FOUNDATIONS FOR SEX 



39 



both sets of data are small and toward the 

 male side. As Westergaard points out, the 

 strains used by these investigators are of 

 different geographic origins. The evolution- 

 ary history of the two strains may have a 

 bearing on the lesser Y and greater X ef- 

 fects on the sex of the American types. 

 Chromosome changes seem to have occurred 

 in the strains before the studies of Warmke 

 and Westergaard and will be discussed. 



The location of the sex determiners has 

 been studied by both investigators utilizing 

 techniques by which the Y chromosome be- 

 comes broken at different places. These 

 breakages may occur naturally and at fairly 

 high rates in individuals which are Y + 2X 

 + 2A. These facts suggest that the break- 

 age of the Y chromosome occurs in meiosis 

 since the breakage comes in selfed individ- 

 uals of highly inbred stocks where heterozy- 

 gosity is not to be expected, in the Y 

 chromosome which has no homologue thus 

 does not synapse, and in the second meiotic 

 <li vision. Plants showing these initial break- 

 ages seem to have the same constitution in 

 all of the somatic cells of different organs as 

 well as in the germ cells, again pointing to 

 meiosis as the time of breakage. 



In AVarmke, Davidson and LeClerc's 

 (1945) material, the normal offspring which 

 resulted from selfing 2A, 2XY plants, were 

 2A, 2XY (male hermaphrodites), 2A, 2X 

 (female), 2A, XY (male), and 2A, X2Y 

 (suiiermales) , and in addition two abnormal 

 hermaphroditic classes. These were: (1) a 

 type in which the female structures were 

 liighly developed, essentially as well de- 

 veloped as in 2A, 2X females and with nor- 

 mal stamens; and (2) a type in which there 

 was a complete failure of stamen develop- 

 ment shortly after meiosis. Cytologic ex- 

 amination showed these types to be as- 

 sociated with the Y chromosome breaks. 

 The first type occurred when the homol- 

 ogous (synaptic) arm of the Y was de- 

 ficient. The deficiencies ranged in size 

 from a short terminal loss to one which 

 seemed to include the entire, or nearly en- 

 tire, homologous arm. It was of importance 

 that the degree of abnormality was not pro- 

 l)ortional to the length of the deficiency. 

 Once a small terminal segment was lost the 

 change in sex type occurred and larger 



losses seemed to have no more pronounced 

 effect. Chromosomes of this type showed 

 complete asynapsis of the Y chromosome, 

 indicating that its synaptic element compar- 

 able with the X was lost. Loss of as much as 

 one-fifth to one-fourth of the arm prevented 

 synapsis. These results seemed to indicate 

 that the Y chromosomes had lost elements 

 which acted as suppressors to the female de- 

 velopment. 



The second type observed by Warmke 

 was associated with a break in the differ- 

 ential arm of the Y chromosome. A 

 small terminal loss of this differential 

 arm was sufficient to cause male devel- 

 opment to be arrested and sterility to re- 

 sult. The plants that had lost as little as 

 one-fourth the differential arm were there- 

 fore male sterile and were also indistin- 

 guishable from plants that had lost both 

 arms. The altered X chromosomes retained 

 their centromeres and were carried through 

 mitotic growth divisions to every cell of the 

 ])lant. Only in rare cases, and with very 

 small fragments, was there evidence that 

 somatic loss may have occurred. From these 

 results Warmke concluded that the Y chro- 

 mosome contained at least three gene com- 

 plexes which operated in the development 

 of maleness. First there was one near the 

 centromere and present in the smallest frag- 

 ment of the Y chromosome which initiated 

 male development. The stamens developed 

 but only just past meiosis. The second fac- 

 tor was found near the end of the differ- 

 ential arm of the Y chromosome and in- 

 fiuenced complete male development. When 

 the entire differential arm of the Y chromo- 

 some was present, full male development 

 resulted. The third element appeared on the 

 terminal fourth of the homologous arm of 

 the Y chromosome and suppressed female 

 development. Whether this was in the pair- 

 ing segment or close to it was uncertain. In 

 individuals with entire Y chromosomes, 

 plus two X chromosomes, the female struc- 

 tures were underdeveloped with only a 

 small percentage of the blossoms capable 

 of setting capsules with seed. When the 

 homologous arm was deficient the female 

 development was complete and every blos- 

 som produced seed-filled capsules, again 

 supporting the conclusion that this part of 



