22 



BIOLOGIC BASIS OF SEX 



by frequent gynandromorphs, XO males, 

 and dominant lethals among the rod and 

 ring zygotes. It has been suggested that the 

 instability is due to heterochromatic ele- 

 ments. Hinton (1959) has observed the 

 chromosome behavior of these types in 

 Feulgen mounts of whole eggs that were in 

 cleavages 3 to 8. He found strikingly ab- 

 normal chromosome behavior in these 

 cleaving nuclei. For some cell divisions 

 chromosome reproduction was interpreted 

 as being through chromatid-type breakage 

 fusion bridge cycles. As a result of this be- 

 havior mosaics are formed which are inter- 

 mediate between those of the half gynan- 

 dromorphs and those which occur much 

 later because of somatic crossing over. In 

 terms of volume of cells included, the ab- 

 normal types may include only a few cells 

 of the total organism, a fair proportion of 

 the cells, or a full half of the whole body. 

 These unstable ring chromosome mosaics 

 may be a part of the secondary reproduc- 

 tive system or for that matter any other 

 region of the body. When the mosaic cells 

 are incorporated in the region of sex organ 

 differentiation male or female type organs 

 or parts of organs may develop as governed 

 by the cell nuclei being X, XX or some frac- 

 tion thereof. 



Gynandromor|)lis appear sporadically and 

 rarely in many species but in some in- 

 stances genes which activate mechanisms 

 for their formation are known. In the pres- 

 ence of recessive homozygous claret in the 

 eggs of D. siniulans, gynandromorphs con- 

 stitute a noticeable percentage of the 

 emerging adults. The gene nearly always 

 operates on the X received from the mother 

 causing it to be eliminated from the cell. 

 The resulting gynandromorphs are similar 

 to those of D. melanogaster. The fact that 

 the claret gene should affect the X and a 

 particular X chromosome is suggestive of 

 the manner in which given chromosomes arc 

 eliminated in Sciara. Other types of sex 

 mosaics will be found in the descriptions of 

 other species, i)articularly in the Hyme- 

 noptci'a. 



C. PARTHENO(iENESIS IN DROSOPHILA 



Parthenogenesis is of interest as it 

 changes the sex ratios in families and brings 

 to light new sex types and novel methods 



for their development (Stalker, 1954), A 

 survey of 28 species of Drosophila showed a 

 low rate of parthenogenesis in 23 species. 

 Adult progeny were obtained for only 3 

 species. For D. 'parthenogenetica the origi- 

 nal rate was 8 in 10,000 whereas that for D. 

 polymorpha was 1 in 19,000. These rates 

 could be increased by selection of higher 

 rate parents: 151 and 70 per 10,000 unfer- 

 tilized eggs of the first and second species 

 respectively. 



D. parthenogenetica diploid virgins pro- 

 duced diploid and triploid daughters as well 

 as rare XO sterile diploid males. Triploid 

 virgins produced diploid and triploid fe- 

 males and large numbers, 40 per cent, of 

 sterile XO diploid males. Diploid virgins 

 heterozygous for sex-linked recessive garnet 

 produced homozygous and heterozygous 

 diploid females as well as +/+/g and 

 + /'g/g triploid females. No homozygous 

 wild-type or homozygous garnet triploid fe- 

 males or garnet mosaics were found. Dip- 

 loid females crossed to fertile diploid males 

 produced few if any polyploid progeny or 

 jjrimary X chromosome exceptional types. 

 Of the unfertilized eggs from diploid virgins 

 which started development, 80 per cent died 

 in late embryonic or early larval stages. 

 The i)arthenogenesis in diploid females de- 

 pended on two normal meiotic divisions fol- 

 lowed by fusion of two of the derived hap- 

 loid nuclei to form diploid progeny, or the 

 fusion of three such nuclei to form triploitl 

 progeny. In the triploid virgins similar fu- 

 sions of the maturation nuclei may produce 

 diploid and triploid females but the large 

 number of dii)loid XO sterile males were 

 picsunicd to be the result of cleavage with- 

 out prior nuclear fusion. Such cleavages 

 without fusion in eggs of dijiloid virgins 

 would lead to the production of haploid 

 embryos. They were presumed i-esponsible 

 for the large early larval and embryonic 

 <h'atlis. These obser\ali()ns have been con- 

 firmed by the study of S|)rackling (1960) 

 in\-olving some 2200 eggs at various stages 

 of cleavage. Evidence from XXY diploid 

 virgins indicated that biiuiclcar fusion in 

 unfertilized eggs involved two terminal 

 haploid nuclei or two central nuclei. The 

 fact that tetraploids were not observed as 

 progeny of ti'iploid vii'gins was considered 

 indicative of relative inviability of this 



