Variation in Karyotype and Ploidy Level Among Field Isolates of Claviceps purpurea

Karyotype analysis by pulsed-field gel electrophoresis of several field isolates of Claviceps purpurea showed extensive variation in size and number of chromosomes. Analysis of F1-ascospore-lines of a cross between two field isolates showing chromosome-length polymorphisms (CLPs) indicate that recombination during meiosis contributes considerably to this variability. In addition, cytofluorometrical estimation of DNA content of single nuclei indicate that the C. purpurea strains vary in their ploidy level: most strains probably are ± diploid, some appear to be haploid, others ± triploid (with varying degrees of aneuploidy). Benomyl treatment (as used, e.g. for 'haploidization' in the parasexual cycle of imperfect fungi) of some field isolates confirmed that the differences in 4'-6-diamidino-2-phenylindol (DAPI)fluorescence correspond to different ploidy levels: derivatives with lower nuclear DNA content could be isolated from the putatively nonhaploid strains; a minimal value (even after repeated benomyl treatment) was obtained, corresponding to the lowest value of field isolates, probably representing the haploid status. Most of these strains are not impaired in fitness and aggressiveness and therefore can be used for the selection of recessive mutants and for functional molecular studies.