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.