58 SEX IN iMICROORGANISJVlS 



by Couch (1926), who found various intergrades in addition to $ 

 and 9 strains among the progeny of a c^ by 9 cross. These prelimi- 

 nary findings in conjunction \\'ith the many reports of multiple sexual 

 strains collected from nature strongly indicate the random recombina- 

 tion of multiple sexual factors during the meiotic process. Further 

 investigation of the segregative pattern, however, is needed to place 

 this type of sexuality on the sound experimental basis shared by other 

 patterns. 



Hervhtphroditis'ui mith lucovipatibility Factors at a Single Locus. 

 This type of heterothallic differentiation produces two self-sterile 

 and cross-fertile strains, each morphologically and functionally her- 

 maphroditic, and depends upon the equal segregation at meiosis of 

 extrasexual determiners or incompatibility factors. Mycelia of each 

 of the two strains characteristically produce both i and 9 sexual 

 organs; sexual fusion is accomplished, however, only between the 

 c^ gametes or the 6 gametangia of one strain with the 9 elements 

 of the opposite and compatible strain (Dodge, 1932; Drayton, 1932, 

 1934; Lindegren, 1932; Shear and Dodge, 1927; Wilcox, 1928). 



The majority of the heterothallic species of the Ascomycetes, 

 with the exception of the heterothallic yeasts, as well as many species 

 of the rusts of the Basidiomycetes, exhibit this basic pattern of sex- 

 uality. In many Ascomycetes, Neiirospora for example, the $ , or 

 fertilizing, element is characteristically the microconidium or sperma- 

 tium, the 9 , an ascogonium. In other Ascomycetes differentiated 

 gametes arc not formed, and fusion occurs bet\^'een morphologically 

 distinct gametangia, antheridia and ascogonia, respectively. In rusts 

 exhibiting this t\"pe of sexuality, spermatia bring about fertilization 

 when brought into contact with receptive elements of the compatible 

 mycelium, usually a specialized organ known as the "flexuous hypha" 

 (Buller, 1950; Craigie, 1927, 1931, 1942). 



The differentiation of sexual cells in plants having this type of 

 heterothallism is phenotypic, and there are numerous cases in w'hich 

 differentiated sexual cells, spermatia or microconidia, of the Ascomy- 

 cetes particularly (Dodge, 1932), have been shown to be capable of 

 purely vegetatixc dc\clopment. Conversely, in a number of forms, 

 fertilization is accomplished w ith equal facility by microconidia, by 

 asexual macroconidia, or, for that matter, by any cell of the vegeta- 

 tive thallus (Backus, 1939), and in some the ability to produce dif- 

 ferentiated S cells appears to have been lost, the function of fertiliza- 



