COCHLIOBOLUS SATIVUS: V. HETEROKARYOSIS AND PARASEXUALITY

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Abstract
Multiply marked strains of Cochliobolus sativus were used to synthesize heterokaryons. The heterokaryons apparently were formed by hyphal anastomosis and nuclear migration. Some conidia (not more than 6%) from heterokaryotic cultures perpetuated the heterokaryons. Probably this low frequency of heterokaryotic conidia was due to a non-random distribution of nuclei in the conidia. Variation in strains due to heterokaryosis may be reduced by the isolation of single spores or hyphal tips in several successive generations. Heterozygous diploids were selected from two heterokaryons on the basis of their phenotypic differences: heterokaryons grew slowly and intermittently; diploids grew rapidly and uniformly. Heterokaryons but not diploids readily dissociated into their components. Diploid spores were consistently larger but not twice as large in volume as haploid ones.Dominant–recessive relationships were expressed in heterokaryons and diploids; white was recessive to tan spore color; nutritional requirements were recessive to the wild type alleles; and resistance to anisomycin was semidominant. Segregants were obtained from a diploid strain by various techniques. Equal numbers of heterozygous and "presumptive" homozygous segregants were found and most of them were recombinants. A parasexual cycle therefore occurs in this heterothallic ascomycete.

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