310 PROTOZOA 



crosses of variety 9 showed much chromosomal abnormality at 

 meiosis. Some survivors were obtained from crosses of popula- 

 tions of variety 6. Again the role of aging in these results is 

 unknown. Further work may well reveal other marked differences 

 among the varieties in their breeding systems. 



Enplotes. There is a group of morphologically distinct species 

 which are, however, much like E. patella. In three of these species 

 (E. aediculatus, E. eurystomus, E. patella) and in the less closely 

 related E. harpa, mating types forming one or more varieties 

 are known, but there has been no effort to discover the full 

 number of varieties in any region in any of these species. Knowl- 

 edge is chiefly confined to one variety of E. patella, indeed to 

 the progeny of two wild individuals from neighboring ponds. 

 These were of different mating type and yielded four other 

 mating types among their sexual progeny. Doubtless more, pos- 

 sibly many more, mating types of this variety exist in nature. 



The method of mating type determination is entirely different 

 from the ones thus far described. The six types are determined 

 by the three homozygous and three heterozygous combinations 

 of three codominant alleles. Because of the genotypic identity of 

 the two exconjugants of a pair, each synclone is uniform in mat- 

 ing type. Mating is normally controlled by the action of three 

 hormones secreted into the medium, each hormone being under 

 the control of one allele. An animal is activated, or put into 

 mating condition, by exposure to a hormone which it cannot 

 itself produce. Animals thus activated can mate with each other 

 regardless of whether their mating types are alike or different. 

 Thus, "mating type" has a somewhat different meaning in this 

 material. 



E. patella is an outbreeder in spite of sonic superficial indica- 

 tions to the contrary. Since any two activated animals can mate 

 with each other, mixtures of two mating types lead to much 

 selfing. But in nature, the lesser population densities and the 

 large volume of diluent probably restrict hormone action to 

 contiguous pairs, and these could conjugate only il each acti- 

 vated the other, a result which is possible only if they arc of 

 certain different mating types. Tin's is then actually an adapta- 



