GYNANDROMORPHISM 29 



If this explanation is valid, then gynandromorphism in 

 Drosophila melanogaster is the result of the elimination of an 

 X-chromosome from the nucleus of one of the cells pro- 

 duced by the first cleavage division of a female zygote (XX). 

 XO tissue is male tissue. If such elimination occurs at the 

 second cleavage division a quarter of the body becomes XO 

 in constitution and male in characterization. The later in 

 embryonic development this elimination occurs the less will 

 be the amount of male tissue in such an individual. When 

 in an elimination gynandromorph the abdomen is affected 

 and is male-type, the individual behaves as a male but is 

 invariably sterile. 



Morgan and Bridges (19 19) found in a series of about a 

 hundred that the maternal X was eliminated about as 

 frequently as was the paternal. 



Doncaster (1914) noted that in the moth Abraxas there 

 was occasionally to be encountered an egg with two separate 

 maturation spindles and two female pronuclei each about to 

 be fertilized by a separate sperm. If each of these pronuclei 

 was fertilized by a separate spermatozoon, and if one of these 

 was X-chromosome-bearing, the other Y-chromosome- 

 bearing, such double fertilization could yield a gynandro- 

 morph. Bridges (Morgan, Bridges and Sturtevant, 1925) 

 came across such a dizygotic gynandromorph in Drosophila 

 melanogaster in a back cross involving the recessive char- 

 acters speck and vestigial, the genes for which are resident 

 in the second chromosome. The right side of the body was 

 predominantly female and displayed the character speck; 

 the left side was mostly male and exhibited the vestigial 

 wing character. The ovum had two nuclei, in each of which 

 was an X and a 2nd chromosome. One of these nuclei united 

 with a sperm carrying an X and a 2nd, the other with a 

 sperm with a Y and a 2nd. From each of these fertilized 

 nuclei one side of the body developed. 



Egg nucleus Sperm 



Right side X 2nd speck X 2nd vestigial speck 



Left side X vestigial Y vestigial speck 



Only rarely was it found that these dizygotic mosaics differed 

 laterally in respect of the sex-characters. 



