FOUNDATIONS FOR SEX 



31 



(DNA) as the all inclusive agent in inherit- 

 ance for they are clearly DNA-positive. 



These cytogenetic observations clear up 

 most of the events leading to sex differentia- 

 tion and the delayed time in embryologic 

 development when it takes place, but, as 

 INIetz points out, other puzzling questions 

 are raised. Observed chromosome extrusions 

 from cell nuclei are rarities today even 

 though discovered for Ascaris 60 years ago. 

 Why should this mechanism be so well de- 

 fined in Sciara? Why should the mode of 

 elimination be so accurately timed and yet 

 be different in method and time for the 

 soma and germ cell lines? Two of the three 

 X chromosomes in the fertilized egg are 

 sisters from the father and should be identi- 

 cal. What is special in the 7th or 8th cleav- 

 age that causes elimination in the soma 

 plasm but not in the germ plasm? Why 

 should the casting out of one of these same 

 paternal X's in the germ plasm of both 

 sexes be reserved for a much later stage in 

 cleavage and by a different procedure? 



The observations of Grouse (1943, 1960a), 

 obtained from translocations and species 

 crosses, are critical in showing that the 

 chromosome first moA'ing in its entirety to 

 the first differentiating pole of the second 

 spermatocyte division is the X and is the 

 one taking part in the later postfertilization 

 sex chromosome elimination of the 7th or 

 8th cleavage. The X heterochromatic end of 

 the chromosome and not the centromere re- 

 gion seems to be responsible for the unique 

 behavior of this chromosome (Grouse, 

 1960b). Induced nondisjunctional eggs of 

 X'X (female-producing) mothers having as 

 a consequence no sex chromosomes, when 

 fertilized by the normal sperm bringing in 

 XX chromosomes, develop into embryos 

 eliminating one of these X's at the 7th or 

 8th cleavage, as expected of the eggs of 

 X'X mothers, and become "exceptional" 

 males with X + 2A soma and XX paternal 

 germ line. Imagoes of these embryos become 

 functional but partially sterile males trans- 

 mitting the paternal X, a reversal of the 

 normal condition. Similarly the nondis- 

 junctional eggs retaining both X's, XX 

 from male-producing mothers, and having 

 4 X's on fertilization eliminate the two pa- 

 ternal X's, as occurs normally in XXX em- 

 l)ryos, to become "exceptional" females with 



2X or sometimes in species hybrids 3X 

 soma. 



Phenotypic sex in Sciara on this evidence 

 depends on the adjustment of the X chro- 

 mosome numbers and their contained genes 

 with this adjustment regularly taking place, 

 not at fertilization as in most forms like 

 Drosophila, but at the 7th or 8th cell cleav- 

 age of embryologic development. The X' vs. 

 X chromosomes of the mother predispose 

 her haploid egg cytoplasm to time specific 

 chromosome elimination. The germ line cells 

 on the other hand may regularly have other 

 chromosome numbers than the soma or ex- 

 ceptionally tolerate other numbers as extra 

 X's (Grouse, 1943), or extra sets (Metz, 

 1959). 



Triploids of 3X and 3A soma have not 

 been found in pure Sciara species. Salivary 

 gland cells of a triploid hybrid S. ocellaris x 

 aS. reynoldsi have two sets of S. ocellaris and 

 one of S. reynoldsi chromosomes (Metz, 

 1959). Ghromosome banding in 2X -I- 3A, 

 3X + 3A larval glands showed the »S. ocel- 

 laris homologues were heterozygous. The 

 3X + 3A type chromosomes were of typical 

 female appearance. The 2X:3A-type had 

 X chromosomes of typical male type, very 

 thick, pale and diffuse. The mosaic sali- 

 varies were a mixture of the two types of 

 cells. These results suggest that the 3X:3A 

 would be a typical female. The intersexes 

 2X -I- 3A conceivably have a range in male 

 and female organ development depending 

 on how the S. ocellaris and S. reynoldsi 

 chromosomes act. If the chromosomes are 

 equivalent, the gene expression would be 

 expected to be that of a 2X to 3A or a 

 phenotypic intersex. If they act separately 

 the S. ocellaris genes would be in balance for 

 female determination, whereas the S. reyn- 

 oldsi autosomal genes would have no X S. 

 reynoldsi genes to balance them and pre- 

 sumably would be overwhelmingly male in 

 effect. The phenotypic outcome would be 

 in doubt. 



The maternally governed cleavage and 

 chromosomal sequence occurring before the 

 8th embryonic cell division, although re- 

 lated to the subsequent events leading to the 

 particular sex, is unique to Sciara and its 

 relatives. Phenotypic sex expression appar- 

 ently starts with the somatic nuclei having 

 either X + 2A or XX + 2A as their chro- 



