PROTOZOA 



oithin a local population, that is, mating between different syn- 

 clones, would require a mechanism for preventing mating until 

 there has been time for dispersal, i.e., by a sufficient immature 

 period, or for a mechanism to make the synclone uniform in mat- 

 ing type. 



In fact, both groups A and B have mechanisms for bringing 

 about synclonal uniformity in mating type. In group B, the mech- 

 anism is massive cytoplasmic exchange between mates. It has 

 been studied to some extent in variety 2 (with intermediate 

 breeding system), but most fully in variety 4, a strong inbreeder. 

 In this variety, the mechanism usually gives a synclone of mating 

 type VIII at high temperatures, while at low temperatures both 

 types of synclones are produced in high frequency (Nanney, 

 19.54 ) . Remarkably, there are strain differences in the capacity to 

 experience cytoplasmic exchange during mating. Wood ( 1953 ) 

 found a single major gene difference between the two kinds of 

 strains. He also found that the gene for cytoplasmic exchange 

 has much higher penetrance if conjugation takes place at low 

 temperatures and if the conjugating clones are very young. These 

 facts could well be of importance in relation to the breeding sys- 

 tem. Outbreeders would be expected to have the genotype for 

 cytoplasmic exchange and to conjugate in nature under condi- 

 tions favorable for the penetrance of the relevant gene. Inbreed- 

 ers would be expected to lack this gene or to conjugate in nature 

 under conditions in which it rarely comes to expression. It is not 

 known whether these alternatives actually occur in association 

 with inbreeding and outbreeding. 



The mechanism in group A is quite different. Here cytoplasmic 

 exchange plays no part whatever, for there is in group A no cy- 

 toplasmic difference between the two mating types. Each car- 

 yonide is independently determined for mating type and the 

 probability for being determined as the even-numbered mating 

 type increases linearly with the temperature prevailing during 

 pre- and postzygotic reorganization (Sonneborn, 1939). At high 

 temperatures, the even type has so high a probability that a con- 

 siderable proportion of synclones are uniform for it. However, it 

 seems likely that at the temperatures prevailing in nature, the 



