506 The Determination of Sex 



sex chromosomes. In other words, there is no visibly hetero- 

 morphic pair of chromosomes in either sex. It does not neces- 

 sarily mean that there are no chromosomes in these plants that 

 have an important effect on sex determination but merely that 

 any difference that does occur between the chromosomes of the 

 two sexes is not evident from the morphology of the chromo- 

 somes. There may be sex chromosomes that are different from 

 one another physiologically even though not morphologically. 



In plants that have the XY mechanism one might easily sup- 

 pose that the X chromosome might contain female-tendency 

 genes and the autosomes male-tendency genes as in Drosophila 

 melanogaster. As in the fruit fly, evidence on this point might 

 easily be tested if heteroploid types could be produced similar to 

 Bridges's series of intersexes and supersexes. In fact, the greater 

 readiness with which heteroploid types are found in plants would 

 indicate that such studies would be even more promising in them 

 than in Drosophila. Studies of this nature were carried out in 

 Rumex acetosa by Ono and by Yamamoto, who found that as in 

 Drosophila female-tendency genes are in the X chromosome and 

 male-tendency genes in the autosomes. A slight difference from 

 Drosophila was pointed out by Yamamoto, w^ho obtained good 

 evidence that two of the autosomes tended to produce femaleness 

 even though the tendency of the autosomes as a whole was to 

 maleness. 



Very interesting results have been obtained in species of Lych- 

 nis (Melandrium) by Warmke and Blakeslee and independently 

 by Westergaard. In dioecious species of this genus the male is 

 heterogametic. Since the ratio of X chromosomes to autosomes 

 is of the greatest importance in Drosophila melanogaster, natu- 

 rally it was examined in the diploids and the synthesized poly- 

 ploids of Lychnis. Plants with a ratio of one X chromosome to 

 one set of autosomes result from the genotypes 2A -f XX, 3A -j- 

 XXX, and 4A + XXXX and are female as in Drosophila. Two 

 types with ratios of 1.25 (4A + XXXXX) and 1.5 (2A + XXX) 

 are also female. The 1.5 type is a superfemale in Drosophila. 

 One type with a ratio of 0.75 (4A + XXX) and one with a ratio 

 of 0.67 (3A + XX) are females although they ^re intersexes 

 in Drosophila. Finally, plants with four sets of autosomes and 

 two X chromosomes and, therefore, a ratio of 0.5 are female, 

 although flies with a similar ratio are males. Since all these 



