Genetic Factors 



429 



tendency and half male. This plant could therefore be regarded as hav- 

 ing the XY type of sex inheritance and to be comparable to Drosophila. 

 The later discovery by Blackburn (1923) of an unequal pair of chromo- 

 somes in Melandrium strongly supported this conclusion, and it is now 

 generally accepted. It has been strengthened by the fact that definitely 

 sex-linked traits have been found here, notably a difference in leaf shape 

 (Fig. 19-7). 



Sex chromosomes are not confined to vascular plants. In the liverwort 

 Sphaerocarpos Donnellii Allen (1919) reported that the four spores of 

 each tetrad produce two male and two female gametophytes. The females 

 have a very large chromosome, apparently the X, and the males its much 

 smaller homolog, apparently the Y ( Fig. 19-8 ) . This is the XY type of 



Fig. 19-7. A sex-linked trait in Melandrium. At left, normal plant. At right, a narrow- 

 leaved mutant, the gene for which is located in the X chromosome. ( From Shull. ) 



sex determination but expressed in the gametophyte generation. A good 

 many other dioecious liverworts and mosses have been found to possess 

 a similar pair of sex chromosomes. It is significant that when such 

 gametophytes are made diploid they become monoecious, evidently be- 

 cause they now possess both types of chromosomes, though in such 

 cases the gametes usually fail to function. The genetics of bryophytes has 

 been reviewed by Allen ( 1935, 1945 ) . 



There are a number of complications in the chromosome theory of sex 

 determination in the higher plants, however. Bryonia, which, like 

 Melandrium, is clearly male-heterozygous on breeding evidence, has no 

 unequal chromosome pair, and it turns out that visibly unequal chromo- 

 somes, presumably sex chromosomes, are present in only about half 

 the genera of dioecious plants. In some cases, also, like Dioscorea, the 

 female is XX and the male XO, with one chromosome less than the female. 



