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White clover (Trifolium repens L.) is an important temperate pasture legume that plays a key role as a companion to grass species, such as perennial ryegrass (Lolium perenne L.). Due to the outbreeding nature of white clover, cultivars are highly heterogeneous. Genetic diversity was assessed using 16 elite cultivars from Europe, North and South America, Australia, and New Zealand. Fifteen simple sequence repeat markers that detect single, codominant polymorphic genetic loci were selected for the study. The genetic relationships among individuals were compared using phenetic clustering, and those among cultivars were compared using nonmetric multidimensional scaling. Intrapopula tion variability exceeded interpopulation variability, with substantial overlap among populations and weak interpopula tion differentiation. No obvious or significant differentiation was observed on the basis of morphology or geographic origin of the cultivars. The number of parental genotypes used to derive each cultivar was not a major determinant of genome-wide genetic diversity. The outcomes of this assessment of genetic variation in elite white clover germplasm pools have important implications for the feasibility of molecular marker-based cultivar discrimination, and will be used to assist the design of linkage disequilibrium mapping strategies for marker-trait association.Key words: white clover, allotetraploid, genetic diversity, polymorphism, simple sequence repeat, cluster analysis, nonmetric multidimensional scaling.

The causative organism of crown rust in ryegrasses (Puccinia coronata f.sp. lolii) is an obligate biotroph that causes significant economic losses within the temperate grazing industries of dairy, meat, and wool production. This study reports on the development, transferability, and utility of gene-associated simple sequence repeat (SSR) molecular markers for crown rust. Analysis of 1100 expressed sequence tag (EST) sequences from a urediniospore-derived cDNA library detected 55 SSR loci. The majority of EST-SSR arrays contained perfect trinucleotide repeats with consistently low repeat numbers, and the motifs (ACC)n and (CAT)n were most commonly represented. DNA extraction from single pustules, in conjunction with multiple displacement amplification, provided the basis for PCR-based screening to evalu- ate genetic marker performance. An example of the identification of intraspecific genetic diversity was obtained from the analysis of 16 P. coronata isolates originating from the United Kingdom, Australia, New Zealand, and Japan. A subset of 12 robust EST-SSR markers was informative for determination of pathogen diversity within and between these localities. It was also demonstrated that crown rust EST-SSR markers were capable of cross-amplification in closely related fungal taxa (Puccinia spp.) and filamentous fungi within the Ascomycota. Resume : L'agent causal de la rouille couronnee chez les ray-grass (P. coronata f. sp. lolii) est un biotrophe obliga- toire qui occasionne des pertes economiques importantes dans les zones temperees de production fourragere pour les fins de production de lait, de viande et de laine. Cette etude rapporte le developpement, la transportabilite et l'utilite de marqueurs microsatellites (SSR) associes a des genes chez la rouille couronnee. L'analyse de 1 100 etiquettes de sequences exprimees (EST) issus d'une banque d'ADNc provenant d'uredospores a decele 55 SSR. La plupart de ces EST-SSR contenaient des repetitions trinucleotidiques parfaites dont le nombre de repetitions etait constamment faible et chez lesquelles les motifs (ACC)n et (CAT)n etaient les plus communs. L'extraction d'ADN de pustules individuels, en conjonction avec une amplification a deplacement multiple (MDA), a rendu possible un crible PCR pour evaluer la performance des marqueurs genetiques. Un exemple de detection de diversite genetique intraspecifique a ete obtenu en analysant 16 isolats du P. coronata provenant du Royaume-Uni, de l'Australie, de la Nouvelle-Zelande et du Japon. Un sous-ensemble de 12 marqueurs EST-SSR robustes s'est avere utilisable chez des taxons fongiques proches (Puccinia spp.) ainsi que chez des champignons filamenteux appartenant aux ascomycetes. Mots cles : Puccinia coronata, microsatellite, etiquette de sequence exprimee, uredospore, diversite genetique, pathogene.