Expression ofcis-regulatory mutations of thewhitelocus in metafemales ofDrosophila melanogaster

Abstract

At the white eye colour locus, there are a number of alleles that have altered expression between males and females. To test these regulatory mutations of the white eye colour locus for their phenotypic expression in metafemales (3X; 2A) compared to diploid females and males, eleven alleles or transduced copies of white were analysed. Two alleles that exhibit dosage compensation between males and females {apricot, blood) also exhibit dosage compensation in metafemales. White-ivory and white-eosin, which fail to dosage compensate in males compared to females, but that are distinct physical lesions, also show a dosage effect in metafemales. Two alleles with greater expression in males than females (spotted, spotted-55) exhibit even lower expression in metafemales. Lastly, five transduced copies of white carrying three different lengths of the white promoter, but that all exhibit higher expression in males, show reduced expression in metafemales, exhibiting an inverse correlation between the level of expression and the dosage of the X chromosome. Because these alleles of white respond to dosage compensation in metafemales as a continuum of the male and female responses, it is concluded that the same basic mechanism of dosage compensation is involved and that the dosage of the X chromosome conditions the sexually dimorphic expression. * Present address: 117 Tucker, University of Missouri, Colombia, MO 65211.

Biological consequences of dosage dependent gene regulatory systems

“…Thus, the two effects occurring simultaneously cancel to produce dosage compensation. This type of mechanism has been recapitulated for cases of dosage compensation in Drosophila (14)(15)(16)(17) and recent evidence suggests an involvement of balance in cases of dosage compensation in plants and for sex chromosomes in Drosophila, C. elegans and mammals (18).…”

Dosage compensation and inverse effects in triple X metafemales of Drosophila

“…The protein components of this complex are associated with the X chromosome in males but not in females (1,(3)(4)(5). However, there is also evidence for dosage compensation of X chromosomal genes in triple X metafemales (6)(7)(8)(9)(10), which have no MSL complex. When the MSL2 protein was ectopically expressed in females and metafemales, no changes of X chromosome expression were observed (10).…”

“…From these data, it was suggested that the MSL complex did not act as the direct mediator of dosage compensation. In contrast, inverse dosage effects produced by genomic imbalance are believed to play an important role in modulation of X expression and some other genotypes with compensation (8)(9)(10)(15)(16)(17).…”

Commonalities in compensation

“…The new results require a re-evaluation of the function of these complexes. In Drosophila, dosage compensation occurs under some conditions in which the MSL complex is not present such as triple X metafemales, (18)(19)(20) and as noted above, the male germline. (2) In metafemales, three X chromosomes are present in an otherwise diploid genotype.…”