MORPHOGENESIS OF SCHIZOPHYLLUM COMMUNE I. MORPHOLOGICAL VARIATION AND MATING BEHAVIOR OF THE THIN MUTATION

A single gene, recessive mutation of Schizophyllum commune, designated as thin, is characterized by its sparse growth on agar and unusual hyphal morphology. Many hyphae exhibit various degrees of "waviness," although some branches of these wavy hyphae appear normal. Differences in cell size and branch development betweeni normal and thin strains are also noted. Most of the characteristics attributed to thin vary quantitatively betweein strains. In addition, conisiderable variability in hyphal characteristics can be found in a single strain. Thin mutants behave as bilateral maters. In matings where the normal allele is present, genetic complementation occurs and normal clamped cells are produced. Fruit bodies with viable spores are formed on both the thin and normal sides of the mating. Thin X thin matings give a stable dikaryon, but they retain the thin morphology and do not produce fruit bodies. Evidence that there is only one locus for the thin mutation is presented. C'ertain aspects of the variability of thins, the possible role of extracellular polysaccharide, and the potential uses for thin strains in studies of hyphal and fruit body development are diseussed. SCHIZOPHYLLUM COMMUNE Fries is a tetrapolar basidiomyeete which has been used in numerous studies of the sexual mechanisms of the hymenomyeetes. A large number of different morphological mutations have been isolated and partially described from S. commune (Raper and Miles, 1958). One of these, the so-called thin mutation, differs from the other morphological mutants in that it is isolated with unusually high frequency from stock homokaryons (Raper, San Antonio, and Miles, 1958) and dikaryons (Miles, 1964). It is also commonly found in common-A heterokaryons as are a number of other mutants (Raper et al., 1958). The mutation was termed thin because of its sparse growth on agar. In addition, it displays a distinctive hyphal morphology generally described as "wavy" or "helical." It has been reported that the cells of thin are wider than those of normal strains and that fewer branches are found (Leary, 1964; Raper and Esser, 1964). Leary also reported that the cells of the thin strain which he studied did not vary significantly in length from normal strains. Raper and Esser reported that the cell length of thin is greater than that of normal cells. They also found that the mycelium of thin grew faster than the mycelium of normal strains. Leary obtained variable results on the relative growth 'Received for publication May 6, 1966. This investigation was supported in part by Research Gralnt A1-06570 from the National Institute of Allergy and Infectious Diseases of the U. S. Public Health Service and by Grant No. GB-3613 of the National Science Foundation. rates. In addition, Raper and Esser reported no difference in the angle of branching between thin and normal strains. The thin mutants show considerable potential usefulness in studies of hyphal development in regard to such aspects as the regulation of septum formation, branch initiation, hyphal structure and the role of extracellular polysaccharide, which Leary (1964) and others have reported as not produced by the thin mutants. The first part of this paper will deal with a more complete discussion of the morphology of the thin mutation. It has been reported that most of the morphological mnutations of S. commune, including thin, mate unilaterally (Raper and Miles, 1958; Leary, 1964). Unilateral maters donate nuclei to a compatible mycelium but do not allow the establishment of a dikaryon within their own hyphae. Raper and Miles (1958) found that by heavy seeding of a compatible thin X feather mating a few dikaryotic hyphae could be obtained. They coneluded that unilaterality appears to be caused by a failure of nuclear migration and not by a failure of formation of hyphal anastomoses. During a related study we discovered that the basic assumption that thin mutants are unilateral maters is incorrect; consequently, a more intensive investigation of the mating behavior of the thin mutants was undertaken, and the results of these studies are also reported here. MATERIALS AND METHODS-The 23 homokaryotic strains of S. commune used in this study