214 PROTOZOA 



strains. For example, Beale (1952) found more than 60% F2 

 mortality after crossing strains 60 and 90. Field studies by 

 Beale ( 1954, and unpublished ) indicate that heterozygotes occur 

 in nature in variety 1 less commonly than expected from random 

 mating. Both close inbreeding and some measure of outbreeding 

 doubtless occur in these three varieties. Evaluation of the net 

 effect of the complex of their features on the breeding system 

 must await fuller study; but it is already evident that these 

 three varieties have breeding systems intermediate between 

 those of the most extreme inbreeders and those of the most ex- 

 treme outbreeders. 



5. Varieties 5, 6, and 13. Nothing is yet known with pre- 

 cision about some of the most important features of the life cycle 

 in these three varieties, e.g., their periods of immaturity and ma- 

 turity. Variety 5 belongs to group A, variety 6 to group B, and 

 variety 13 cannot yet be assigned to either group. Every caryo- 

 nide of variety 13 becomes a selfer; 6 most caryonides of variety 5 

 also do; and so do many caryonides of variety 6. However, the 

 frequency of selfing pairs in the selfing caryonides of varieties 

 5 and 6 is usually very low, while it is very high in variety 13. 

 Further, the predominant mating type in selfing caryonides of 

 varieties 5 and 6 remains constant for a caryonide, but it does 

 not in variety 13. Finally, autogamy occurs in varieties 5 and 6, 

 but it has not yet been found in variety 13. 6 On the basis of the 

 widespread occurrence of selfing in these varieties, they may all 

 be considered as strongly predisposed to inbreeding; but, since 

 selfing occurs universally in all caryonides of variety 13 and with 

 high frequency in each of them, variety 13 is by far the most ex- 

 treme inbreeder of this group. This picture may have to be 

 changed somewhat when variety 13 becomes more fully studied. 1 ' 

 The possibility has not yet been excluded that a short immature 

 period exists and that the frequency of selfing increases with age, 

 though rather rapidly. If these things do occur, they would op- 

 erate to some extent toward favoring early outbreeding. 



6. Varieties 4 and 8. These last two varieties to be taken up 

 are clearly very strong inbreeders, almost as extreme in this re- 



