FOUNDATIONS FOR SEX 



17 



rise to 6 chromosomally different types of 

 offspring: diploid males, diploid females, 

 triploid females, intersexes, females carry- 

 ing a Y and a male limited 4th chromosome 

 hut otherwise diploid, and tetraploid fe- 

 males. All intersexes cytologically show a 

 Y and a 4th chromosome present. Intersexes 

 without the Y are presumed also w^ithout 

 male limited 4th chromosomes and would 

 be expected to be inviable or very weak. 

 No supermales or superfemales, that w^ould 

 correspond with those found in D. melano- 

 gaster, were observed so are presumed to 

 l)e inviable due to unbalance for the 4th 

 chromosomes. Among 948 progeny of trip- 

 loid females X diploid males there were 9 

 individuals that were phenotypically abnor- 

 mal females. They had slightly spread, ven- 

 trally curved wings with slightly enlarged 

 wing cells. In 8 of the 9 the first section of 

 the costal vein was shortened so that no 

 junction was made with the first vein at the 

 distal costal break. Heads were large with 

 rough eyes, thoraxes shortened, legs fre- 

 (luently malformed, and abdomens small 

 with unusually wide 7th sternites. Genitalia 

 were apparently normal with well devel- 

 oped ovaries. This type carries three doses 

 of any genes contained in the 4th chromo- 

 some to two doses of the genes in the other 

 chromosomes. Its phenotype represents a 

 trisomic condition. 



The sex characteristics of the flies ob- 

 served in D. americana seem to follow the 

 same patterns as those of D. melanogaster 

 as judged by the numbers of the X chromo- 

 somes and autosomes. A Y chromosome in 

 the intersex was not observed to affect sex 

 expression. The intersexes could be grouped 

 into six classes ranging from extreme male 

 type to the most female type. The male 

 type showed largely male organs, courted 

 females, and had motile but nonfunctional 

 sperm. The most female type had nearly 

 normal ovipositor plates, well developed 

 uterus, ventral receptacle, spermathecae 

 and oviducts. At least one gonad showed 

 egg strings, although a small patch of 

 orange-red tissue was present at the tip. 

 Two types of chromosomes were observed 

 in the nuclei of these extreme female type 

 intersexes; those like the chromosomes in 

 any normal diploid cells and some which 



were so swollen as to be almost unrecog- 

 nizable. Such swollen chromosomes were 

 not found in the other classes of intersexes 

 or in diploid or triploid individuals. They 

 are suggestive of some noted by Metz 

 (1959) in Sciara. Most of the intersexes 

 were of the male type, 45 per cent, with de- 

 creasing numbers for each of the other five 

 classes until those in the most female class 

 constituted only about 4 per cent of the 

 total. 



D. LOCATION OF SEX-DETERMINING GENES 



The problems of isolating and determin- 

 ing the modes of action of the factors nor- 

 mally operating in sex determination have 

 received extensive study since they were 

 reviewed by Bridges in 1939: Patterson, 

 Stone and Bedichek (19371, Patterson 

 (1938), Burdette (1940), Pipkin (1940- 

 1942, 1947, 1959), Poulson (1940), Stone 

 (1942), Dobzhansky and Holz (1943), 

 Crow (1946), and Goldschmidt (1955). 

 From his work on Lymantria dispar, Gold- 

 schmidt concluded that sexual differentia- 

 tion was controlled by a major male factor, 

 M in the Z chromosome and a factor F 

 directing development toward the female 

 and at first assumed to be in the cytoplasm 

 but later considered to be in the W chromo- 

 some. The heterogametic female of this 

 species would then be FM and the male be 

 MM. In considering this problem, Gold- 

 schmidt attempted to distinguish between 

 the sex determiners responsible for the F/M 

 balance and modifiers affecting special de- 

 velopmental processes (Goldschmidt, 1955). 

 At the other extreme Bridges' study of trip- 

 loids led him to consider that sex in Droso- 

 phila was determined by the interaction of 

 a number of female tendency genes found 

 largely in the X chromosome and of genes 

 having male bias located largely in the au- 

 tosomes. These numerous genes were con- 

 sidered as being distributed throughout the 

 whole inheritance complex. Search for the 

 more exact locations of these genes within 

 the different chromosomes of Drosophila 

 has largely taken the form of determining 

 the variation in sex types as induced by the 

 addition or deletion of various pieces of the 

 different chromosomes to either the normal 

 male, normal female, or triploid complexes. 



