Variation in karyotype and ploidy level among field isolates of Claviceps purpurea

Karyotype analysis by pulsed-field gel electrophoresis of several field iso lates of Claviceps purpurea showed extensive variation in size and number o f chromosomes. Analysis of F1-ascospore-lines of a cross between two field isolates showing chromosome-length polymorphisms (CLPs) indicate that recom bination during meiosis contributes considerably to this variability. In ad dition, cytofluorometrical estimation of DNA content of single nuclei indic ate that the C. purpurea strains vary in their ploidy level: most strains p robably are +/- diploid, some appear to be haploid, others +/- triploid (wi th varying degrees of aneuploidy). Benomyl treatment (as used, e.g. for 'ha ploidization' in the parasexual cycle of imperfect fungi) of some field iso lates 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 strai ns; a minimal value (even after repeated benomyl treatment) was obtained, c orresponding to the lowest value of field isolates, probably representing t he haploid status. Most of these strains are not impaired in fitness and ag gressiveness and therefore can be used for the selection of recessive mutan ts and for functional molecular studies.