54 SEX IN MICROORGANISMS 



breeding cannot be accurately assessed. The widespread occurrence 

 of homothallism in fungi, however, is eloquent testimony of the evo- 

 lutionary success of this pattern of sexuality among these forms. 



Superficially it would appear that in homothallic species, typi- 

 cally uniting genetically identical nuclei, the usual sexual endeavor 

 approaches a total sacrifice of quality for quantity; the exceptional 

 cases which prevent the accomplishment of this biological absurdity 

 appear to provide sufficient variability to allow for necessary adapta- 

 tion and survival. 



Heterothallism 



Six basic patterns of sexuality have been called responsible for 

 heterothallism among fungi. Beyond the single requirement for het- 

 erothallism, that the sexual act involve two individuals, these several 

 patterns are quite distinct. 



The distinctions between the various basic patterns imposing 

 intermycelial mating reactions have been recognized by many au- 

 thors, several of whom have proposed new terms for one or more of 

 the seemingly coordinate patterns to distinguish them from hetero- 

 thallism as originally defined. Some of these terms have been widely 

 accepted and now constitute useful components of our working vo- 

 cabulary; others have probably deserved the oblivion to which they 

 have been relegated. It is certainly not the purpose here to add to 

 this burden of awkward descriptive terms, but rather to differentiate 

 as concisely as possible between a number of patterns which are based 

 upon distinct genetic devices, are quite similar superficially, and which 

 accomplish a common purpose. 



The basic segregative mechanisms responsible for the six differ- 

 ent heterothallic patterns are diagramed in Fig. 3. The order within 

 the comparative listing here is not intended to convey any intimations 

 of phylogenetic or evolutionary significance. 



In typical heterothallic species the immediate products of meio- 

 sis, spores of one sort or another in most cases, differ among them- 

 selves in respect to either sexual sign, or incompatibility factors, or, 

 in one known case, both sexual sign and incompatibility specificity. 



The necessary use here of both sexual and incompatibility factors 

 forces upon the reviewer the most unwelcome chore of attempting 

 to distinguish concisely between the two; the onerous fact that a 



