MOLECULAR GENETICS OF GENE DOSAGE COMPENSATION AND SEX

In Drosophila sex is determined by the ratio of the number of X
chromosomes to the number of sets of autosomes. Cells with one
X chromosome and two sets of automsomes are males and cells
with 2 X chromosomes and two sets of autosomes are females.
Since males have the same requirements for X-linked gene
products as females, they respond by hyperactivating the X
chromosome at the transcriptional level to compensate for the
difference in gene dosage. This project is aimed at two questions:
how do male cells double the transcription rate of virtually every
gene on the X chromosome, and how do cells count the number of
X chromosomes. The first question will be addressed by using the
gene for 6-phosphogluconate dehydrogenase (Pgd) as a model gene
for the X chromosome. In vitro manipulation of the gene and its
promoter, followed by reintegration into the Drosophila genome,
will be used to find the cis-acting regulatory site of the Pgd gene
that is responsible for the dosage compensation response. Dosage
compensation is an important topic in the study of gene regulation
since it changes the expression levels of a large battery of genes
by exactly two-fold. This subtle regulation may be controlled by
a novel mechanism, or by novel application of conventional
mechanisms. Understanding dosage compensation will be an
important step in understanding the coordinated regulation of a
large group of genes, which is also a significant facet of
development and neoplasia. The question of how the X
chromosomes are counted will be attacked by cloning the Tp1
gene either by a chromosome walk or transposon tagging, and then
studying the expression of the gene. Tpl is a locus on chromosome
3 which is extremely dosage sensitive. Drosophila requires
exaclty 2 doses of Tpl for survival. Individuals bearing 1 or 3
doses die as embryos. It has been suggested that Tpl is involved in
counting the X chromosomes for two reasons. One would expect a
gene that titrates the X chromosomes against itself to be dosage
sensitive, and in addition, Tpl appears to interact with the X in
aneuploids. If this hypothesis turns out be incorrect, then Tpl
must perform a vital function in embryogenesis or cellular
metabolism. In either case the study of Tpl has for-reaching
implications for sex determination, the consequences of
aneuploidy, and embryogenesis. Drosophila is one of very few
organisms in which such a gene can be studied since it has a
dominant lethal phenotype.