ASEXUAL SELECTION IN NEUROSPORA CRASSA

Abstract

H ETEROCARYOSIS, the association of genetically different nuclei in a single cytoplasmic unit, has been recognized in asexually reproducing fungi as a mechanism of natural variation since the work of HANSEN and SMITH (1932) on the "dual phenomenon." They discovered that wild isolates of Botrytis cinerea were unstable upon single conidial transfer, the wild isolate giving rise to two stable "conidial" and "mycelial" strains, as well as the unstable, intermediate wild type. Later investigations (HANSEN 1938) showed that this behavior was widespread among the Fungi Imperfecti. In 1942. HANSEN and SNYDER discovered the dual phenomenon in the sexually reproducing Ascomycete, Hypomyces solani. By standard genetic tests, it was shown that the "conidial" and "mycelial" characters were controlled by a single pair of alleles, and that the dual phenomenon was doubtless the result of the assortment of two different nuclear types of a heterocaryon during the formation of multinucleate conidia. The investigations of DODGE (1942) , and of BEADLE and COONRADT (1944) with various mutant strains of Neurospora species have revealed a great deal concerning the physiological consequences of heterocaryosis. In most cases, it was found that heterocaryotic mycelia, constituted of two nuclear types carrying non-allelic mutations would display a normal phenotype. They felt that the mutant gene carried by one nuclear type was compensated for by the action of its wild type allele in the other nuclear type. Though not strictly comparable with dominance in diploid systems, the complementary action of wild type genes over their mutant alleles in heterocaryons indicated a dominant-recessive relationship. The occurrence of the dual phenomenon in nature and the experimental investigations of heterocaryosis indicate that filamentous fungi possess a more adaptable genetic system than do unicellular haploid organisms. Because many species of fungi reproduce by multinucleate conidia, a heterocaryotic condition may be maintained not only during the growth of mycelia, but also in their asexual reproduction. The plasticity of such a system would be somewhat akin to that of diploids, because a reserve of variability could accumulate in a nuclear population by mutation, shielded from adverse selection by more advantageous genomes.