68 SEX IN MICROORGANISMS 



1938), or the gametangia may fuse without further differentiation, as 

 in numerous monoflagellate Phycomycetes, such as Siphoiiaria ((3 — 

 7—12)) (Karling, 1945; Wager, 1913). 



In yet other forms the sexual activity is relegated to gametangia 

 which originate de novo as extra-vegetative structures. Three dif- 

 ferent patterns of further sexual development are found in these 

 forms: gametangia may produce gametes which fuse in pairs, as in 

 Allomyces ((3—8—9—10)) (Emerson, 1941; Kniep, 1929); 

 gametangia of one sexual sign may produce differentiated gametes 

 which react sexually with gametangia of the opposite sexual 

 sign, as in Achlya (Bary, 1881; Raper, 1939) ((3—8—9—11)) or 

 Neurospora (Backus, 1939; Shear and Dodge, 1927), and many rusts 

 ((3—8—9—11—13)) (Buller, 1950; Craigie, 1942); the game- 

 tangia, morphologically differentiated in respect to sexual sign or not 

 depending upon the species, may fuse directly with one another, as 

 in Mucor and Rhizopus ((3—8—12)) (Blakeslee, 1904, 1920; Bur- 

 geff, 1924; Krafczyk, 1935) or Fyronejna (Claussen, 1912) and 

 Ascobohis (Dodge, 1920) ((3—8—12—13)). The developmental 

 pattern of Phycomycetes and most of the lower Ascomycetes (Hem- 

 iascomycetes) differs from that of the higher Ascomycetes (Eu- 

 ascomycetes) and Basidiomycetes following plasmogamy in that their 

 nuclei fuse immediately, whereas in the higher groups dicaryons are 

 regularly formed. 



The dozen or so developmental sexual histories and sexual mecha- 

 nisms sketched here are the more common types encountered among 

 the fungi. Most forms fit comfortably in one or the other of these 

 patterns, but there are a number of cases that would be categorized 

 variously according to the preferred interpretation of structural and 

 behavioral characteristics. 



Relatively little is known of the underlying physiological and 

 biochemical aspects of sexual development and sexual activity. It 

 has long been recognized that an intimate relationship exists between 

 nutritional requirements and metabolic processes on the one hand and 

 sexual differentiation and activity on the other (Coker, 1923; Dodge, 

 1920; Klebs, 1898, 1899, 1900; Molliard, 1903; Raper, 1952). The 

 knowledge of such relationship, however, has commonly been arrived 

 at quite empirically, and only in a few cases is there a glimmer of 

 the underlying mechanism. 



Intraspecific chemical regulators of sexual processes, sexual hor- 



