LIFF. CYCLES, SEXUALFFV, AND SF.XUAL .MFCHAXISAIS 59 



rion being completely assumed bv asexual or vegetative elements 

 (Dow ding, 1933; Dowding and Bullet, 1940). 



Secondatv homothallism, seiisu \\'hitehouse, a phenomenon of 

 fairly common occurrence in this group and shared to a greater or 

 lesser degree by forms of other patterns in which incompatibility 

 factors constitute the critical determination of mating behavior, re- 

 sults from the regular inclusion in the spore of two nuclei carrying 

 opposed incompatibility factors (Ames, 1934; Dodge, 1927; Dowding, 

 1931; Dowding and BuUer, 1934; Sass, 1929). Binucleate spores of 

 this sort give rise to heterocaryotic mycelia which are self -fertile; in 

 some species, sexual organs are present and appear to be essential, in 

 other species sexual organs may be absent or greatly reduced and 

 apparently non-essential. In several cases, occasional irregularities 

 during spore production yield small, uninucleate spores, each of 

 \\hich develops into a self-sterile but cross-fertile mycelium which 

 behaves exactly as do the individual mycelia of heterothallic species 

 (Ames, 1934; Dowding, 1931). The initial binucleate condition thus 

 predetermines a composite heterocaryon the net reaction of which is 

 homothallic. 



Alternate Sexual Factors ucith Incovipatibility Factors at a Single 

 Locus. A pattern of sexuahty involving the independent assortment 

 of sexual factors and incompatibility factors has been demonstrated 

 to date in only a single species, Hypoviyces solani f. cucurbitae, an 

 ascomycetous fungus. The segregation of four distinct mating strains, 

 $ of compatibility type A, 2 of A, $ of a, and 2 of a, constitutes 

 the basic pattern of sexuahty in this species (Hansen and Snyder, 

 1946). Four additional strains, however, appear among the progeny 

 of certain crosses with a frequency of ca. 25 per cent; these are her- 

 maphrodites and neuters of both incompatibility types. Cytological 

 investigations (Hirsch, 1949) have revealed the mechanism that 

 accounts for these anomalous strains. Frequent nondisjunction at 

 meiosis of the two homologous chromosomes carrying the sexual 

 factors yields two additional classes as regards sexual factors: one 

 containing both, the other containing neither. The incompatibility 

 alleles, segregating independently of the sexual factors and presum- 

 ably located on different chromosomes, combine with these tw^o 

 classes at random to yield the four observed strains. Thus a total of 

 eight different strains constitutes the array of distinct mating types in 

 this species. Matings of 2 x 5 , 62 x 5 , and 2 x 52 , of the proper 



