T. M. SONNEBORN 229 



served linear relation between temperature and the frequency of 

 differentiation of macronuclei determining the even-numbered 

 mating type would be explicable as due to the rise in relative 

 concentration of the corresponding cytoplasmic differentiator 

 with increasing temperature. Exactly the same rise has been 

 noted by Nanney ( 1954 ) in group B when conjugants exchange 

 cytoplasm and thus set up an approximately equal concentration 

 of the two cytoplasmic differentiators. However, because of the 

 steady state mechanism in group B, a rise in the concentration of 

 the even-numbered differentiator leads to a steady state in which 

 it greatly predominates. Finally, Sonneborn (unpublished) and 

 Butzel (1953) showed that different genotypes in group A result 

 in determination of different proportions of the mating types at 

 the same temperature. Hence, the genes seem to be determining 

 something on which temperature also acts. On the present hy- 

 pothesis, this something is the relative concentrations of the 

 cytoplasmic differentiators. 



Viewed in this way, the evolution of the differences between 

 the two mating type systems needs involve but few changes: the 

 establishment or loss of the steady state condition for the cyto- 

 plasmic differentiators, and the predominant control of these 

 differentiators by the expressed mating type genes versus control 

 by genes which are identical in both mating types. If Nanney's 

 ( 1956b ) conclusion as to the existence in both groups of varieties 

 of an intranuclear steady state among the products of the mating 

 type genes is correct, the extension of this steady state mecha- 

 nism into the cytoplasm in group B and not in group A may rep- 

 resent a relatively simple difference. 



The group B system seems to be the more ancient one, occur- 

 ring in all investigated varieties of other species of Paramecium 

 and in one variety of Tetrahymena pyriformis. It is associated in 

 some of these organisms with outbreeding and, as already men- 

 tioned, outbreeders were probably the ancestors of current in- 

 breeders. However, one variety of T. pyriformis shows the group 

 A system. Thus, either we are on the wrong track in supposing 

 groups A and B of P. aurelia diverged early, or such divergence 

 has occurred repeatedly and independently in different genera. 



