44 



SEX IN A'lICROORGANISMS 



Other, and a unique nuclear association, known as the dicaryon, 

 greatly increases the range of life cycle variability. 



Since the dicaryon effects important changes in the life cycle 

 and is peculiar to the fungi, it deserves a brief description and illus- 

 tration at this point (Fig. 1.) The essential portion of the sexual 

 process is initiated by the fusion of two sexual cells or organs, each 

 containing one or more haploid nuclei (N). This fusion has been 

 termed plasinogamy or cy to gamy. The nuclei provided by the fusing 

 elements may retain their individuality and become associated in one 



Haploid (N) {^.^ (y 

 Plasmogamy '!>. 



Dicaryon (8) 



Conjugate 



Division H ° 



Caryogamy 

 Diploid (D) V € 



Fig. 1. Schematic representa- 

 tion of the initiation, multipHca- 

 tion, and termination of the di- 

 caryotic association of compatible 

 nuclei. The dicarvon occurs in the 

 higher Ascomycetes in the ascog- 

 enous hyphae and universally in 

 the Basidiomycetes in the "second- 

 ary" (dicaryotic) mycelium. 



or more pairs, each pair known as a dicaryon (B). The dicaryon may 

 be propagated for a short or for an indefinite period of time by re- 

 peated, simultaneous mitotic divisions of its members, the division 

 figures of the two components commonly lying side by side. This 

 process is termed conjugate division. Fusion of the two associated 

 nuclei, or caryogamy, eventually occurs in terminal binucleate cells 

 to establish the diploid phase (D). The dicaryotic phase thus serves, 

 when present, to effect a temporal and spatial separation of plas- 

 mogamy and caryogamy. It also serves, because of the repeated 

 divisions of its component nuclei, to increase greatly the produc- 

 tivity per sexual fusion both in numbers and in possible genetic 

 recombinations. 



