FOUNDATIONS FOR SEX 



29 



forms some of which were created as in- 

 duced developmental, chromosomal, or hor- 

 monal abnormalities. The complexities re- 

 sponsible for the remarkable facts were 

 analyzed by Metz and his collaborators. 

 The following brief review is based upon 

 Metz's (1938) summarization of the chro- 

 mosome behavior problem to which the 

 reader is referred for further information. 

 Of Sciara species studied 12 out of 14 have 

 their basic chromosome groups composed of 

 three types of chromosomes: autosomes, 

 sex chromosomes, and "limited" chromo- 

 somes. Two species, S. ocellaris Comstock 

 and S. reynoldsi Metz, lack the "limited" 

 chromosomes. Two sets of autosomes and 

 three sex chromosomes are found in the 

 zygotes of all Sciara species at the comple- 

 tion of fertilization. The behavior of these 

 chromosomal types will be considered in 

 sequence. 



The autosomes behave normally in so- 

 matic mitosis and in oogenesis. There is 

 cytologic and genetic evidence for synapsis, 

 crossing over, random segregation, and reg- 

 ular distribution of chromosomes and genes 

 in the female. 



In spermatogenesis the story is quite dif- 

 ferent. The first maturation division is uni- 

 polar. Genetic evidence shows a complete, 

 selective segregation of the maternally de- 

 rived autosomes and sex chromosomes from 

 those of paternal origin. The paternal homo- 

 logues move away from the pole, are 

 extruded, and degenerate. The second 

 spermatocyte division is likewise unequal 

 and one of the products, a bud, degenerates. 

 Thus a spermatogonial cell passing through 

 meiosis gives rise to only one sperm. At the 

 second spermatocyte division all of the 

 chromosomes (maternal homologues) except 

 the sex chromosome undergo an equational 

 division but both halves of the sex chromo- 

 some enter into the same nucleus. This nu- 

 cleus becomes the sperm nucleus. The chro- 

 mosomes at the opposite pole form a bud 

 and degenerate. Fertilization of the Sciara 

 egg is normally monospermic not poly- 

 spermic. 



The sex chromosomes XXX of the fer- 

 tilization nucleus are derived, two from the 

 sperm and one from the oocyte. Their des- 

 tiny depends on whether the egg they are 



in develops into a male or a female imago. 

 In male early development, those nuclei 

 which are to become male soma lose the two 

 sex chromosomes XX, contributed by the 

 father's sperm, to give X + 2A soma. These 

 chromosomes fail to complete mitosis at 

 the 7th or 8th cleavage division and are left 

 to degenerate in the general cytoplasm when 

 there are no true cells in the soma region 

 and no membranes surround the nuclei. 

 Those embryos which are to become female 

 soma at the same cleavage cycle eliminate 

 but one paternal X chromosome. On a chro- 

 mosome basis the soma cells respectively 

 become X, plus two sets of autosomes, give 

 rise to males on differentiation, or become 

 XX + 2A and develop female organs (Du 

 Bois, 1932). The germ line nuclei for each 

 sex, on the other hand, remain unrestricted 

 in their development. They retain their 

 XXX constitutions until the first day of 

 larval life, or about 6 hours before the for- 

 mation of the left and right gonads (Berry, 

 1941), when they eliminate a single pater- 

 nally derived X. The X which is rejected or 

 makes its own exit is always one of two 

 sister chromosomes contributed by the 

 father. The process of loss is strikingly dif- 

 ferent from that noted in the soma-building 

 nuclei. Both cell walls and nuclear mem- 

 branes are present. The path of the X chro- 

 mosome is through the nuclear membrane 

 into the cytoplasm where degeneration 

 eventually takes place. The loss occurs at 

 a time when there is no mitotic activity and 

 the chromosomes are separated from the 

 cytoplasm by an intact nuclear membrane. 

 Some Sciara species are characterized by 

 females which produce only unisexual fam- 

 ilies — practically all females or practically 

 all males. Genetically, the sex of progeny is 

 accounted for by sex chromosomes, X' and 

 X, which are so designated because of their 

 physiologic properties. Mothers having only 

 female progeny are characterized by always 

 having the X' chromosome in all their cells, 

 X'X + 2A, whereas mothers whose progeny 

 are all males are XX + 2A (Moses and 

 Metz, 1928). The all female and all male 

 broods are observed in about equal num- 

 bers. The male has no influence on sex ratio. 

 Normally the progeny in all female broods 

 will be X'X + 2A or XX + 2A in soma 



