IDENTIFICATION, CHARACTERIZATION, AND MAPPING OF ECM1, A LOCUS AFFECTING EXTRACELLULAR-MATRIX PRODUCTION AND LESION SIZE IN COCHLIOBOLUS-HETEROSTROPHUS

A mutant of Cochliobolus heterostrophus lacking the outer layer of ext racellular matrix around its germ tubes and hyphae was obtained by mut agenizing protoplastc;. The mutant not only lacks the outer matrix, bu t also produces much smaller lesions on corn leaves than nonmutant str ains; the area of mutant lesions averages 0.6 mm(2) compared with 5.8 mm(2) for nonmutant lesions. Genetic analysis demonstrated that the fa ilure to produce the outer matrix cosegregates with the reduced lesion size, indicating that the two traits are controlled by the same locus , designated Ecm1 (extracellular matrix deficient). The mutant retains normal growth on media and normal abilities to germinate, form appres soria, and penetrate corn leaves. This indicates that the outer matrix is not necessary for infection prior to entrance of the fungus into t he leaf. It also indicates that the pathogenicity defect in this mutan t is manifested after penetration. To facilitate future tests of wheth er the pathogenicity defect is caused by the lack of the outer matrix, Ecm1 was mapped. Seven markers linked to Ecm1 were found by analysis of amplified fragment length polymorphisms. Ecm1 maps to chromosome 4; the closest markers to Ecm1 are 5 cM distant, which is estimated to r epresent about 115 kb.