S6 SEX IN MICROORGANISMS 



plex of interbreeding species of the green alga, Chlaviydoinonas, 

 which comprises heterogamous, anisogamous, and isogamous forms 

 (Moewus, 1950). In other cases where sexual sign cannot be tested 

 by cross-breeding with sexually differentiated forms, it is impossible 

 to make a certain distinction between sexual and incompatibility 

 control of mating behavior in the absence of clear morphological dif- 

 ferences. This difficulty will be apparent in the following description 

 of segregative patterns. 



Single Alternate Sexual Factors. The simplest pattern of sexual 

 differentiation yields two classes of progeny, each of which is either 

 immediately distinguishable as .5 or 9 or bears differentiated ($ or ? 

 sexual organs or gametes respectively or both. Sexual dimorphism is 

 typically rigid in plants belonging to this category. 



Relatively few groups of fungi contain species which unques- 

 tionably show this type of differentiation. Among the more primitive 

 monoflagellated aquatic fungi numerous species produce thalli which, 

 at maturity, are differentiated into single gametangia with clear mor- 

 phological distinction between S and ? (Couch, 1942; Harder and 

 Sorgel, 1938; Sparrow, 1943). Such forms are obviously heterothallic; 

 whether the differentiation of the indvidual as a S or as a ? is phe- 

 notypically or genotypically determined, however, remains uncertain 

 although intensive efforts have been made to resolve the problem 

 (Cantino and Hyatt, 1953; Emerson, 1950). Certain groups among 

 these primitive fungi, particularly Blastocladiella of the Blastocladi- 

 ales, constitute series grading from clear distinction between $ and 

 9 thalli to forms that show no morphological difference between the 

 two mating types (Stiiben, 1939). The sure distinction here between 

 sexual factors and incompatibility factors is not possible, but it would 

 seem to the reviewer, in disagreement with the views of Whitehouse, 

 that here as elsewhere a common pattern of sexuality most probably is 

 shared by the members of a closely related group. 



Sexual dimorphism is also known among the members of a few 

 groups of Ascomycetes, in Stromatinia narcissi of the Discomycetes 

 (Drayton and Groves, 1952), in many species of the Laboulbeniales 

 (Benjamin and Shanor, 1950; Thaxter, 1908), and in Pericystis 

 (Clausen, 1921). 



The heterothallic members of the Mucorales, to which "black 

 bread mold" belongs and in which heterothallism was first discovered, 

 have long been cited as the classic examples of sexual segregation 



