The world's oldest and largest Medicago truncatula collection is housed at the South Australian Research and Development Institute (SARDI). We used six simple sequence repeat (SSR) loci to analyse the genetic diversity and relationships between randomly selected individuals from 192 accessions in the core collection. M. truncatula is composed of three subspecies (ssp.): ssp. truncatula, ssp. longeaculeata, and ssp. tricycla. Analysis at the level of six SSR loci supports the concept of ssp. tricycla, all the samples of which showed unique alleles at two loci. Contingency Chi-squared tests were significant between ssp. tricycla and ssp. truncatula at four loci, suggesting a barrier to gene flow between these subspecies. In accessions defined as ssp. longeaculeata, no unique allelic distribution or diagnostic sizes were observed, suggesting this apparent ssp. is a morphological variant of ssp. truncatula. The data also suggest M. truncatula that exhibits unusually wide genotype dispersal throughout its native Mediterranean region, possibly due to animal and trade-related movements. Our results showed the collection to be highly diverse, exhibiting an average of 25 SSR alleles per locus, with over 90% of individuals showing discrete genotypes. The rich diversity of the SARDI collection provides an invaluable resource for studying natural allelic variation of M. truncatula. To efficiently exploit the variation in the SARDI collection, we have defined a subset of accessions (n = 61) that maximises the diversity.

Research Summary• Medicago truncatula (barrel medic) has emerged as a model legume and accession A17 is the reference genotype selected for the sequencing of the genome. In the present study we compare the A17 chromosomal configuration with that of other accessions by examining pollen viability and genetic maps of intraspecific hybrids.• Hybrids derived from crosses between M. truncatula accessions, representative of the large genetic variation within the germplasm collection, were evaluated for pollen viability using Alexander's stain. Genetic maps were generated for the following crosses: SA27063 × SA3054 ( n = 94), SA27063 × A17 ( n = 92), A17 × Borung ( n = 99) and A17 × A20 ( n = 69).• All F 1 individuals derived from crosses involving A17 showed 50% pollen viability or less. Examination of the recombination frequencies between markers of chromosomes 4 and 8 revealed an apparent genetic linkage between the lower arms of these chromosomes in genetic maps derived from A17.• Semisterility and unexpected linkage relationship are both good indicators of a reciprocal translocation. The implications of the A17 distinctive chromosomal rearrangement on studies of genetic mapping, genome sequencing and synteny between species are discussed.New Phytologist (2007) 174 : 299 -303

Powdery mildew isa major disease of barley (Hordeum vulgare L.) for which breeders have traditionally relied on dominant, pathogen race-specific resistance genes for genetic control. Directional selection pressures in extensive monocultures invariably result in such genes being overcome as the pathogen mutates to evade recognition. This has led to a widespread reliance on fungicides and a single broad-spectrum recessive resistance provided by the mlo gene. The range of resistance genes and alleles found in wild crop relatives and landraces has been reduced in agricultural cultivars through an erosion of genetic diversity during domestication and selective breeding. Three novel major-effect adult plant resistance (APR) genes from landraces, designated Resistance to Blumeria graminis f. sp. hordei (Rbgh1 to Rbgh3), were identified in the terminal regions of barley chromosomes 5HL, 7HS, and 1HS, respectively. The phenotype of the new APR genes showed neither pronounced penetration resistance, nor the spontaneous necrosis and mesophyll cell death typical of mlo resistance, nor a whole epidermal cell hypersensitive response, typical of racespecific resistance. Instead, resistance was localized to the site of attempted penetration in an epidermal cell and was associated with cell wall appositions and cytosolic vesicle-like bodies, and lacked strong induction of reactive oxygen species. The APR genes exhibited differences in vesicle-like body sizes, their distribution, and the extent of localized 3,3-diaminobenzidine staining in individual doubled haploid lines. The results revealed a set of unique basal penetration resistance genes that offer opportunities for combining different resistance mechanisms in breeding programs for robust mildew resistance.

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