Microsatellite retrieval in lettuce (Lactuca sativa L.)

By using enriched genomic libraries, microsatellite-containing sequences were isolated from lettuce (Lactuca sativa) with high efficiency. With this approach, a sizeable fraction (up to 55%) of the clones contained a microsatellite. In about half of these clones, primers could be designed for PCR amplification of the microsatellite. This yielded 28 primer sets amplifying unambiguously scorable products, of which 26 showed polymorphisms in a test set of six lettuce varieties. Practically all microsatellite-amplifying primer sets yielded products in lettuce's nearest relative, L. serriola, but only half of the primer sets yielded products in the more distant species L. saligna and L. virosa. An average polymorphism information content (PIC) value of 0.55 and an average number of 3.5 alleles per locus were in the normal range for a self-fertilizing species like lettuce. In addition, the incidental cloning of a microsatellite-containing repeat family, apparently specific for Lactuca, is reported and the implications for the efficient retrieval of single-locus microsatellite sequences are discussed. The microsatellite loci isolated will be useful for distinguishing lettuce cultivars and for screening diversity of genetic resources.