Association of AFLP markers with downy mildew resistance in autotetraploidalfalfa

The amplified fragment length polymorphism (AFLP) assay is an efficient met hod for the identification of molecular markers useful in the improvement o f numerous crop species. The identification of AFLP markers linked to disea se resistance genes has been shown in segregating populations from crosses of inbred lines. The development of inbred lines in alfalfa is not possible , but existing breeding programs have produced populations selected for res istance to a single pest. Two such populations, UC-123 and UC-143, differin g only in selection for resistance to downy mildew (Peronospora trifoliorum de Bary) isolate I-8, were used in this study. Thirty-six resistant plants from UC-143, and 36 susceptible plants from UC-123 were screened for DNA p olymorphisms using fourteen AFLP primer combinations. Four AFLP fragment ma rkers, ACACTC(208), ACACTC(150), ACACAT(216) and ACACTC(486), were found to be significantly associated with disease susceptibility or resistance. Res istant and susceptible plants were crossed in a diallel scheme and the prog eny were screened for resistance to P. trifoliorum isolate I8. Two of the A FLP markers, ACACTC(208) and ACACTC(486) were significantly associated with resistance in the F-1 and S-1 progeny. The utilization of two populations, comprised of 36 resistant and 36 susceptible plants, for the identificatio n of DNA fragments associated with disease resistance proved successful. Se venty-two plants is a very manageable number and provides a starting point for further refinement of marker-trait associations.