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The Chinese chestnut ( Castanea mollissima) carries resistance to Cryphonectria parasitica, the fungal pathogen inciting chestnut blight. The pathogen, introduced from Asia, devastated the American chestnut ( Castanea dentata) throughout its native range early in the twentieth century. A highly informative genetic map of Chinese chestnut was constructed to extend genomic studies in the Fagaceae and to aid the introgression of Chinese chestnut blight resistance genes into American chestnut. Two mapping populations were established with three Chinese chestnut parents, 'Mahogany', 'Nanking', and 'Vanuxem', totaling 337 progeny. The transcriptome-based genetic map was created with 329 simple sequence repeat and 1,064 single nucleotide polymorphism markers all derived from expressed sequence tag sequences. Genetic maps for each parent were developed and combined to establish 12 consensus linkage groups spanning 742 cM, providing the the most comprehensive genetic map for a Fagaceae species to date. Over 75 % of the mapped markers from the Chinese chestnut consensus genetic map were placed on the physical map using overgo hybridization, providing a fully integrated genetic and physical map resource for Castanea spp. About half (57 %) of the Chinese chestnut genetic map could be assigned to regions of segmental homology with 58 % of the peach ( Prunus persica) genome assembly. A three quantitative trait loci (QTL) model for blight resistance was verified using the new genetic markers and an existing interspecies ( C. mollissima × C. dentata) F mapping population. Two of the blight resistance QTLs in chestnut shared synteny with two QTLs for powdery mildew resistance in peach, indicating the potential conservation of disease resistance genes at these loci. [ABSTRACT FROM AUTHOR]

A genome-wide framework physical map of peach was constructed using high-information content fingerprinting (HICF) and FPC software. The resulting HICF assembly contained 2,138 contigs composed of 15,655 clones (4.3× peach genome equivalents) from two complementary bacterial artificial chromosome libraries. The total physical length of all contigs is estimated at 303 Mb or 104.5% of the peach genome. The framework physical map is anchored on the Prunus genetic reference map and integrated with the peach transcriptome map. The physical length of anchored contigs is estimated at 45.0 Mb or 15.5% of the genome. Altogether, 2,636 markers, i.e., genetic markers, peach unigene expressed sequence tags, and gene-specific and overgo probes, were incorporated into the physical framework and supported the accuracy of contig assembly. [ABSTRACT FROM AUTHOR]

Amplified fragment length polymorphisms (AFLP) and targeted simple sequence repeats (SSR) were employed to assess genetic similarity of North American apricots having natural resistance to plum pox virus (PPV) within diversified germplasm including six nondomesticated apricot species. On a dendrogram constructed from 231 AFLP loci, the position of the North American cultivars reflects relatedness to the European apricots and introgression of non-European germplasm as well. The occurrence of diagnostic AFLP markers supports an introgression of Chinese germplasm into the North American PPV resistant assortment and supports a different breeding history for ‘Stark Early Orange’ (SEO) and Goldrich-Harlayne lineages. Five SSR loci linked to the PPV resistance region on G1 provided evidence that the investigated lineages (SEO and ‘Harlayne’–‘Goldrich’) have the same or related source of resistance introduced presumably from Northern China. Possible introgression of genetic material from nondomesticated apricots P. mandshurica sp, P. sibirica var. davidiana and P. mume sp. was detected and discussed. [ABSTRACT FROM AUTHOR]

Plum pox virus (sharka; PPV) can cause severe crop loss in economically important Prunus species such as peach, plum, apricot, and cherry. Of these species, certain apricot cultivars (‘Stark Early Orange’, ‘Goldrich’, ‘Harlayne’) display significant levels of resistance to the disease and are the genetic substrate for studies of several xlaboratories working cooperatively to genetically characterize and mark the resistance locus or loci for marker-assisted breeding. The goals of the work presented in this communication are the characterization of the genetics of PPV resistance in ‘Stark Early Orange’ and the development of co-dominant molecular markers for marker-assisted selection (MAS) in PPV resistance breeding. We present the first genetic linkage map for an apricot backcross population of ‘Stark Early Orange’ and the susceptible cultivar ‘Vestar’ that segregates for resistance to PPV. This map is comprised of 357 loci (330 amplified fragment length polymorphisms (AFLPs), 26 simple sequence repeats (SSRs), and 1 morphological marker for PPV resistance) assigned to eight linkage groups. Twenty-two of the mapped SSRs are shared in common with genetic reference map for Prunus (T × E; Joobeur et al. 1998) and anchor our apricot map to the general Prunus map. A PPV resistance locus was mapped in linkage group 1 and four AFLP markers segregating with the PPV resistance trait, identified through bulk segregant analysis, facilitated the development of SSRs in this region. [ABSTRACT FROM AUTHOR]

Thirty SSR primer combinations, developed from peach SSR-enriched genomic libraries and BAC libraries of peach [Prunus persica (L.) Batsch.], were tested for cross amplification with 74 apricot (Prunus armeniaca L.) germplasm accessions. Twelve primer pairs amplified 14 polymorphic SSR loci useful for discriminating most apricot cultivars, as well as for investigating patterns of variation in apricot germplasm. Levels of polymorphism were higher than the levels described using other codominant marker systems (i.e., isozymes, RFLP markers). Overall, 107 alleles were identified, and all but 11 accessions were unambiguously discriminated. Genetic differentiation of native germplasm into traditional ecogeographical groups was low, with a high level of genetic identity (> 0.75) between the groups. However, neighbor joining cluster analysis of marker distances between cultivars reflected the complex history of apricot domestication, producing groupings not evidently based on the geographical origin of the cultivars. Distant positioning of Chinese cultivars on UPGMA and neighbor joining dendrograms supports the authors' consideration of Chinese apricots as subspecies, Prunus armeniaca var.ansu Maxim., rather than a separate species. [ABSTRACT FROM AUTHOR]