50 SEX IN AIICROORGANISMS 



sexuality in most of the members of the Mucorales, the order of 

 fungi to which "black bread mold" belongs. It is of interest in the 

 present discussion that all species of this group having a sexual stage 

 were unambiguously divisible into heterothallic and homothallic 

 groups, and that in each heterothallic species no irregularities in re- 

 spect to sexual sign were encountered, although individual isolates 

 often varied widely in sexual potency. 



In the half century that has elapsed since Blakeslee's first demon- 

 stration of obligatory intermycelial reaction for sexual reproduction, 

 similar conditions have been reported for some members of every 

 major group of fungi. The necessity for intermycelial reaction, how- 

 ever, is the only feature common to all cases: in some, sexual differ- 

 ences are clearly evident; in others, sexual differences equally clearly 

 do not account for the pattern of self-sterility and cross-fertility. 

 With the discovery of the several patterns of interactions, each differ- 

 ing in some important respect from that oricrinally described in the 

 Mucorales and termed heterothallism, a number of proposals have 

 been made either to differentiate, by appropriate terms, these cases 

 from heterothallism as originally defined or to redefine heterothallism 

 in a more broadly inclusive manner. The chief result of these efforts 

 has been to indicate the degree of prevailing confusion rather than 

 to contribute to a unified scheme of categorization which was rea- 

 sonably free of damning objections. 



Whitehouse (1949), in a very comprehensive and excellent re- 

 view, has recently advanced a logical system of differentiation which 

 promises to clarify considerably the entire subject of sexuality in the 

 fungi. He retains, on rational grounds and with historical justification, 

 the term heterothallism to include all those cases in which inter- 

 mycelial reaction is a requisite for sexual fusion. Two major types of 

 heterothallism are distinguished: (1) viorphological beterothaUisni, to 

 include those cases in which the two interacting thalli differ by pro- 

 duction of morphologically dissimilar sexual organs or gametes which 

 are identifiable as i and 9, and (2) physiological heterothallism, to 

 include those cases in which the interacting thalli differ in mating 

 type, or compatibility, irrespective of the presence or absence of sex- 

 ual organs or differentiated gametes per se. Homothallism is retained 

 in the original sense: sexual fusion between elements of the same 

 thallus or, in unicellular ors^anisms, between individuals of the same 



