i.N.S 



( IIAI'TER 39 



described in terms of the apr allele. Fe- 

 males having apr in triple dose (the extra 

 apr [OCUS is carried in another chromosome ) 

 have darker apricot eyes than apr apr fe- 

 males, demonstrating not only the hypo- 

 morphic nature of the mutant but also the 

 direction of dosage compensation — namely, 

 suppression of eye pigment formation in 

 apr apr females and the eonsequent level- 

 ing-ofi of pigment formation at the level 

 produced by one apr locus present in the 

 X chromosome of a male. Males carrying 

 an extra apr locus have apricot eyes even 

 darker than those of females with triple apr. 



Since XO and XY males and XX, XXY, 

 and XXYY females — all pure for apr — 

 have the same eye color, the Y chromo- 

 some cannot be responsible for dosage com- 

 pensation. In addition, males transformed 

 from females (X" p ' X""'. tratra) with or 

 without a Y chromosome have the same 

 eye color as an X'"" Y male. If maleness 

 as such prevents the suppression of gene 

 action leading to dosage compensation, the 

 transformed-from-female male with a double 

 dose of apr should have a darker apricot 

 eye than a male with a single dose of apr. 

 Thus, dosage compensation is not dependent 

 upon male or female phenotype. 



What is the genotypic basis for dosage 

 compensation in Drosophila? That the 

 male (transformed female) with two full X 

 chromosomes shows dosage compensation, 

 while the male with one X opr and another 

 apr — either in a grossly deleted X or in- 

 serted into an autosome — does not, suggests 

 that the X chromosome itself contains dosage 

 compensator genes, a single dose of such 

 genes present in males and a double dose in 

 females. One complete X chromosome ap- 

 parently carries several dosage compensators 

 to suppress one apr gene. In a regular X""' 

 Y male, this suppression permits only apri- 

 cot eye color. Note that females with the 

 apr region in one X deleted, and apr present 

 in the other have a light apricot eye color. 



Pure apr males or females hyperploid for 

 different short segments of X can be ob- 

 tained and scored for eye color and sex. 

 Such experiments show that: 



1 . Different X segments have a positive 

 or negative dosage compensating effect 

 on eye color with an overall effect of 

 suppression 



2. The effects of these segments on eye 

 color are not correlated with their ef- 

 fects on sex differentiation. 



Moreover, the net dosage compensation ef- 

 fects exerted by individual segments are dif- 

 ferent when other loci showing dosage 

 compensation are investigated. In conclu- 

 sion, therefore, we find: 



1. No correlation between a set of com- 

 pensator genes and their effect on sex 

 differentiation 



2. Suppression of different genes exhibit- 

 ing dosage compensation either by dif- 

 ferent groups of dosage compensator 

 genes or by the same groups of com- 

 pensator genes whose action varies 

 with the locus to be compensated. 



X-linked genes or alleles without dosage 

 compensation may be so new in terms of 

 evolution that dosage compensator genes 

 may not yet have had an opportunity to be- 

 come established. Supporting this view is 

 evidence that eosin (W ) and ivory (w 1 ) 

 which do not show dosage compensation are 

 nonallelic to apr '- and, therefore, may be 

 mutants of a more recently-evolved locus. 

 Additional support comes from the study of 

 mutants in the X chromosome of D. psendo- 

 obscura which is V-shaped with one arm 

 homologous to the X, the other to the left 

 arm of chromosome III of D. melanogaster. 

 If we assume that most mutants are hypo- 

 morphs, we find more mutants in the arm 

 homologous to the melanogaster X showing 



12 From work of M. M. Green (1959). 



