FOUNDATIONS FOR SKX 



11 



ratio is 1 for the perfect female and 0.5 for 

 the perfect male. Between these values in- 

 tersexual conditions develop. Beyond the 

 value 1, development is overbalanced by- 

 excess female genes resulting in the super- 

 female. Values less than 0.5 create a de- 

 ficiency in the female elements or excess of 

 male elements and a supermale results. 

 Schrader and Sturtevant (1923) proposed 

 another system. Instead of the ratio of X 

 clu-omosomes to autosomes, they suggested 

 that a straight difference between the prod- 

 ucts of the female determining elements of 

 the sex chromosomes and the male effects of 

 the autosomes causes the sex changes. 

 ]3ridges criticized this system on the basis 

 of the fact that progressive polyploidy did 

 not change the sex type or ratio between the 

 X and autosomes, whereas the numerical 

 difference between them would be progres- 

 sively increased. While keeping the X/A 

 ratio as descriptive of the ultimate effects 

 of the genes in these chromosomes, he modi- 

 fied their proposed weight from 1 : 1 for 

 X:A to 1 for the X and 0.80 for the A. 



All formulations for explaining sexual 

 differences are beset with a lack of an un- 

 biased quantitative scale by which these 

 differences can be measured. The estimates 

 of the changes in sexuality are left to the 

 insight of the observer. It seems reasonable 

 to suppose that the quantitative relations 

 between the male and female sex determin- 

 ing elements should have intermediate 

 values when the specimens under observa- 

 tion show a mixture of organs of either sex. 

 This agrees with Bridges' considerations of 

 this problem. It is not so clear, however, 

 that the so-called supersexes^ really are 

 what the names may connote to many 

 readers. The superfemales, with their three 

 X chromosomes and two sets of autosomes, 

 are quite inviable; small in size, wings re- 

 duced and irregularly cut on the margins, 

 ovaries developed to only the early pupal 

 stage, and reproductive tracts much re- 

 duced in size.^ The supermales with one X 



- Recently termed metafemales bj' Stern (1959b). 



'^ Further development of the ovaries is able to 

 take place in normal XX + 2A hosts. Larval 

 XXX + 2A ovaries transplanted into fes/fes hosts 

 IM-oduce eggs in the recipient host which on fer- 

 tilization are capable of developing into adult ima- 

 goes. These imagoes show that there is a low per- 



chromosomc and three sets of autosomes 

 are described as resembling males but are 

 sterile. The wings arc somewhat spread and 

 bristles less in size. They are late emerging 

 and poorly viable. Neither type can be re- 

 ferred to as superior to normal female or 

 normal male in anatomic develoi)ment or 

 physiologic functioning. A new type, re- 

 cently described by Frost (1960j, empha- 

 sizes this difficulty. Females, called triploid 

 metafemales, Table 1.1 had 4X chromosomes 

 and 3 second, 3 third, and 2 fourth auto- 

 somes. Viability was greater than superfe- 

 males but still low. Fertility was about 10 

 per cent. Progeny per female about 10. The 

 flies were like triploids in bristles, eye and 

 wing cells large, sex combs absent. They 

 showed characteristics of superfemales in 

 rough eyes, narrow wings without inner 

 margins, and smaller body build. The dis- 

 tribution of these different Drosophila sex 

 types (Table 1.1) showed that optimal 

 development comes when the X/A values 

 are 1.0 and 0.5. Any deviation away from 

 these values tends to make the sex system 

 less rather than more efficient. 



In normal Drosophila sex differences are 

 probably expressed in every cell making up 

 their bodies. These differences are made 

 visible to us only under special conditions. 

 In adult organ differentiation, the sexual 

 differences are manifest through such 

 things as the body size of the males being 

 about three-fourths that of the females, 

 differences in coloration of the tergites, the 

 appearance of sex combs, the development 

 of the gonads into ovaries and testes, and 

 the formation of a secondary reproductive 

 system composed of several glands and 

 ducts. The origin of these last elements is 

 of particular interest. The ovaries can be 

 distinguished from the testes as early as 

 the second instar through their size and 

 position within the fat body. They are lo- 

 cated about two-thirds the larval length 

 back from the mouth parts. The sex combs, 

 on the other hand, take their origin from 

 imaginal discs which are located in the 

 head region, possibly one-third back from 

 the mouth parts. The secondary reproduc- 



centage of crossing over and high nondisjunction 

 rate in the XXX + 2A ovaries and a high mortality 

 rate in the offspring (Beadle and Ephrussi, 1937). 



