T. M. SONNEBORN 195 



University where it was isolated about 1941. This 15-year-old 

 culture is the well-known strain long maintained in axenic cul- 

 ture under the name of P. multimicwnucleatum by Dr. Willis 

 Johnson (1952), who gave it to me. Both of these strains I 

 found at once to be predominantly mating type XXIX but selfers, 

 and in both all exconjugants died. Dr. Ruth Dippell stained 

 these for me and found, as I suspected, that both are now ami- 

 cronucleate. These strains are thus genetically dead. How long 

 they have been so, I do not know. But it is clear that the total 

 life cycle in variety 15 is at least 15 years and that life persists, 

 perhaps very long, after a culture has become genetically dead. 

 As mentioned above, the same sort of relation exists on a smaller 

 scale in varieties with shorter life cycles, e.g., clones of variety 

 4 may live for about 100 fissions after they are unable to survive 

 sexual processes. In both cases, deterioration and loss of the 

 micronuclei, the germ plasm, mark genetic death which precedes 

 somatic death by a considerable period. Whether variety 15 and 

 variety 16 clones ever reach the stage of somatic extinction is as 

 yet unknown. If they do, it must be after ages measured in dec- 

 ades and therefore rarely if ever discovered by an investigator. 



Should the Varieties of P. aurelia Be Assigned Species Names? 

 The preceding detailed account of the known and, so far as can 

 now be foreseen, the knowable differences among the varieties 

 of P. aurelia had as its first objective marshalling all evidence 

 which might be pertinent to the question: Can the varieties of 

 P. aurelia be readily identified? A positive answer to this question 

 would justify changing the status of the varieties to species and 

 assigning to each a species name. A negative answer would seem 

 to require that we abstain from assigning species names even if 

 we regard the varieties as species. 



So far as now known or now foreseeable, all the 16 varieties 

 could be uniquely identified either by mating type tests alone 

 or by serologic means alone, but not by any other single charac- 

 teristic. To carry out the mating type tests, it is necessary to 

 employ successfully standard cultures of each of the 34 mating 

 types. This involves having available enough strains to yield 

 all these types, being able to obtain and isolate the various mating 



