T. M. SONNEBORN 219 



would tend to wipe out the intruder by reason of its small propor- 

 tion in the population, if extensive mating with the indigenous 

 strain took place. The degree to which such interbreeding would 

 occur depends upon the mating type frequencies in the two 

 strains. If some does occur, the progeny will include some viable 

 combinations, and these will give rise in further fertilizations to a 

 supply of genotypes and karyotypes for selection to work upon. 

 Eventually this could lead to a new relatively homogeneous popu- 

 lation of a single selected best adapted karyotype or to a balanced 

 mixture of diverse karyotypes and genotypes. Until field studies 

 of local populations of strongly inbreeding varieties and labora- 

 tory studies of mixtures of diverse strains of such varieties are 

 available, it is not worth pursuing the possibilities further. 



If a migrant enters a new body of water not already occupied 

 by its variety or one with which it can mate, there is no difficulty 

 in seeing how both an inbreeder and an outbreeder could soon 

 give rise to a population containing both mating types. Both the 

 group A and group B methods of mating type inheritance provide 

 ready-made mechanisms for this except in the exceedingly rare 

 cases in group A (varieties 1 and 7) of strains pure for one mat- 

 ing type. The only real problem raised by migrations of this sort 

 is how to account for the development of a distinctive karyotype 

 by the inbreeders. This may happen very slowly by selection of 

 rare spontaneously arising chromosomal aberrations. In the latter 

 case, two factors present themselves as possibilities. First, one 

 thinks of the invariable production of chromosomal aberrations 

 with advancing senility (Sonneborn, 1955; Dippell, 1955). Ordi- 

 narily these would not be expected to occur in nature. In estab- 

 lished populations, famine would be expected to set in long before 

 the age at which the aberrations occur. This would lead to ferti- 

 lization and thereby to setting the age clock to zero (Sonneborn, 

 1954a). However, if the migrant is already no longer young and 

 the new waters provide food for much reproduction by a single 

 intruder (or very few), the requisite age might be reached. A sec- 

 ond possibility arises from some indications that fertilization at 

 extreme temperatures (10° and 35°) also results in chromosomal 

 aberrations in certain variety 4 stocks. If paramecia migrate via 



