20 



BIOLOGIC BASIS OF SEX 



determining functions. It would further 

 show that autosomes may behave differ- 

 ently with regard to their sex-determining 

 properties according to the chance distribu- 

 tion of sex genes which happen to fall 

 within them as they do in Rumex (Yama- 

 moto, 1938j. The finding that the chromo- 

 some IV has a bias toward female tenden- 

 cies further strengthens Bridges' multiple 

 sex gene theory and weakens the theory of 

 an all-or-none action of the whole X chro- 



IV. Sex under Special Conditions 



A. SPECIES HYBRIDITY 



Hybrid progeny coming from species 

 crosses are apt to represent but a very few 

 of the possible genotypes of the total num- 

 ber that conceivably could come from the 

 gene pool. The hybrid phenotypes may dis- 

 play three kinds of characteristics. The 

 common set is that derived from genes in 

 either or both parents through ordinary 

 meiotic segregations and dominance. The 

 second set shows intermediate develop- 

 ment of the characters found in the two 

 parent species. The third set of characters 

 that complete the animal is new to those 

 observed in either parent species. These new 

 characters may be the loss of a few dorso- 

 central and scutellar bristles, broken or 

 missing cross veins, or abnormal bands in 

 the abdomen as in D. simulans x D. melano- 

 gaster hybrids (Sturtevant, 1920b), extra 

 antigenic substances as found in dove hy- 

 brids (Irwin and Cole, 1936), or more nu- 

 merous characteristics as in the mule. Fre- 

 quent among these new characteristics is 

 sterility. The sterility may extend to either 

 or both sexes and affect the secondary sex 

 ratios. As Sturtevant (1920b) points out, 

 crosses between the domestic cow and male 

 bison give male offspring with humps de- 

 rived from the bison which are so large as 

 to prevent their being born alive. The fe- 

 male hybrids lack these humps and are con- 

 se(iuently born normally. The abnormal sex 

 ratio observed at time of birth is due to 

 causes external to the hybrid itself and at- 

 tributable to the structure of the mothers. 



A comparable case was found during the 

 study of female sterility in interspecies hy- 

 brids of Drosophila pseudoobscura in which 



Mampell (1941) showed that in the hybrids 

 of certain strains, the females produced no 

 or few offspring because of interspecies le- 

 thal genes connected with a maternal effect. 

 Comparable cases as well as those depend- 

 ent on other mechanisms are known for 

 other groups. The progeny may also be al- 

 tered to give new sex types, generally inter- 

 sexes. These intersexes often replace either 

 the male or the female sex group. However, 

 despite their apparent relation, the changes 

 in the sex ratio and the appearance of in- 

 tersexes can have different causes. D. sim- 

 vlans X D. melanogaster hybrids emphasize 

 that there may be no relation between the 

 peculiar hybrid sex ratios and the intersexes 

 since extreme differences in sex ratio occur 

 but no intersexual types. 



Species, however, may have natural dif- 

 ferences in the sex potencies of their X 

 chromosomes and/or their autosomes. In 

 crosses between D. repleta and D. neo- 

 repleta involving a sex-linked recessive 

 white-eyed mutant type of D. repleta Stur- 

 tevant (1946) obtained about 15 per cent 

 fertile matings in 500 mass cultures, a total 

 of 532 females to 635 males. All progeny as 

 expected were wild type in character. The 

 males, however, had long narrow testes and 

 were totally sterile, a condition later shown 

 to be due to a gene in the X chromosome 

 located near the white locus. Females sug- 

 gested intersexuality in having three anal 

 plates instead of the usual two. Mating of 

 Fi hybrid females to white D. repleta males 

 gave 9 per cent fertility, the 179 offspring 

 being distributed as 70 wild type females, 

 9 white females, 42 wild type males, and 58 

 white males, although the expectation for 

 the classes was equality. Evidence indicates 

 that some of the 9 white females were in- 

 tersexes as were possibly some of the white 

 males. The wild-type males again had the 

 long narrow testes and sterility of the Fi 

 male progeny. Wild type females were mod- 

 erately fertile. By continued backcrossing 

 to 1). repleta males having white or white- 

 singed, a female line was picked up which 

 continued to have the unusual sex ratios 

 but had more fertility. It was presumed 

 that the D. neorepleta gene responsible for 

 the unusual ratios was originally associated 

 with MHotlier gene that decreased fertility 



