PROTOZOA 



moderately short maturity. The little information available may 

 appear to be somewhat conflicting. Purity of a strain for one 

 mating type limits its conjugation to outbreeding. Yet it can 

 live on indefinitely without conjugation. I have had this strain 

 for 18 years during which it has undergone successive autogamies 

 at intervals of some 30 to 40 fissions. It is still as vigorous as ever. 

 Thus, purity for one mating type in a strain that undergoes autog- 

 amy has double significance: it conjugates only by outbreeding, 

 but without conjugation it undergoes the most intense form of 

 inbreeding. Further, the purity for type XIII depends upon a 

 recessive gene. Therefore, once an outbreeding has taken place, 

 this purity disappears except in segregants in later generations, 

 and these at once have available for inbreeding plenty of re- 

 lated segregants of complementary mating type. On the whole, 

 therefore, purity for one mating type in this context cannot be 

 considered as an important outbreeding feature. All other avail- 

 able facts (group A, no immaturity, short maturity, narrow 

 range ) indicate that variety 7 is prevailingly an inbreeder. 



Variety 9 has been found only in western Europe. It belongs 

 to group A, has a moderately short mature period, with senility 

 beginning by about the 50th fission, and it undergoes autogamy. 

 With so short a period until the onset of senility, immaturity, if 

 it occurs, could only be brief. These features point to an in- 

 breeder. In this case, the inference can be supported by analysis 

 of Pringle's (1956) field studies. He made collections from a 

 small part of a large pond at approximately 20-day intervals over 

 a period of 15 months. At each collection, 24 to 55 samples of 

 30 ml. each were brought into the laboratory and searched for 

 variety 9. Not more than one animal of this variety per sample 

 was studied. It and its descendants were tested for the alleles 

 present at two loci. At each locus three alleles were found, but 

 one was very rare. The results on the two loci were similar and 

 may be combined. For the two principal alleles at each of these 

 loci, 151 homozygotes and 7 heterozygotes were found. At one 

 of the two loci, no heterozygotes were found. In a randomly 

 mating population, altogether 116 homozygotes and 42 heterozy- 

 gotes would be expected. Pringle cautiously remarks that his de- 



