48 SEX IN MICROORGANISMS 



serve to sort out the original dicaryotic components into liaploid, 

 vegetative mycelia. Similar cells produced on haploid mycelia behave 

 in an identical manner. 



Dicaryotic Cycle 



The extreme development of the dicaryotic phase is exemplified 

 by the cycle in which the immediate products of meiosis, ascospores 

 or basidiospores, fuse to initiate the dicaryotic phase. Both haploid 

 and true diploid phases are thus reduced to single nuclear generations. 

 This type of cycle is occasionally seen in the yeasts (Guillermond, 

 1940) and is of common occurrence among the smuts (Kniep, 1926). 



The distinction made here between the haploid-dicaryotic and 

 the dicaryotic cycles emphasizes the two extremes in what, in all 

 probability, is a continuous series. Chance juxtaposition of compatible 

 germinating spores of the haploid-dicaryotic type might result in the 

 typical dicaryotic cycle; on the other hand, the experimental pro- 

 longation of the haploid phase, as sprout mycelia in the smuts for 

 example, converts the typical dicaryotic cycle into the haploid- 

 dicaryotic. 



Of particular interest in this and the haploid-dicaryotic cycle is 

 the failure of the dicaryon in many cases to constitute a physiological 

 summation of its haploid components. This phenomenon is reflected 

 in ( 1 ) the host specificity of the two phases in the heteroecious rusts 

 (for example, the haploid phase of the "black stem rust" of wheat is 

 an obligate parasite of the barberry, whereas the dicaryotic phase is 

 an obligate parasite of grasses); (2) the saprophytic habit of the 

 haploid phase versus the obligate parasitic habit of the dicaryotic 

 phase of many smuts (Christensen and Rodenhiser, 1940); and (3) 

 the complex pattern of fruiting requirements of the dicaryotic phase 

 of certain Hymenomycetes as compared to the nutritional require- 

 ments of the component homocaryons (Schopfer and Blumer, 1940). 



Haploid-Diploid Cycle 



The alternation of haploid and true diploid generations, a com- 

 mon type of cycle in the algae and in the higher plants, occurs among 

 fungi only in certain members of the aquatic phycomycetous order, 

 the Blastocladiales, with Allomyces the best known example (Couch, 



