CHROMOSOMES AND SEX 387 



quartet are of four types, each one reacting with only one of the other 

 three. 



A parallel situation is found in ascomycetes. For example, in Neuro- 

 spo7'a tetrasperma the four spores usually developed in a single ascus are 

 binucleate and produce "homothallic" mycelia, while occasional uninu- 

 cleate spores produce "heterothallic" ones. In the ascus of N. sitopMla 

 there are regularly eight spores, of which four are "plus" and four 

 "minus," as indicated by the interactions of the mycelia they form. 

 By studying the relative positions in which the spores with the various 

 tendencies are borne in the ascus, it is possible to show that the factors 

 concerned in the differentiation of these tendencies may be segregated at 

 either the first or the second meiotic mitosis and that this segregation is 

 independent of that of certain other factors. ^^ 



In certain heterothallic phycomycetes, notably in Phycomyces nitens, 

 the spores in a germ sporangium develop two kinds of mycelia, zygospores 

 being formed only when gametangia borne on the plus and minus strains 

 are brought together. ^^ 



It has been generally assumed that the plus and minus strains in the 

 fungi represent the two sexes, the factors segregated at meiosis being 

 "sex factors" analogous to those in bryophytes. It was to this sexually 

 dioecious condition that the term heterothallism was originally applied 

 (Blakeslee, 1906). When it was found that the mycelia in certain basi- 

 diomycetes were of four types rather than two, some observers concluded 

 that there must be four grades of sex, this "multipolar" condition being 

 determined by the independent segregation of two or more pairs of sex 

 factors in meiosis. 



A number of recent researches in this field tend to support another 

 interpretation of such phenomena. In certain species, representing both 

 ascomycetes and basidiomycetes, it has been shown that the plus and 

 minus strains are not unisexual: the mycelium from a single spore bears 

 functional organs of both sexes," but sexual unions occur only between 

 those of opposite strain. From this it is inferred that the meiotic segrega- 

 tion in such cases is not one of sex factors but rather one of factors 

 inducing self-sterility and cross-fertility between certain classes of 



(1927, 1928), R. F. Allen (1929, 1930, 1932). Kniep (1928) and M. Hartmann (19296c, 

 1930) review the subject of sexuality in lower plants. See also the experiments of 

 Harder (1927) (p. 419). 



26 B. O. Dodge (1927, 1928a6, 19296), WUcox (1928), Lindegren (1932). See also 

 Dowding (1931) and Ames (1932). 



26 Blakeslee (1906, 1915, 19206), Burgeff (1913, 1914, 1915), Couch (1926). See, 

 further, footnote 28. 



2^ E.g. : Antheridia and ascogonia in Ascobolus (Gwynne-Vaughan and Williamson, 

 1932); spermatia (microconidia) and ascogonia in Pleurage (Ames, 1932) and Sclero- 

 tinia (Drayton, 1932); and spermatia and receptive hyphae in Puccinia (R. F. Allen, 

 1932). 



