376 CLASS BASIDIOMYCETEAE 



heterothallism as originally applied refers to mycelia representing differ- 

 ent sexes it seems inadvisable to use the terms heterothallic and homo- 

 thallic in these higher fungi, at least without qualification. 



Apparently the factors governing the compatibility are two allelo- 

 morphic factors borne on different chromosome pairs. Kniep (1928) and 

 others have pointed out that only those unions of cells lead to diploidiza- 

 tion that bring about a combination that is heterozygous for both these 

 sets of factors. This need not involve a union of the nuclei into a diploid 

 nucleus, a process that occurs only in the basidium, but concerns merely 

 the bringing together, without union, of two haploid nuclei in the same 

 cell, so that so far as the cell is concerned diploidization has taken place 

 even though the two haploid nuclei remain separate for the present. Thus 

 a monocaryon mycelium with the compatibility factors A and B (these 

 being on separate chromosomes) could unite with another monocaryon 

 mycelium of the formula ab; similarly a mycelium with the formula aB 

 could unite with one of the formula Ab. In either case the formula for the 

 resultant cells would be AaBb, a condition heterozygous for both sets of 

 characters and fulfilling the requirements for the mating of the mycelia. 

 In the basidium the diploid nucleus undergoes two meiotic divisions to 

 consummate the reduction process. If both pairs of chromosomes bearing 

 the factors for incompatibility or compatibility undergo disjunction in 

 the first division it will be a matter of chance whether the resultant 

 daughter nuclei will be Ab and aB or AB and ab. Since each nucleus di- 

 vides again, this time by splitting the chromosome, there will be four 

 nuclei (one for each spore), two each of the formula Ab and aB or AB and 

 ab respectively. If the disjunction does not occur in either chromosome 

 pair until the second meiotic division the first division may represent 

 merely the splitting of the chromosome so that the two daughter nuclei 

 will be like the parent nucleus with the formula AaBb. When these divide 

 by the disjunction division it will give this time Ab and aB or AB and ab, 

 for each of the two nuclei. If the separation of the chromosomes or the 

 chromosome pairs occurs in the same direction in both nuclei there will 

 result four nuclei, alike two by two, but this separation may be in opposite 

 directions in the two dividing nuclei for one of the chromosome pairs so 

 that the four nuclei may all be different. If disjunction of one chromosome 

 pair occurs at the first division and of the other pair at the second division 

 the four resultant nuclei will also be all different, viz., Ab, aB, AB, ab. 

 Miss Newton (1926) showed that in Coprinus lagopus all of the foregoing 

 arrangements may be found in the various basidia of the same hymenium. 

 Apparently disjunction occurs more often at the first division for both 

 sets of chromosomes, for in the majority of cases two of the basidiospores 

 of a given basidium will be of one sexual phase and the other two of the 

 opposite phase. In some species only two sexual phases appear to occur. 



