EVIDENCE FOR A HETEROTHALLIC MATING SYSTEM IN TAPESIA-ACUFORMIS USINGBENOMYL SENSITIVITY AND ESTERASE ISOENZYME PROFILES

Ascospore progenies from eight apothecia of Tapesia sp., collected fro m wheat stubble in Belgium and Germany, yielded strains characteristic of T. acuformis and strains characteristic of T. yallundae. The colon y and conidium morphology, esterase isoenzyme profiles and sensitivity to benomyl, diethofencarb and triadimenol of these and of isolates fr om eyespot lesions were compared. Differences were detected amongst th e ascospore offspring from single apothecia using these markers, but a ll strains from a single apothecium were always of a single species of Tapesia. Single-ascospore strains from a single ascus could always be grouped into four pairs using a combination of markers. In vitro pair wise crossings between four representative single-ascospore strains of T. acuformis yielded some mature and fertile apothecia on moist wheat straw inoculated with mycelium and incubated in test tubes under a mi xture of nuv light and daylight at 8-12 degrees C for at least 6 month s. Recombination was observed between the parental esterase zymograms and sensitivities to benomyl, leading to new phenotypes in the progeny . The results of pairing reference single-ascospore strains of both T. acuformis and T. yallundae with 115 isolates from eyespot lesions str engthened the contention that T. acuformis exhibits a two-allele heter othallism and that T. acuformis and T. yallundae are sexually incompat ible species.