BIPOLAR HETEROTHALLISM IN TAPESIA-YALLUNDAE FOR THE 2 VARIETIES OF THE ANAMORPH PSEUDOCERCOSPORELLA-HERPOTRICHOIDES

Apothecia or apothecial primordia of Tapesia yallundae were produced i n vitro on wheat straw incubated under artificial light between 5 and 12 degrees C. The genetic control of the sexual reproduction was initi ally investigated by pairwise combination of single-ascospore strains obtained from field-collected apothecia, representatives of two asci o f the anamorph Pseudocercosporella herpotrichoides var. herpotrichoide s and one ascus of the anamorph P. h. var. acuformis. Subsequently, tw o compatible single-ascospore strains of each variety of P. herpotrich oides were paired with isolates from lesions on plants collected in a naturally infested field in Belgium. The patterns of pairings between isolates of P. h. var. herpotrichoides in which mature apothecia were formed is consistent with bipolar heterothallism as the mechanism cont rolling sexual compatibility in T. yallundae. The pairings between sin gle-ascospore strains of P. h. var. acuformis produced only apothecial primordia, but also in a manner typical of bipolar heterothallism. So me primordia developed into mature but sterile apothecia in one of tho se pairings. In further tests under similar conditions no apothecia or primordia were obtained in the pairings between single-ascospore stra ins and isolates of the latter variety, suggesting that ascocarp induc tion in P. h. var. acuformis requires some hitherto unknown environmen tal trigger, while P. h. var. herpotrichoides is less demanding. No pa irings between strains of the two varieties of P. herpotrichoides prod uced apothecia or apothecial primordia.