Mechanisms of Variation in thanatephorus Auaumeris and t. Pratioolus

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Abstract
Mechanisms of variation occurring in T. cucumeris and T. praticolus have been studied in the laboratory and an attempt made to determine how these mechanisms operate under natural conditions. Variation occurs through mutation, heterocaryon formation, and meiosis. Field isolates are usually heterocaryotic. This condition is revealed by the wide variation in physiological characteristics among single-basidiospore cultures from the one parent, and among single-cell cultures derived from older hyphae where the number of nuclei per cell is reduced from 8-10 to 2-4. The occurrence of mutation is revealed by sectoring in single-basidiospore cultures derived from uninucleate single spores. Mutation is also exhibited in the variation among F2 single-spore cultures from a homo-caryotic, homothallic, F1 single-spore culture. These mechanisms lead to variation in a wide range of cultural characteristics, and also to differences in virulence on different hosts. The only indication of a change in pathogenicity or host range was that a few single-spore isolates of the root-attacking strain of T. cucumeris apparently failed to attack the roots of wheat seedlings. Anastomosis between different pathogenic strains was investigated, but the death in 12-36 hr of the anastomosed cells prevented the ready formation of heterocaryons. Such death of cells was investigated in single-spore cultures and appears to due to one or more factors which segregate during spore formatioa Variation through anastomosis and basidiospore formation may be rare in the soil, but mutation probably occurs with the same frequency as in other fungi. Some isolates of both species of Thanatephorus appear to be homothallic. The other isolates either have sterility factors inhibiting the re-formation of basidia in single-spore cultures, or are heterothallic or require specific and as yet undetermined environmental conditions for fruiting.

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