70 SEX IN MICROORGANISMS 



In two heterothallic species of Achlya, a common genus of the 

 aquatic Phycomycetes, it lias been demonstrated that the initiation 

 and coordination of each of a chain of interdependent reactions, 

 which together constitute the sexual process, depend upon one or 

 more specific chemical agents (Raper, 1940, 1951, 1952). The hor- 

 monal mechanism, as currently interpreted in these plants, is shown 

 in Fig. 8. Essentially, the mechanism consists of a minimum of seven 

 distinct hormones, four secreted by the $ and three by the 2 , which 

 induce and regulate a series of reactions alternately in the $ and 5 ; 

 each reaction is chemically dependent upon and quantitatively regu- 

 lated by the reaction immediately preceding. The entire sexual 

 process, with the exception of the physical transfer of $ nuclei in 

 the act of fertilization, has been shown to be coordinated in this 

 manner. None of the sexual hormones has been either isolated in 

 chemically pure form or identified. 



CORRELATIONS OF LIFE CYCLES, SEXUALITY, AND 

 SEXUAL MECHANISMS 



The three principal facets of sex in fungi having been examined 

 in some detail, it should now be possible to attempt some correlation 

 between them and to approach some sort of integrated picture of 

 the problem in its entirety. Such a correlation is attempted in Table 

 L In this table are shown the more frequent combinations of life 

 cycle, sexuality, and developmental sexual history, as well as examples 

 of these, chosen wherever possible, from those fungi which are rela- 

 tively well known to biologists other than mycologists. Patterns of 

 sexuality and developmental histories have been bracketed within 

 each type of life cycle; indication of the actual combinations which 

 are known to occur would serve only to obscure the important con- 

 clusions that may be drawn from this body of information. 



The most striking fact that emerges here is also one of consider- 

 able significance, namely, no rigid and inclusive correlation exists be- 

 tween the various combinations of sexual features and the universally 

 accepted phylogenetic groupings. To illustrate this: homothallism, 

 possibly the most primitive of the various pattern of sexuality, occurs 

 in conjunction with all types of life cycles, with practically all devel- 

 opmental histories, and in every major grouping from the most 

 primitive Phycomycetes to the most highly evolved Basidiomycetes. 



