256 PROTOZOA 



The regular exchange of cytoplasmic differentiators during 

 conjugation in P. bursaria is one of its key adaptations to out- 

 breeding. It almost completely prevents the occurrence of mating 

 within a synclone during maturity: only 2.6% of the synclones 

 are exceptions to this rule. This feature, the long period of 

 immaturity, the long life cycle, multiple mating types and the 

 high mortality from sib crosses, assure a highly developed system 

 of outbreeding. Variations from population to population occur, 

 but the populations are prevented from progressive divergence 

 by the forced interbreeding. A common gene pool is thus main- 

 tained over a wide range. 



As a consequence, speciation occurs on a much smaller scale 

 than in the inbreeders, P. aurelia and P. caudatum. Only 3 va- 

 rieties of P. bursaria have been found in the United States; they 

 were discovered by 1939, within two years after the discovery 

 of mating types, and no additional ones have been found in the 

 large number of collections subsequently examined. At the same 

 time, the first three varieties of P. aurelia were found; but in the 

 following years ten additional varieties were discovered in the 

 United States. Probably few if any additional varieties of P. 

 bursaria will be found in this region; but doubtless more, possibly 

 many more, varieties of P. aurelia will be discovered, for several 

 new ones turned up within the past year. The difference is ac- 

 centuated by the existence of karyotypic differences between lo- 

 cal populations of P. aurelia and their regular production of F2 

 mortality after crosses between representatives of different popu- 

 lations. Speciation is indeed rich in the inbreeder, poor in the 

 outbreeder. 



Tetrah ymena pyriformis 



Mating Types and Varieties and Their Distribution. Like the 

 other Ciliates which have been studied in this connection, T. 

 pyriformis consists of a number of sexually isolated varieties. 

 Elliott and Gruchy (1952) discovered variety 1; Gruchy (1955) 

 reported varieties 2 through 8; and variety 9, which seems not to 

 occur in North America, was found by Elliott and Hayes (1955) 

 in Colombia and in the vicinity of the Canal Zone. In the latter 



