Simple sequence repeats for germplasm analysis and mapping in maize

Simple sequence repeats (SSRs) are a relatively new class of DNA markers consisting of short runs of tandemly repeated sequence motifs evenly distributed throughout eukaryotic genomes. Owing to the high rate of variation in the number of repeat units, the polymorphism level shown by SSRs is high. Furthermore, they are easy to analyze by means of the polymerase chain reaction, using flanking unique sequence primers. In order to establish the utility of SSR markers for genetic mapping and for the analysis of corn germplasm, corn genomic libraries were constructed and screened for clones containing dinucleotide and trinucleotide repeats. One hundred and fifty clones were isolated and 34 of them were used in this study to analyze 15 (AG)(n) repeats, 15 (AC)(n) repeats, and 4 trinucleotide repeats. Twelve corn inbred lines, representing 87% of the RFLP alleles present in a collection of public corn cultivars, were used to assess the information content of the SSR markers. The expected heterozygosity of each SSR marker was compared with the expected heterozygosity of 100 different RFLP markers. The stability of SSRs was also tested through segregation analysis on an existing mapping population.