66 



SEX IN MICROORGANISMS 



The four basic types of sexual apparatuses, as exemplified in 

 four well-known representative species, are shown, in surface view, 

 in Fig. 6. Each of these types may be found in a wide array of mor- 

 phological variations, but the basic aspects of the sexual progression, 

 nuclear behavior, and so on are relatively constant in the variants of 

 each type. 



MEIOSIS= 



COPULATION, Gometic 

 PLASMOGAMY 



CARYOGAMY. 



Gametangia 

 ^^ Primordia 



Gametangia 



Gomete- 

 Gometanqiol Gometdnqial 



Somatic 



Fig. 7. Summary diagram of developmental sexual histories in fungi. The 

 numbered lines trace developmental variations relating the critical events at 

 meiosis, sexual fusion, and nuclear fusion. 



( 3 ) After the fusion of the two sexual elements there exist two 

 possibilities with regard to the subsequent activity of the paired 

 compatible nuclei: they may fuse immediately to establish the diploid 

 phase, or they may become associated in one or more pairs and 

 divide conjugately by mitosis to provide ultimately a large number of 

 paired nuclei which finally fuse to form the definitive nuclei in the 

 asci or basidia. 



The developmental patterns relating these cardinal stages in the 

 sexual cycle may be represented by the various pathways indicated 

 in Fig. 7. The specific combination of events at the critical points in 



