270 PROTOZOA 



individuals. Third, inbreeding nonviability tends to eliminate 

 those which do inbreed; and, if there are present simultaneously 

 outbred competitors, these would have an advantage in subse- 

 quent population increase. 



Looking at the picture as a whole, therefore, I am inclined 

 to interpret T. pyriformis as an outbreeder, but as one that has 

 an already highly developed potential for evolving inbreeding 

 descendants. The fact that Nanney has been able in the labora- 

 tory to select inbred lines of variety 1 that give virtually no 

 exconjugant death shows that the organism could do the same 

 in nature, if conditions were propitious for it. The fact that some 

 selfing clones which give viable sexual progeny are found in na- 

 ture could mean that the same sort of selection in nature pre- 

 ceded their origin; but I am rather of the opinion that this 

 represents a last resort for old clones which have failed to out- 

 cross. Finally, it is of much interest that Corliss (1952) has in 

 fact discovered a species of Tetrahymena (T. rostrata) which 

 regularly responds to starvation by going 100% into autogamy — 

 the closest type of inbreeding. T. pyriformis is in fact of great 

 interest in our present discussion because it shows how an out- 

 breeder is preadapted, as it were, to become an inbreeder on 

 demand. This may be considered as strengthening our view con- 

 cerning the evolutionary derivation of inbreeders from out- 

 breeders. The mechanisms needed for an inbreeder are present, 

 but counterbalanced so that they operate relatively poorly; remove 

 the restraints and an inbreeder results. The restraints actually 

 seem to be developed to different degrees in different varieties, 

 as indicated, for example, by variations in the length of the 

 immature period and in the mode of mating type inheritance. 



As the varieties of T. pyriformis are compared in more detail, 

 it may well be discovered that the different mechanisms for in- 

 breeding and outbreeding are quite differently developed and 

 balanced among them, as among the varieties of P. aurelia. In 

 this connection, comparison between variety 1 and variety 2 is 

 of special interest. The latter is clearly more of an outbreeder 

 than the former. It shows synclonal uniformity in mating type, 

 while variety 1 does not. For this reason, the report that its imma- 



