T. M. SONNEBORN 257 



places and in Mexico, they also found variety 2, which is widely 

 distributed from Canada to Central America. The other seven 

 varieties are known only from North America. Gruchy (1955) 

 observes that varieties 1, 4, 5, and 8 have been found only in the 

 northern states; that variety 3 seems to occur mainly in running 

 water; and that varieties 1, 5, 6, and 8 have been found only in 

 standing water. In striking contrast to the varieties of P. aurelia, 

 of which two or more are often found together in the same body 

 of water, the varieties of T. pijriformis usually occur alone, two 

 varieties in the same collection having been found only once 

 (Elliott and Hayes, 1955). Altogether, cultures from about 

 eighty collections in the Americas have been identified as to 

 variety; no collections from other continents have yet been re- 

 ported upon. 



The varieties of T. pijriformis are diagnosed primarily on the 

 basis of sexual reactions. If a clone from a new collection mates 

 well with a standard mating type culture, it is assumed to belong 

 to the same variety as the standard. Variations in intensity of the 

 mating reaction appear not to occur under optimal cultural con- 

 ditions (Gruchy, 1955). In the case of P. aurelia, it will be re- 

 called, varieties 4 and 8 interact sexually with almost normal 

 maximal intensity. It is therefore possible, but perhaps not likely, 

 that in T. pijriformis some different varieties have been, on the 

 basis of strong intermating, misclassified as belonging to the same 

 variety. Tests of genetic isolation, which have proved important 

 in P. aurelia and P. bursaria, have not yet been carried out ex- 

 tensively in T. pijriformis. (There are practical difficulties here, 

 as will appear. ) For this reason, the status of the nine reported 

 varieties, particularly in relation to the problem of gene flow 

 with which we are concerned here, is still dubious. However, it 

 seems clear that there are at least nine varieties among the ma- 

 terials examined, for these are all reported to be isolated by 

 failure to intermate. The study of gene flow within these varie- 

 ties could serve only to increase their number, if it changes the 

 situation at all. 



Another systematic exclusion of possible additional varieties 

 comes from the practice of ignoring all isolates from nature which 



