Preservation of the Heterozygous Diploid Condition in Industrial Micro-organisms

Abstract

THE discovery of parasexuality in Aspergillus nidulans¹ has led to the possibility of breeding strains of asexual micro-organisms for increased yields of a desired product². It has been reported that in Penicillium chrysogenum a synthesized heterozygous diploid gave a higher yield of penicillin than either haploid parent, perhaps through an effect of heterosis². It might sometimes be desirable therefore to use a heterozygous diploid of P. chrysogenum or other micro-organism in an industrial fermentation. A heterozygous diploid, however, does not breed true, but can give rise to segregants as the result of mitotic crossing-over or non-disjunction of chromosomes³. The rate of segregation may vary in different organisms, and if it were high then the accumulation of segregants during the course of a fermentation could have an adverse effect on the yield of a product. To prevent this or to avoid the consequences there are two possibilities, namely, to reduce the rate of segregation, or to select against segregants as they are formed.