216 PROTOZOA 



various conditions. Such information should be of great impor- 

 tance in estimating the relative frequency of occurrence of con- 

 jugation and autogamy. 



Unlike the extreme inbreeders ( varieties 10 and 14 ) which have 

 been found once only, variety 8 has been found in numerous 

 localities, all but one in the extreme southern part of the United 

 States; and variety 4 occurs abundantly in cool to warm parts of 

 the United States, South America, Japan, and Australia. The mate- 

 rials for outbreeding are thus much greater in variety 8 than in 

 varieties 10 and 14, and they are enormously greater in variety 4. 



Nevertheless, in both varieties crosses between strains from 

 different sources invariably lead to considerable mortality in the 

 F2 obtained by autogamy. In variety 4 this varies from about 15% 

 to about 98%, depending upon which strains have been crossed. 

 This is in striking contrast to the results of repeated autogamies 

 within a strain. Sonneborn and Sclmeller (1955) passed a strain 

 of variety 4 through 50 successive autogamies without an inter- 

 vening conjugation and without increasing mortality which re- 

 mained throughout virtually negligible (0.7%). Variety 4 thus 

 survives the closest inbreeding very well, but invariably it suffers 

 from outbreeding. This imposes a strong selective disadvantage 

 on crosses between different local populations. 



The probable basis of F2 mortality following crosses of repre- 

 sentatives of different populations of variety 4 was revealed by 

 Dippell's (1954) karyological studies. Every strain of variety 4 

 that she examined had a different karyotype. The karyotypes dif- 

 fered in number of chromosomes in an aneuploid series from 

 about 33 to 51 chromosomes in the haploid set. Further, there 

 were regularly differences in the shapes and sizes of the chromo- 

 somes of different strains. In the laboratory, a strain remains as a 

 rule constant for all features of its karyotype. Whether variations 

 occur in detectable frequency within a natural population remains 

 unknown. Finally, Dippell roughly correlated the degree of F2 

 mortality with the degree of karyotypic diversity. 



If, as now appears, F2 mortality has this basis, then the high 

 F2 mortality found in other varieties, such as varieties 1 and 8, 

 may be taken as prima facie evidence of the existence in those 



