The Cytogenetics of the Conidium in Microsporum gypseum and of Pleomorphism and the Dual Phenomenon in Fungi

Abstract

SUMMARY The cytogenetics of the macroconidium was studied in wild-type and mutant strains of Nannizzia gypsea and N. incurvata, the two perfect states of Microsporum gypseum. One mutant (ng) arose spontaneously in N. gypsea as a result of chromosomal aberration, and the other (ni) in N. incurvata as a single gene mutation. The developing macroconidium of the wild type passes through three developmental stages: The first is marked by active nuclear division in a swollen hyphal tip that grows into a macroconidium. The second stage is marked by vacuolation and redistribution of the nuclei, due to the enlargement of the macroconidium. The third is a stage of septation, further enlargement and thickening of the wall. In the mutant ng, the macroconidium passes through the first stage with no noticeable variation. However, in the second and third stage variations occur. The nuclei are scattered and continue to divide, frequently forming chromatin threads, and septation often fails to occur or stops at the 2-3 septate condition. Occasionally, the macroconidia proliferate forming protuberances, long fine hyphae or secondary macroconidia. In the mutant ni, the macroconidia are highly distorted and their development is too irregular to describe. The cytogenetics of these so-called "pleomorphic" mutants was compared with that of the M-type mutants in Hypomyces solani f. cucurbitae, the perfect state of Fusarium solani. In the latter organism the macroconidium arises as a single uninucleate cell in which each nuclear division is followed by a septum formation and the resultant cells are always uninucleate. It is concluded that, regardless of growth pattern and nuclear condition, the M-type mutants resemble closely the "pleomorphs," and that mutation in the broad sense is the underlying mechanism of "pleomorphism" and the "dual phenomenon" in fungi. The last two terms, however, are inadequate and should be discarded. Although fungus variability is no longer a riddle to be solved, it is still a subject on which much attention is focused and a phenomenon described in various terms. Of the various terms coined, pleomorphism and the dual phenomenon deserve special attention. Pleomorphism is a type of