LIFL. CYCLES, SEXUALITY, AND SEXUAL MECHANISAIS 57 



rlic developmental cycle, meiosis, fusion of sexual elements, and 

 nuclear fusion, defines about as well as is possible the sexual mecha- 

 nism tor any given species. A few well-known forms will be used 

 liere to illustrate the range of possibilities and also to demonstrate 

 a shorthand system for designating the sexual mechanism and develop- 

 mental sequence. 



The production of gametes as the immediate products of meiosis 

 may possibly occur in a very few species, members of the Blasto- 

 cladiales of the aquatic Phycomycetes. A single mitotic division, how- 

 ever, has been reported as interposed between meiosis and the differ- 

 entiation of gametes in the single species which has been cytologically 

 investigated (\\'ilson, 1952). 



Most species, if not all, produce spores immediately after meiosis, 

 and the further developmental sequence is extremely variable. In a 

 number of cases the fusion of these differentiated spores constitute 

 the sexual act. Of common occurrence in the yeasts is the fusion of 

 ascospores in pairs while still in the ascus to reestablish the diploid 

 phase ((2 — 10)) (Winge and Laustsen, 1939) or, rarely, a dicaryon 

 ( (2 — 1 — 1 3 ) ) (Guillermond, 1 940) ; a similar sexual fusion is known 

 in many smuts, in which sporidia, or basidiospores, fuse to establish 

 a stable dicaryon ((^1 — 13)) (Bauch, 1925; Kniep, 1926). 



The spores give rise in other fungi to vegetative thalli or clones 

 of vegetative cells prior to sexual activity. Vegetative cells may par- 

 ticipate without any discernible sexual differentiation in either of two 

 ways. In clonal, unicellular forms, such as many of the haploid yeasts, 

 each individual cell is functionally a gamete, and fusion between such 

 cells may be considered a gametic copulatory process ((3 — 5 — 10)) 

 (Guillermond, 1940). In a large number of extensively developed 

 mycelial forms, including the majority of the species of the Basidi- 

 omycetes, all vegetative cells of the thallus are capable of reciprocal 

 somatic copulation to initiate the dicaryon ((3 — 6 — 13)) (BuUer, 

 1924; Kniep, 1920, 1922). 



Remaining fungi produce sexual organs or gametangia, and these 

 are almost invariably essential for sexual activity. 



The entire vegetative thallus may be differentiated at maturity 

 into one or more gametangia, which may develop further in either 

 of two different ways. The gametangia may undergo internal dif- 

 ferentiation to produce uninucleate gametes which fuse in pairs, as 

 m Blastocladiella ((3—7—9—10)) (Couch, 1942; Harder and Sorgel, 



