MUTATIONS MODIFYING SEXUAL MORPHOGENESIS IN SCHIZOPHYLLUMl

HE morphogenetic sequence leading to the establishment of the fertile dikaryon from two self-sterile homokaryons in the tetrapolar Hymenomycete Schizophyllum commune consists of a number of distinct stages. After hyphal fusion between compatible homokaryons, (a) reciprocal nuclear exchange and migration convert both homokaryons into a dikaryon by (b) pairing of the compatible nuclei, ( c ) conjugate nuclear division, (d) the formation of hookcells, lateral hyphal outgrowths associated with conjugate division, (e) hook-cell and hyphal septation, and ( f ) hook-cell fusion. The dikaryon, capable of indefinite propagation, normally bears fruiting bodies, within which occur karyogamy, meiosis, and the formation of haploid spores to complete the life cycle. This process is common to all tetrapolar fungi; its nature and regulation appears to be identical in Schizophyllum, Coprinus, Collybia, Pleurotus, etc. Relevant information from species other than S. commune is accordingly included (for comprehensive review see RAPER 1966). All of the events from nuclear migration through hook-cell fusion are regulated jointly by the A and B incompatibility factors. The two factors assort independently and are each constituted of two linked loci, (Y and ,8, each with multiple alleles (RAPER, BAXTER, and MIDDLETON 1958; RAPER, BAXTER, and ELLINGBOE 1960). The entire developmental sequence can occur only when the paired homokaryons carry different alleles in at least one of the loci of the A factor and in one of the loci of the B factor. The role of each factor is evident from the sequences of events that occur in the two hemicompatible heterokaryons. In the establishment of the heterokaryon in which the B factors are different but the A factor is identical, A= BZ, only nuclear migration occurs. This is the major event of the B sequence. In the establishment of the heterokaryon in which the A factors are different but the B factor is identical, A# B=, nuclear migration either does not occur or is severely restricted (SNIDER 1965), but the events beginning with nuclear pairing and ending with hook-cell septation proceed normally. This is termed the A sequence. Pairing and conjugate division of the two parental nuclei occur in growing apical cells as part of the A sequence; but the subsequent failure of hook-cell fusion in each apical cell prevents the entrapped daughter nucleus from passing from the hook-cell to the newly formed subapical cell, which then contains a daughter nucleus of the other parental type. The establishment of a