30 



BIOLOGIC BASIS OF SEX 



and germ lines, whereas the all male prog- 

 eny broods will be only X + 2A in the soma 

 and XX + 2A in the germ line. 



Studies of Grouse (1943, 1960a, b) show 

 nondisjunction may occur and eggs entirely 

 lacking sex chromosomes or having double 

 the normal number may be produced. When 

 the nondisjunctional eggs lacking X chro- 

 mosomes are from X'X females and are fer- 

 tilized by sperm contributing two sister X 

 chromosomes, one X (as expected of X'X 

 mothers, not two as expected of XX cells) is 

 eliminated at the 7th or 8th cleavage to give 

 cells which develop into "exceptional" males 

 with XO + 2A soma and XX + 2A germ 

 line, all X chromosomes being of paternal 

 origin. The casting out of the single X chro- 

 mosome is consequently controlled by the 

 characteristics of the egg cytoplasm derived 

 from the X'X mother rather than by the 

 kind of chromosomes present in the nuclei 

 at the 7th or 8th cleavage stage of the em- 

 l)ryo. 



Similarly when nondisjunctional eggs 

 having both XX chromosomes of the XX 

 mothers are fertilized by the normal sperm- 

 bearing XX chromosomes, the zygotes are 

 XXXX. At the 7th or 8th cleavage both 

 paternal X chromosomes are eliminated and 

 the resulting soma develops into an ''excep- 

 tional" female, XX + 2A, often with 3 X's 

 in her germ line. These observations con- 

 firm the earlier interpretations that the X'X 

 or XX constitution of the mother conditions 

 her eggs respectively to cast out 1 or 2 

 parental X chromosomes at the 7th or 8th 

 cleavage according to the cytoplasmic con- 

 stitution of the egg. Somatic sex develop- 

 ment is visually determined only after this 

 stage when the chromosomes of the somatic 

 cells take on the familiar aspect of either 

 XX + 2A for the females or X + 2A for the 

 males. 



Although at present there are few exam- 

 ples in other species, this cytoplasmic con- 

 ditioning of early embryologic development 

 by the mother's genotyj)e may be a common 

 rather than a rare occurrence of develop- 

 ment. The expression of the events to which 

 this control may lead may be quite differ- 

 ent in different cases. The spiral of snail 

 shells was an early recognized case (Sturtc- 

 vant, 1923) . In Drosophila, a gene acting 

 through the mother's genome allows onlv 



male embryos to survive (Gowen and Nel- 

 son, 1942), suggesting that this gene acts 

 through her cytoplasm in a manner com- 

 parable to that of the X' and X chromo- 

 somes of Sciara. In either case the geno- 

 type would not be considered a primary sex 

 factor, profound as the effect on sex may 

 ultimately be. 



Sciara also has strains or species where 

 the progeny of single matings are of both 

 sexes, bisexual, yet the chromosomal elimi- 

 nation mechanisms remain as described 

 above save for the fact that they now op- 

 erate within broods rather than between 

 broods. Sex ratios for the families within 

 these bisexual strains are extremely varia- 

 ble, 1:0 or 0:1 ratios not being infrequent 

 and 1 : 1 ratios being the exception. An in- 

 stance of a bisexual strain arising from a 

 unisexual line has been analyzed by Reyn- 

 olds (1938). The females were found to be 

 3X in their germ line and produced 2X eggs 

 which developed into daughters and X eggs 

 which became sons. Loss of the extra X 

 from this line resulted in the line's return to 

 unisexual reproduction. 



In general, bisexuality as contrasted with 

 unisexuality seems to be caused by differ- 

 ences in the X'X-XX mechanisms, either 

 specific for the mechanism, or induced by 

 modifying genes which cause different em- 

 bryos within the same progeny to cast out 

 sometimes one or sometimes two X chromo- 

 somes from all their somatic nuclei. This 

 casting out occurs uniformly for all nuclei 

 of a given embryo as is shown by the fact 

 that sex mosaics or gynandromorphs are as 

 seldom observed in bisexual as unisexual 

 strains. 



Some species of Sciara have large chro- 

 mosomes which are in essence "limited" to 

 the germ line since they are lost from the 

 somatic nuclei at the 5th or 6th cleavage 

 division. Other species lack these chromo- 

 somes, yet agree with the rest in the be- 

 havior of their remaining chromosomes and 

 in sex determination. So far as is now 

 known the "limited" chromosomes seem 

 empty of inheritance factors influencing ei- 

 ther sex or unisexual vs. bisexual broods or 

 for that matter other characteristics (their 

 persistence seems to make this latter un- 

 likely I. Their presence does lead to a ques- 

 tion of the efficacv of desoxvribonucleic acid 



