminary result may be the first step in the future development of marker-assisted selection for peach fruit sucrose and soluble solid content.