PROTOZOA 



differences in these rates in two mates leads to abnormality 

 (Sonneborn, 1954b). Such abnormality would put the hybrids 

 between the two types at a selective disadvantage. 



The role of the killer traits in bringing about isolation is per- 

 haps not as great as might at first be supposed. Killers that liber- 

 ate paramecin might be expected to eliminate or reduce other 

 strains in the population, especially when the population density 

 is high. To the degree that mating is at random, the excess of 

 killers would foster inbreeding among the killers, but outbreed- 

 ing among surviving sensitives of the same variety. The mate- 

 killers, known only in variety 8, present a special situation, which 

 has been discussed by Siegel ( 1954 ) . 4 Gene flow occurs readily 

 from sensitive to killer, but not directly in the reverse direction. 

 However, since mate-killers readily outgrow the cytoplasmic 

 basis, [J-, of the trait sensitives of the same strain are likely to be 

 present and through them some gene flow from the killer strain 

 to other sensitive strains would be possible. Even so, gene flow in 

 the two directions would probably be very unequal unless the 

 proportion of mate-killers in the strain were low. 



Whatever degree of isolation is due to killers is cytoplasmic in 

 basis; and cytoplasmic involvement in isolation has also been 

 found in another case. Levine (1953) showed that the death of 

 hybrids between varieties 4 and 8 is due to the lethal action of 

 the cytoplasm of each variety on the nuclei of the other. This is 

 also to be noted as an additional evidence of the importance of 

 the cytoplasm in the varieties of group B. I doubt whether the 

 same sort of mechanism plays any role in isolation of varieties of 

 group A. Butzel (1953) was unable to find any trace of such 

 cytoplasmic action in hybrids between varieties 1 and 7. Cyto- 

 plasmic differentiation may regularly bo a stage in genetic isola- 

 tion in organisms with well-developed cytoplasmic genetic sys- 

 tems like group B, not in other organisms like group A. 



Probably the most important isolating factor in P. aurelia, aside 

 horn geographical isolation, is the widespread and strong in- 

 breeding habit. This is so important in relation to the species 

 problem that the two are considered together in the next section. 



Breeding Systems and flic Species Problem. Among the L6 



