ADJUSTMliNT AND BALANCE 



completeness of the parental contributions, the greater should be the 

 loss of vigour and fertility in the hybrid. 



One consequence of this rule is to be seen in crosses between 

 sexually differentiated species, hi such cases the sexes very commonly 

 differ in their vigour and fertility; and, as Haldane pointed out, it 

 is the XY or heterogametic sex which is less vigorous and less fertile 

 and even less frequent. The difference may be so extreme that only 

 one sex survives to maturity, as in the moth crosses Chacrocampa 

 clpenor X Mctopsiius porcellus and Deildphila euphorhiae X -D. galli. 

 Here Federley has found that although the males survive the hetero- 

 gametic females die as pupae. Or the difference may not appear 

 until germ cell formation, as in Drosophila pscudoohscura X D. 

 persimilis, where the females arc fertile but the males, otherwise 

 normal, are made sterile by a wholly irregular process of meiosis. 



Why should disharmony affect the heterogametic sex more 

 strongly l In the homogametic sex both autosomes and sex chromo- 

 somes are derived equally from the two parents. In the heterogametic 

 sex they are derived unequally; indeed the sex chromosome 

 contribution may come entirely from one parent. The sex-autosome 

 balance can be neither that of one parent, nor intermediate between 

 those of the two. It is much more likely, therefore, to be out of gear 

 in the heterogametic than in the homogametic sex (Fig. 54). 



The difference between the two is thus an expression of the same 

 kind of balance between sex chromosomes and autosomes as that 

 which is responsible for a DwsopJiila with two X chromosomes 

 growing into a female if diploid, but into an intersex if triploid, for 

 the autosomes (Fig. 55). 



Now comparison shows that the composition of the sex chromo- 

 somes varies between species. One arm in the X chromosome of 

 both D. pscudoohscura and D. persimilis, for example, corresponds 

 to an arm of an autosome in D. mclanogaster: there has been a 

 translocation in the history of the species. Smaller structural changes 

 and genie changes will be even more characteristic of the relations 

 between the sex chromosomes of different species. The balance of 

 sex chromosomes and autosomes must therefore often differ between 

 species, and we might expect to fmd that reciprocal species crosses 

 give similar results in regard to the liomogamctic sex, but differ in 

 their heterogametic offspring (Fig. 54). This is in fact true. In 



230 



