226 PROTOZOA 



achieves its primary function of self -perpetuation. In view of its 

 surpassing importance for every species, no surprise or suspicion 

 should be aroused concerning the apparent convergence of so 

 many aspects of the biology of P. aurelia upon it. 



Evolution of Groups A and B and Their Varieties. ( 1 ) Origi- 

 nally groups A and B were recognized only on the basis of their 

 different systems of mating type inheritance ( Sonneborn and Dip- 

 pell, 1946). In the discussion of the serotypes, it was pointed out 

 that all the group A varieties examined (varieties 1, 3, and 9) show 

 one system of serotypes and that all the group B varieties ex- 

 amined (varieties 2, 4, and 8) show a different system. Thus, 

 group A varieties may equally well be distinguished from group 

 B varieties by either the mating type or the serotype system. 

 There is also a third difference between the two groups (Sonne- 

 born, 1956c). Killers are common in all group B varieties in 

 which many strains have been observed, and have been found in 

 every group B variety of which more than one strain is available, 

 i.e., varieties 2, 4, 6, and 8. Not a single killer strain has been 

 found among the large number examined in the varieties of group 

 A. 4 Curiously, these three independent sets of traits show in their 

 inheritance a conspicuous cytoplasmic involvement in group B, 

 not in group A; but the mode of cytoplasmic involvement seems 

 different for each of the three sets of traits. One can hardly re- 

 frain from suspecting some at present unrecognized common ele- 

 ment in the situation which leads to this nonrandom concentra- 

 tion of cytoplasmically inherited traits in group B. 



The differences between group A and group B varieties in three 

 sets of traits suggest that the cleavage into these two groups of 

 varieties represents an ancient and important evolutionary diver- 

 gence. If so, then one might expect the differences between the 

 group A and group B alternatives for each of the three sets of 

 traits to be rather simply derivable from one another. The al- 

 ternatives with respect to the killer trait are genetically simple. 

 The presence of this trait depends upon the presence of one major 

 gene (Sonneborn, 1943) and a few minor genes (Sonneborn, 

 unpublished; Balbinder, unpublished). Without these genes, the 

 cytoplasmic particles, kappa, ran, or pi, which determine the 



