T. M. SONNEBORN 277 



interbreeding mating types in an organism kindly identified for 

 me as Colpidium truncatum by Corliss. This organism, which is 

 closely related to Tetrahymena, likewise has its breeding system 

 much complicated by selfing. Downs (1952) reported a system 

 of five interbreeding mating types in Stylonychia putrina col- 

 lected from a single source and he ( 1956 ) now finds two varie- 

 ties with eleven types in one variety and fifteen in another. In 

 S. putrina, selfing is rare; only one clone of one mating type 

 selfed among 41 clones of the first five types studied. 



In Oxytricha bifaria, Siegel ( 1956 ) reported a system of nine 

 interbreeding mating types. He investigated the possibility that 

 O. bifaria might manifest relations like those in Euplotes because 

 these organisms are both hypotrichous Ciliates and so are more 

 closely related to each other than they are to other (holotri- 

 chous) Ciliates in which mating types have been found. Siegel 

 looked for action of sex hormones, but could find no evidence of 

 their existence. He also devised means of discovering whether 

 the conjugating pairs in mixtures of two mating types were true 

 crosses or selfers and showed that they were regularly true 

 crosses. Indeed, selfing appears to be exceptional in his material. 

 Only one mating type (III) selfed, and this frequently showed 

 about 1% of the animals conjugating even when not mixed with 

 another type. The two members of selfing pairs were separated 

 before becoming firmly united. Both gave rise to cultures like 

 the parent, that is, they were predominantly or entirely type III, 

 but selfed slightly. 



Crosses of certain mating types gave types among the progeny 

 which sometimes differed from both parents; but the genetic 

 details were not reported. Whether, as in Euplotes, mating types 

 are determined by a series of multiple alleles is still unknown. 

 The large number of mating types in O. bifaria and in S. putrina 

 suggests that such a basis may well exist. On the other hand, 

 Siegel found the same mating type in several natural sources, 

 and this indicates that the number of existing types may not be 

 great. 



The life cycle of O. bifaria begins with a period of immaturity 

 lasting for variable periods, from less than one month to over 



