Chromosome transfer in dikaryons of a smut fungus

Abstract

THE haploid sporidial stage of the anther smut fungus, Ustilago violacea, consists of yeast-like uninucleate cells of two mating types, a₁ and a₂. When complementary auxotrophs of opposite mating type are conjugated and plated on minimal medium (MM), several types of prototrophic colonies grow up, the most distinctive of which are the translucent colonies of diploids, heterozygous for mating type and formed by karyogamy. These appear with a frequency of about 3 per 10⁴ conjugated cells¹ and have been exploited for mitotic mapping in this species². The other types of prototrophs seem to carry only one mating-type allele and form opaque colonies. These prototrophs form with a frequency of about 1 per 10³ conjugated cells, and we show here that they arise from either conjugant after a number of chromosomes have been gained from the other conjugant. We also show that most of them are not conventional aneuploids, but that they have the extra chromosomes sequestered in such a way that they are lost or gained as an entity. The frequency of these segregants and their stability suggest that this process may play a hitherto unsuspected part in the origin of new physiological races in this species and perhaps also in other fungi.