Dikaryon Formation in Coprinus cinereus: Selection and Identification of B Factor Mutants

Abstract

~Mutations in the B incompatibility factor of Coprinus cinereus were obtained using two techniques. Both selected for the expression of B-controlled functions (associated with nuclear migration during dikaryon morphogenesis) in heterokaryotic mycelia where these were blocked by having identical B alleles present. The first technique selected for clamp cell fusion in a common B heterokaryon, the second selected for septum dissolution in a heterokaryon following a common B mating. Two mutants, one derived by each of the selection techniques, were studied in detail. In both, mutation was inseparable by recombination from the B locus and resulted in self-compatibility. Mutant monokaryons had uninucleate cells and intact septa. Thus, full regulatory control of morphogenesis in the mutant monokaryon was maintained. However, both mutations promoted clamp cell fusion in dikaryons formed between the mutants and monokaryons having the progenitor B allele or the mutant self allele. Therefore, mutation leads to the loss of regulation of clamp cell fusion. A combination of a B mutation and an A mutation (Amut Bmut) in the same nucleus gave a haploid mycelium which resembled a true dikaryon; thus, both the A and B morphogenetic sequences which are expressed in the dikaryon were operating, However, these dikaryons were not fertile.

Life History and Developmental Processes in the BasidiomyceteCoprinus cinereus

“…Amut homokaryons with only a mutated A locus mimic common B heterokaryons and are characterized by mycelia with unfused ("false") clamp cells at some but usually not all septa. Amut homokaryons may have one, two or occasionally even three nuclei per cell (103,165,452) at frequencies comparable to what we have found in monokaryons (384, 385). Amut homokaryons are hemicompatible and form stable dikaryons with any monokaryotic strain as long it has a different B specificity (103,165,452).…”

“…They mate bilaterally and accept and donate nuclei in crosses with monokaryons as long as these differ in the A mating-type locus. Bmut homokaryons have simple septa like normal monokaryons (165,452). In contrast to the analogous Bmut homokaryons of S. commune (243,361), the dolipore septa are not disrupted in the C. cinereus Bmut homokaryons and the nuclear distribution within the cells is even over the entire hyphal length as in monokaryons (165,452).…”

“…Such strains, commonly called Amut Bmut homokaryons, are fully self-compatible and will initiate fruiting-body development under certain environmental conditions (452; see below). Depending on the general genetic constitution of the strain, fruiting-body initials and primordia will mature and karyogamy, meiosis, and the production of four basidiospores will occur within the gills of the fruiting body (165,452) (Fig. 3).…”

“…To generate Amut Bmut homokaryons, independent mutations were selected in the A and B mating-type loci and combined into the same nucleus by crosses (165,452). Amut homokaryons with only a mutated A locus mimic common B heterokaryons and are characterized by mycelia with unfused ("false") clamp cells at some but usually not all septa.…”

Morphogenetic Effects of Mutations at the A and B Incompatibility Factors in Coprinus cinereus

“…These mutant strains not only resembled dikaryons in various morphological characteristics but were also sexually fertile. More recently, Amut Bmut strains were recovered in Coprinus as well, however, sexual morphogenesis in these was incomplete as they failed to produce mature fruit bodies (Haylock et al, 1980). The present paper describes the isolation and characterization of Amut and Bmut strains in C. cinereus from which Amut Bmut double mutant strains were constructed.…”