T. M. SONNEBORN 263 



Recent discoveries have, however, made it necessary to reevalu- 

 ate the situation. 



The first shock came with the report of Elliott and Nanney 

 (1952) that one of the long-cultivated strains, E, has been ami- 

 cronucleate for at least 20 years. But the full impact of this 

 discovery becomes clear only in connection with other recent 

 discoveries about T. pyriformis and P. aurelia. After mating types 

 were discovered in the former species, efforts were made to ob- 

 tain collections far and wide in the Americas in order to find the 

 mating types and varieties in this region. This search turned up 

 a surprising and, in my opinion, highly significant fact: one-third 

 to more than one-half of the collections of T. pyriformis from 

 nature proved to be amicronucleate. Thus 50 of the 154 collec- 

 tions examined by Gruchy ( 1955 ) and more than half of the 

 collections of Elliott and Hayes (1955) in Central and South 

 America were amicronucleate. To me these facts signify that 

 the amicronucleate condition is, at least in some varieties, a 

 regular and long-lasting stage of life. 



The absence of micronuclei is clearly to be characterized as a 

 senile condition, for two reasons. First, an animal without micro- 

 nuclei is genetically dead. Its genotype, confined to the macro- 

 nucleus, could not be transmitted to sexual progeny, since gamete 

 nuclei cannot arise directly or indirectly from a macronucleus. 

 Second, as more and more amicronucleate cultures have been 

 studied, it has become clear that absence of a micronucleus is 

 invariably associated with inability to mate, i.e., even to unite 

 in pairs (Elliott and Nanney, 1952; Elliott and Hayes, 1955; 

 Gruchy, 1955). Hence, amicronucleates have no mating type 

 and, once they get into this condition, the variety to which they 

 belong can no longer be ascertained. However, one can guess 

 that the amicronucleates found in Central and South America 

 belong to variety 2 or 9 or both, because these are the only 

 varieties thus far found in that region. Since maturity by defini- 

 tion is the stage of life during which fruitful mating can occur, 

 amicronucleates are not mature and cannot later become mature. 

 In short, they are senile. 



The situation is in part parallel to the one existing in variety 



