64 SEX IN MICROORGANISMS 



Furthermore, the factors at the A and B loci appear to exert chiefly 

 a quantitative control over intrastrain fertility and interstrain reaction 

 rather than the qualitative control imposed by both sexual factors 

 and incompatibility factors in truly heterothallic species. They differ 

 from sexual factors in that they do not, in any case, determine uni- 

 sexual strains, and they differ from incompatibility factors in that 

 common factors do not in all cases prevent interstrain mating. 



The pattern of sexuality in Gloinerella c'mguJata is therefore 

 basically different from any other known among the fungi. It is pos- 

 sible that here we have on display a species in the process of evolving 

 from homothallism to heterothallism, or vice versa, and to accept 

 Mather's concept of partial heterothallism may well be the best that 

 can be done at the present time toward integrating this pattern into 

 the general scheme of sexuality in the fungi. 



SEXUAL MECHANISMS 



The final aspect that must be considered to give a comprehensive 

 understanding of sex in fungi is the sexual mechanism, the mechanical 

 means by which compatible elements are brought together under 

 the conditions imposed by the particular life cycle and pattern of 

 sexuality involved. 



The number of possible combinations of basic sexual mechanisms 

 and developmental histories to be found among fungi precludes the 

 consideration of all significant combinations. Let us rather list a few 

 possible variants at certain critical stages and demonstrate by simple 

 developmental histories the range of variety of specific overall pat- 

 terns. 



Sexual mechanisms may be differentiated on the basis of mor- 

 phological differences at three critical points in the life cycle. These 

 points are (1) meiosis, (2) the physical union of compatible sexual 

 elements, and (3) the fusion of compatible nuclei. 



(1) The immediate products of meiosis, with few possible ex- 

 ceptions among the fungi, are spores of various kinds, such as zoo- 

 spores, ascospores, and basidiospores. 



(2) In spite of the almost endless variety of sexual apparatuses 

 among the fungi they may be considered to belong to four basic 

 types, first recognized by Kniep (1928). Each type comprises definite 



