Plant Genome III Conference Plant Genome III Conference
Simple sequence repeats (SSRS) provide the basis for extremely polymorphic genetic marker system. Hexaploid bread wheat (Triticum aestivum) shows relatively low levels of Restriction Fragment Length Polymorphism and, therefore, it is desirable to develop PCR-based marker systems utilizing SSR length variation. We have initiated a project to isolate and characterize SSRs from the wheat genome. Small-insert genomic libraries have been screened for six different types of simple sequence repeat (GA, CA, GAA, CAA, ACG, CAG). Most of the SSRs isolated have contained (GA)n or (CA)n repeats. To date we have assayed PCR primers against approximately 70 SSRS, on wheat varieties and aneuploid stocks. It has been possible to assign approximately 25% of these SSRs to particular wheat chromosomes, Significant problems with repetitive sequences have been encountered, whereby SSR flanking primers do not produce discrete bands on agarose or polyacrylamide gels. This problem is probably due to the high proportion of repetitive sequences in cereal genomes, and it may be necessary to modify cloning procedures to allow preferential isolation of low copy-number sequences. We are currently assaying the extent of SSR repeat length variation and devising cloning strategies for circumventing the problems due to repetitive sequences. Once the current set of SSRs has been fully analysed, we should be able to comment on the frequency of microsatellites in the wheat genome and their utility for mapping in hexaploid wheat and other cereals.