FOUNDATIONS FOR SEX 



23 



type. Successful parthenogenesis was under 

 partial control of the inheritance as 80 gen- 

 erations of selection increased the rate 

 about 20-fold. Similarly outcrossing to bi- 

 sexually reproducing males and reselection 

 resulted in both pronounced increases in 

 and survival of the parthenogenetic types. 



Carson, Wheeler and Heed (1957) and 

 Murdy and Carson (1959) have established 

 a strain of Drosophila mangabeirai with 

 only thelytokous reproduction. Males have 

 been captured in nature but are rare. Fe- 

 cundity of the virgin females is low but the 

 egg hatch is 60 per cent and 80 per cent 

 survive to adult stage. The progeny of the 

 virgins are always diploid. In meiotic spin- 

 dle formation D. mangabeirai differs from 

 other Drosophila species in that its orienta- 

 tion increases the probability for fusion of 

 two haploid nuclei into structurally het- 

 erozygous diploid females. The study of 

 Feulgen whole mounts of freshly laid eggs 

 indicated automictic behavior with two 

 meiotic divisions followed by a fusion of 

 two of the four haploid meiotic products. 

 The absence of adult structural homozy- 

 gotes in wild populations is probabl}^ ex- 

 plained by death during early development 

 and by possible fusion of second division 

 meiotic products derived from different sec- 

 ondary oocytes. Stalker (1956a) postulated 

 such selective fusion in order to account for 

 the heterozygous condition in females of 

 Lonchoptera dubia, an automictic, parthe- 

 nogenetic fly in which males are rare or un- 

 known. 



A case in which jihenotypically rudimen- 

 tary females, supposed homozygous for r/r 

 give rare and unexpected type progeny, has 

 suggested that polar body fusion may also 

 take place in D. melanogaster (Gold- 

 schmiclt, 1957). The rudimentary mothers 

 producing the peculiar type are interpreted 

 as formed by a most unusual series of 

 events: a fertilization nucleus derived from 

 the fusion of an r containing egg fertilized 

 by an r containing sperm and a polar copu- 

 lation nucleus derived from the fusion of a 

 polar body containing an r genome and one 

 having wild type. Both cell types become 

 incorporated into the ovary. The progeny 

 which come from these supposed rudimen- 

 tary mothers are presumed to be derived 

 from the maturation into eggs of the cells 



derived from the heterozygous i)olar copu- 

 lation nuclei. If these progeny-producing 

 rudimentary mothers arise in the presumed 

 manner they give the basis for a partheno- 

 genetic mode of reproduction and the sex 

 types which have been described in other 

 Drosophila species by Stalker and Carson. 

 There are similar, as well as other forms 

 of parthenogenesis which affect sex (Smith, 

 1955) or assist in maintaining trijiloid con- 

 ditions (Smith-White, 1955). A number of 

 these types have been reviewed by Suoma- 

 lainen (1950, 1954). The reader may be re- 

 ferred to this material for other cases and 

 chromosome behaviors. 



D. SEX INFLUENCE OF THE Y CHROMOSOME 



The first function discovered for the Y 

 chromosome in D. melanogaster was that it 

 was necessary to male fertility (Bridges, 

 1916). Two and possibly more Y chromo- 

 some-borne, genetic factors were involved 

 (Stern, 1929). Gamete maturation when 

 these factors were lacking ceased just short 

 of the sperm's becoming motile (Shen, 

 1932). The motility conferred on the sperm 

 by the presence of the Y chromosome fac- 

 tors was fixed for the testes at an early 

 stage of development as transplantation ex- 

 periments, sterile testes to fertile larvae 

 and fertile testes to sterile larvae, showed 

 motility to be a property determined by 

 early localized somatic influences on the 

 developing gametes or predetermined in the 

 diploid phase (Stern and Hadorn, 1938). 

 This Y chromosome function was sex lim- 

 ited, because females without a Y were the 

 normal fertile females and those with an 

 extra Y also were fertile. 



Neuhaus (1939) followed by Cooper 

 (1952, 1959) and Brosseau (1960) further 

 analyzed the Y chromosome for fertility 

 loci. The latter showed at least two fertility 

 loci on the short arm and five on the long 

 arm of the Y chromosome. Data are com- 

 patible with a linear order of the genes. An 

 additional fertility factor common to the X 

 and Y was suggested. The Y chromosome 

 fertility factors consequently fall in line 

 with Bridges' concept of multiple gene loci 

 distributed in a more or less random man- 

 ner which may affect sex. 



The Y chromosome has other attributes 

 which help to explain its significance to sec- 



