Bipolar heterothallism in B-group isolates of Leptosphaeria maculans

Pairwise combinations were carried out between the eight isolates originating from each of three asci of the B-group of Leptosphaeria maculans . Some of the pairings produced mature pseudothecia when incubated on wheat straws at 18°C and under a 12-h photoperiod of blue light. 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 ( MAT1 ) with two alleles MAT 1–1 and MAT 1–2. Under these same conditions, each individual isolate from an A-group ascus originating from Brassica juncea produced pseudothecia with one or other of two compatible A-group testers originating from B. napus . However, attempts to mate these eight A-group single-ascospore isolates with two B-group single-ascospore isolates of opposite mating types remained unsuccessful. This work is the first successful report of in vitro sexual reproduction between B-group isolates, and provides evidence for bipolar heterothallism in B-group isolates of L. maculans . The implications of this mating protocol for genetic studies of B-group isolates and the consequences of this work for the taxonomy of L. maculans are discussed.