Curly top survey in the Western United States.

Curly top in sugar beet continues to be a challenging disease to control in the western United States. To aid in development of host resistance and management options, the curtovirus species composition was investigated by sampling 246 commercial fields along with nursery and field trials in the western United States. DNA was isolated from leaf samples and the species were identified using species-specific polymerase chain reaction primers for the C1 gene. Amplicons from 79 isolates were also sequenced to confirm identifications. Beet severe curly top virus (BSCTV) and Beet mild curly top virus (BMCTV) were widely distributed throughout the western United States, while only a few isolates of Beet curly top virus (BCTV) were found. In phylogenetic analysis, BSCTV, BMCTV, and BCTV isolates formed distinct groups in the dendrogram. Seven isolates not amplifiable with species-specific primers did amplify with curly top coat protein primers, indicating novel curtovirus species or strains may be present. Given the wide host range of the viruses responsible for curly top, frequent co-infections, and genetic diversity within and among species, establishing better host resistance, and controlling curly top will continue to be a challenge.

Medicago truncatula EST-SSRs reveal cross-species genetic markers for Medicago spp.

Expressed sequence tags (ESTs) are important resources for gene discovery and molecular marker development. From over 147,000 ESTs of Medicago truncatula, we have identified 4,384 ESTs containing perfect simple sequence repeats (EST-SSR) of di-, tri-, tetra- or pentanucleotides. Six hundred sixteen primer pairs (PPs) were designed and screened over a panel of eight genotypes representing six Medicago spp. and subspecies. Nearly, 74% (455) of the PPs produced characteristic SSR bands of expected size length in at least one Medicago species. Four hundred six (89%) of these 455 PPs produced SSR bands in all eight genotypes tested. Only 17 PPs were M. truncatula -specific. High levels of polymorphism (>70%) were detected for these markers in alfalfa, M. truncatula, and other annual medics. About 48% of the reported markers are part of gene transcripts linked to putative functions. Our results indicate that the SSR markers developed from M. truncatula ESTs are valuable genetic markers for the Medicago genus. These markers will be useful in establishing the genomic relationships of M. truncatula to important forage legume crops such as alfalfa and other annual medics.

Isolation of EST-derived microsatellite markers for genotyping the A and B genomes of wheat.

Genetic variation present in 64 durum wheat accessions was investigated by using three sources of microsatellite (SSR) markers: EST-derived SSRs (EST-SSRs) and two sources of SSRs isolated from total genomic DNA. Out of 245 SSR primer pairs screened, 22 EST-SSRs and 20 genomic-derived SSRs were polymorphic and used for genotyping. The EST-SSR primers produced high quality markers, but had the lowest level of polymorphism (25%) compared to the other two sources of genomic SSR markers (53%). The 42 SSR markers detected 189 polymorphic alleles with an average number of 4.5 alleles per locus. The coefficient of similarity ranged from 0.28 to 0.70 and the estimates of similarity varied when different sources of SSR markers were used to genotype the accessions. This study showed that EST-derived SSR markers developed in bread wheat are polymorphic in durum wheat when assaying loci of the A and B genomes. A minumum of ten EST-SSRs generated a very low probability of identity (0.36x10(-12)) indicating that these SSRs have a very high discriminatory power. EST-SSR markers directly sample variation in transcribed regions of the genome, which may enhance their value in marker-assisted selection, comparative genetic analysis and for exploiting wheat genetic resources by providing a more-direct estimate of functional diversity.