52 SEX IN MICROORGANISMS 



ular association, initiated in spore formation in certain species, of two 

 kinds of nuclei of dissimilar sexual or incompatibility types in a 

 single thallus which is self-fertile. A form of this kind, in spite of its 

 segregative pattern and the necessity of genetically dissimilar nuclei 

 for sexual fusion, must be termed homothallic because of the self- 

 fertile nature of its thallus. 



A second complication is the possibility of final determination of 

 sexual or mating behavior, in forms lacking this determination at 

 meiosis, by environmental factors during the development of the 

 thallus. A physiological differentiation of this sort between individual 

 cells or groups of cells within a single thallus constitutes typical 

 homothallic behavior; if the final differentiation involves different 

 thalli, however, it must be termed heterothallic because of the self- 

 sterile nature of the sexually mature thalli. Students of different 

 groups of fungi have shown somewhat less than ideal accord in their 

 integration of phenotypic determination with the homo-heterothallism 

 concept. The general acceptance of the concept of the clone, now 

 frequently ignored except in the study of unicellular forms, would 

 resolve the more important discrepancies in interpretation of these 

 phenomena. 



A third mode of deviation from a strict dichotomy between 

 homo- and heterothallism may arise through mutations of factors 

 controlling mating behavior or modifying sexual expression. 



These departures from strict homo- and heterothallism will be 

 considered later in connection with the detailed accounts of the vari- 

 ous patterns of mating behavior. 



HOMOTHALLISM 



Of the several distinct patterns of sexuality to be found among 

 the fungi, homothallism is the most common; it occurs in all major 

 groups and, with very few exceptions, in a majority of species within 

 each group. The critical differentiation of compatible elements is 

 intramycelial and may involve single cells or relatively large groups 

 of cells. The spatial relationship between differentiated elements of 

 the fusing pair is also variable. This variability may best be illustrated 

 by certain species in the aquatic phycomycetous order, the Saproleg- 

 niales: (1) <^ and 9 elements may together constitute a specialized 



