Fine structure and instability of the Ml-a locus in barley

There are many naturally occurring variants at the Ml-a locus in barley that confer resistance to the powdery mildew fungus Erysiphe graminis f. sp. hordei. Since the Ml-a locus is bracketed by Hor-1 and Hor-2, genes that encode storage proteins in the endosperm, the Ml-a locus is amenable to fine structure analysis. Rare susceptible recombinants, as judged by exchange of flanking markers, were recovered in F3 families from the Ml-a10 X Ml-a1, Ml-a1 X Ml-a15 and Ml-a6 X Ml-a13 crosses. Some susceptible recombinants were recovered from the Ml-a6 X Ml-a13 cross that did not fit the expected F3 family segregation ratios. The Ml-a6/Ml-a13 recombinants often reverted to resistance in subsequent generations. No recombinants were recovered in the reciprocal cross, Ml-a13 X Ml-a6. The possibility of a transposable element and a possible linear order of six "alleles" at the Ml-a locus is discussed.