318 Genetics of a Daphnia Hybrid during Parthenogenesis 



regard to its reproductive processes, and these features were maintained 

 throughout the ten generations during which it has been observed. 



(1) Excessive production of sexual eggs. 



(2) Excessive production of males. 



(3) Complete sterility of the males. 



(4) Imperfect fertility of the parthenogenetic eggs. 



(5) Probably normal fertility of the sexual eggs. 



Ten generations of the hybrid clone have been bred for this experi- 

 ment, counting the original hybrid $ as generation 1. Seventeen 

 daughters of this $ were used each to found a line, though owing to 

 the high degree of sterility, a number of lines died out in the first few 

 generations. A condensed summary of the whole hybrid population is 

 shown in the pedigree chart constituting Table IV. It will be under- 

 stood of course that in the original records the individual offspring of 

 individual parents were recorded, but in the chart only the total number 

 of males and females measured in each line in each generation are shown. 



r D 



The means for the ^ ratio of the ^'s, and of the -r ratio of the 2 's, 



of the hybrid clone and of the two parent forms are given in Table V. 

 The male parent of the hybrid belonged to clone R of D. pulex. Very 

 few measurements were made of members of this clone, which was 

 unfortunately allowed to die out. Instead, the figures are given for 

 clone RR, which was descended from a female produced sexually by 

 a straight fertilization of a clone i2 $ by a clone R </ . This clone can 

 therefore be considered, for all practical purposes, identical with clone R. 



TABLE V. 

 ' Mean Ratios for the Hybrid and Parental Clones. 



B 



JO 



males -, females 



Number of Number of 



males females 



measured Mean measured Mean 



D. ohtusa, clone H ... 4 30-8 75 2-17 



D.pulex, clone RR ... 28 6-2 69 2-89 



Hybrid clone 281 lO'l 226 1-97 



Inheritance within the hybrid clone Xc. 



A glance at the results of these experiments is enough to show that 

 segregation of a single pair, or even of only two or three pairs, of factors 

 is not taking place. It is however common in species crosses for any given 

 character difference between the two parents to depend upon a large 



