T. M. SONNEBORN 305 



confined varieties. The group A and group B mating type systems 

 are found in other species of Paramecium and in Tetrahymena 

 pyriformis, suggesting repeated independent evolutionary diver- 

 gence along these lines. A general formulation of the two systems 

 is given from which it appears that one or a few changes in de- 

 tails could shift from one system to the other. The major isolating 

 mechanisms in P. aurelia are listed. 



Paramecium caudatum. This species consists of 16 known va- 

 rieties, with two mating types each. Only four varieties occur on 

 two continents, the rest on one only. The varietal differences 

 include: mating type specificity, geographical distribution, ani- 

 mal size and shape, micronuclear size and shape, optimal tem- 

 peratures for mating, and resistance to a killer of P. aurelia. Only 

 mating type specificity serves to distinguish all varieties. None of 

 the other differences and no combination of them can, so far as 

 present knowledge goes, serve to identify the varieties. Assigning 

 species names would be futile. 



With respect to gene flow, the varieties fall into two groups, 1 

 and 2. The 9 varieties of group 1 are absolutely isolated sexually 

 from all other varieties. The 7 varieties of group 2 show a limited 

 system of interbreeding, but the pattern of mating reactions of 

 each mating type is unique. The possibility of gene flow among 

 them has not yet been excluded. European workers are unable 

 to sort their collections into varieties. They find fair viability 

 after conjugation within a local population; but variable, though 

 usually less, viability after conjugation between different popula- 

 tions. This refers to the Fl only. Little is known about F2 results 

 in P. caudatum after crosses between populations of the same 

 or different varieties. The Fl results on the European material 

 suggest that each local population may be more or less isolated 

 from all others, but the matter is in an unsettled condition. 



Most varieties, if not all, of P. caudatum appear to be inbreed- 

 ers. Immature periods are unknown and, if they exist, must be 

 relatively short. Selfing is common. The group B method of mat- 

 ing type inheritance assures the usual existence of both mating 

 types in a synclone. Some selfing is induced in mixtures of mating 

 types. Autogamy seems to occur in some varieties, but not in 



