T. M. SONNEBORN 223 



that possesses macro-nuclei descended from the same ancestral 

 macronucleus. The clone, embracing the vegetative descendants 

 of a single autogamous individual or a single conjugant, usually 

 includes two carvonides in most varieties but four in varieties 13, 

 15, and 16. A new term, synclone, is a convenient designation for 

 the vegetative descendants of two animals that have conjugated 

 with each other. Each of these three terms, caryonide, clone, and 

 synclone, designates a group of individuals sharing a common 

 genotype. (The two conjugants of a pair normally acquire iden- 

 tical genotypes.) Nevertheless, in spite of genie uniformity only 

 the caryonide is (aside from selfers) always phenotypically uni- 

 form with respect to mating type. The two caryonides of the 

 same clone are often unlike in mating type in group A varieties 

 but usually alike in mating type in group B varieties. The two 

 clones of a synclone are usually unlike in mating type in group B 

 varieties. The only larger unit we need to consider is the local 

 population which consists of unknown numbers of caryonides, 

 clones, and synclones. 



The difficulty in judging the bearing of diverse systems of mat- 

 ing type inheritance on the breeding system stems from the fact 

 that in both the group A and the group B varieties, as well as in 

 the unclassified variety 13, any one synclone as a rule has a rea- 

 sonably high probability of including both mating types. Whether 

 inbreeding will occur or not thus seems to be independent of the 

 method of mating type inheritance at conjugation. The same dif- 

 ficulty is encountered if we start, not with synclones, but with a 

 single caryonide. When it goes into autogamy (and also presuma- 

 bly if it selfs in senility ) , both mating types will arise in varying 

 relative frequencies. The only known exceptions are the very 

 rare strains pure either for mating type I or mating type XIII; as 

 already pointed out, they are thereby forced, not to outbreed, but 

 to inbreed by autogamy. Thus, so far as the methods of mating 

 type inheritance alone are concerned, it would seem that all varie- 

 ties of P. aurelia should be close inbreeders. Outbreeding be- 

 tween local populations would be favored if each whole popula- 

 tion were pure for one mating type, but only if there were also a 

 very long period of maturity and no autogamy. Outbreeding 



