BIPOLAR HETEROTHALLISM IN B-GROUP ISOLATES OF LEPTOSPHAERIA-MACULANS

Pairwise combinations were carried out between the eight isolates orig inating from each of three asci of the B-group of Leptosphaeria macula ns. Some of the pairings produced mature pseudothecia when incubated o n wheat straws at 18 degrees C and under a 12-h photoperiod of blue li ght. Isolates within a given tetrad could be attributed to one of two sexual compatibility groups, on the basis of combinations of isolates leading to fertile crosses. Within each tetrad, mating type segregated in a 1:1 ratio, suggesting the existence of one mating type gene (MAT 1) with two alleles MAT1-1 and MAT1-2. Under these same conditions, ea ch individual isolate from an A-group ascus originating from Brassica juncea produced pseudothecia with one or other of two compatible A-gro up testers originating from B. napus. However, attempts to mate these eight A-group single ascospore isolates with two B-group single-ascosp ore isolates of opposite mating types remained unsuccessful. This work is the first successful report of in vitro sexual reproduction betwee n B-group isolates, and provides evidence for bipolar heterothallism i n B-group isolates of L. maculans. The implications of this mating pro tocol for genetic studies of B-group isolates and the consequences of this work for the taxonomy of L. maculans are discussed.