NUCLEAR BEHAVIOR IN HOMOKARYOTIC AND HETEROKARYOTIC FRUITING OF THANATEPHORUS-CUCUMERIS (RHIZOCTONIA-SOLANI) ANASTOMOSIS GROUP-1, SUBGROUP IC

The soilborne plant pathogenic basidiomycetous fungus Thanatephorus cu cumeris (anamorph: Rhizoctonia solani) is composed of at least 12 anas tomosis groups (AG), which seem to be genetically isolated. The sexual ity of T. cucumeris has been studied for several AG, revealing that AG 1 and 4 are heterokaryotic and possess a bipolar mating system, and t hat AG-4 is heterothallic. To clarify whether AG-1 is also heterothall ic, fruiting of heterokaryotic and homokaryotic single spore-derived i solates was studied and nuclear staining with fluorochromes was perfor med during fruiting. Meiosis preceding spore formation was studied for heterokaryotic and derived homokaryotic isolates. A few homokaryotic isolates did not form fruiting bodies. Most homokaryotic isolates form ed basidia, but showed interrupted meiosis. A few homokaryotic isolate s completed meiosis and formed viable basidiospores. The observed meio tic homokaryotic fruiting in T. cucumeris AG-1-IC differed from the mi totic homokaryotic fruiting of most other basidiomycetes. More than si x chromosomes were observed. Heterokaryons formed by pairing of two no nfruiting homokaryons were able to fruit, and the amount of spores dis charged by fruiting heterokaryons was 500 times higher than for fruiti ng homokaryons. From this it was concluded that T. cucumeris AG-1-IC i s heterothallic.