184 PROTOZOOLOGY 



In certain races of Polytoma pascheri and Chlamydomonas euga- 

 metos, the sexual fusion takes place between members of different 

 clones only. The zygote gives rise as was stated before to four swarm- 

 ers by two divisions, which are evenly divided between the two 

 sexes, which shows that the sex-determining factors are lodged in a 

 single chromosome pair. In a cross between Chlamydomonas para- 

 doxa and C. pseudoparadoxa, both of which produce only one type of 

 gamete in a clone, the majority of the zygotes yield four clones, two 

 producing male gametes and the other two female gametes; but a 

 small number of zygotes gives rise to four clones which contain both 

 gametes. It is considered that this is due to crossing-over that 

 brought the two sex factors (P and M) together into one chromo- 

 some, and hence the "mixed" condition, while the other chromosome 

 which is devoid of the sex factors gives rise to individuals that soon 

 perish. 



In crosses between Chlamydomonas eugametos which possesses a 

 stigma and 10 haploid chromosomes and C. paupera which lacks a 

 stigma and 10 haploid chromosomes, 12 pairs of factors including 

 sex factor are distinguishable. Consequently at least two chromo- 

 somes must have two factors in them. Thus adaptation to acid or 

 alkaline culture media was found to be linked with differences in 

 the number of divisions in zygote. That there occurs a sex-linked in- 

 heritance in Chlamydomonas was demonstrated by crossing stigma- 

 bearing C. eugametos of one sex with stigma-lacking C. paupera of 

 the opposite sex. The progeny that were of the same sex as C. euga- 

 metos parent possessed stigma, while those that were of the same sex 

 as C. paupera parent lacked stigma. Thus it is seen that the sex factor 

 and stigma factor are located in the same chromosome. 



The genetics of conjugation which takes place between two diploid 

 conjugants has been studied by various investigators. Pure-line 

 cultures of exconjugants show that conjugation brings about diverse 

 inherited constitutions in the clones characterized by difference in 

 size, form, division-rate, mortality-rate, vigor, resistance, degene- 

 ration, etc. The diversities brought about by autogamy are not as 

 varied as those produced by conjugation. In Paramecium much in- 

 formation has in recent years been brought to light through the 

 studies of Sonneborn, Jennings, Kimball, and others on the mating 

 type (p. 156). The mating type is as a rule inherited without change to 

 descendants through vegetative reproduction. 



Sonneborn (1939) has made extended studies of variety 1 of 

 Paramecium aurelia (p. 158) and found that genetically diverse ma- 

 terials show different types of inheritance, as follows: 



