Plant Genome III Conference

CHARACTERIZATION AND MAPPING OF SIMPLE SEQUENCE REPEATS (SSR) IN DIPLOID AND TETRAPLOID ALFALFA.

1. USDA-ARS, Beltsville, MD
2. Univ of Georgia, Athens, GA.

Cultivated alfalfa (Medicago sativa) is an autotetraploid (2n=4x=32). However, all three existing alfalfa genetic maps resulted from crosses of diploid alfalfa, In which the parents of the mapping populations were either Cultivated Alfalfa on the Diploid Level (CADL). or wild diploid relatives of alfalfa. These maps showed up to 50% distortion from Mendelian segregation. A study was undertaken to determine ff the level of distortion Is similar In tetraploid alfalfa, and to evaluate the feasibility of SSR markers in alfalfa. SSRs were found to be abundant In the alfalfa genome with an estimated 20,000 (CA)n, (CT)n, (AT)n and (ATT)n repeats In the tetraploid genome. Six SSR loci including two (CT)n, two (CA)n, one (AT)n and one (CTT)n were incorporated into an existing RFLP diploid alfalfa map. The (AT)n locus: MTLEC2A. originated from a Genbank sequence of Medicago truncatula. Of the six loci, two (AFCT32 and MTLEC2A) mapped to the two ends of linkage group 2, two (AFCA11 and AFCT24) mapped into linkage group 3 with more than 5OcM between them, and two (AFCA1 and AFCTT1) mapped into linkage group 4 with more than 40 cM between them. Segregation of these six SSR loci was also analyzed in a tetraploid mapping population. Distortion from Mendelian segregation was found in both populations. The tetraploid population had either three or four alleles in each of the loci examined. Following the segregation of these alleles in the tetraploid F2, mapping population was clear and easy, indicating that SSR markers should be a useful tool for tetraploid mapping.