234 PROTOZOOLOGY 



judged by segregation at first or sometimes second division in excon- 

 jugants and by the influence of temperature during conjugation and 

 the first division. 



(2) Stocks containing only one mating type. No conjugation oc- 

 curs in such stocks. Autogamy does not produce any change in type 

 which is always type I. Stocks that contain type II only have not 

 yet been found. 



(3) Hybrids between stocks containing one and two mating types. 

 When the members of the stock containing both types I and II 

 (two-type condition) conjugate with those of the stock containing 

 one type (one-type condition), all the descendants of the hybrid 

 exconjugants show two-type condition, which shows the dominancy 

 of two-type condition over one-type condition. The factor for the 

 two-type condition may be designated A and that for the one-type 

 condition a. The parent stocks are AA and aa, and all Fi hybrids Aa. 

 When the hybrids (Aa) are backcrossed to recessive parent (aa) 

 (158 conjugating pairs in one experiment), approximately one-half 

 (81) of the pairs give rise to two-type condition (Aa) and the remain- 

 ing one-half (77) of the pairs to one-type condition (aa), thus showing 

 a typical Mendelian result. When Fi hybrids (Aa) were interbred by 

 120 conjugating pairs, each exconjugant in 88 of the pairs gave 

 rise to two- type condition and each exconjugant in 32 pairs pro- 

 duced one-type condition, thus approximating an expected Men- 

 delian ratio of 3 dominants to 1 recessive. That the F 2 dominants 

 are composed of two-thirds heterozygotes (Aa) and one-third homo- 

 zygotes (AA) was confirmed by the results obtained by allowing F 2 

 dominants to conjugate with the recessive parent stock (aa). Of 19 

 pairs of conjugants, 6 pairs gave rise to only dominant progenj^, 

 which shows that they were homozygous (AA) and their progeny 

 heterozygous (Aa), while 13 pairs produced one-half dominants and 

 one-half recessives, which indicates that they were heterozygous 

 (Aa) and their progeny half homozygous (aa) and half heterozygous 

 (Aa). Thus the genie agreement between two conjugants of a pair 

 and the relative frequency of various gene combinations as shown in 

 these experiments confirm definitely the occurrence of meiosis and 

 chromosomal exchange during conjugation which have hitherto been 

 considered only on cytological ground. 



In Euplotes patella, Kimball (1942) made various matings with 

 respect to the inheritance of the mating type. The results obtained 

 can be explained if it is assumed that mating types I, II, and V, are 

 determined by different heterozygous combinations of three allelic 

 genes which if homozygous determine mating types III, IV, and VI. 



