Dosage compensation and sex determination in Drosophila: mechanism of measurement of the X/A ratio.
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Date
1982-09
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Abstract
We propose a molecular mechanism for the intra-cellular measurement of the
ratio of the number of X chromosomes to the number of sets of autosomes, a process central to
both sex determination and dosage compensation in Drosophila melanogaster. In addition to
the two loci, da and Sxl, which have been shown by Cline (Genetics, 90, 683, 1978)and others
to be involved in these processes, we postulate two other loci, one autosomal (ω) and the
other, X-linked (π). The product of the autosomal locus da stimulates ω and initiates
synthesis of a limited quantity of repressor. Sxl and π ,both of which are X-linked, compete for
this repressor as well as for RNA polymerase. It is assumed that Sxl has lower affinity than π
for repressor as well as polymerase and that the binding of polymerase to one of these sites
modulates the binding affinity of the other site for the enzyme. It can be shown that as a result
of these postulated interactions transcription from the Sxl site is proportional to the X/A ratio
such that the levels of Sxl+ product are low in males, high in females and intermediate in the
intersexes. If, as proposed by Cline, the Sxl- product is an inhibitor of X chromosome activity,
this would result in dosage compensation. The model leads to the conclusion that high levels
of Sxl+ product promote a female phenotype and low levels, a male phenotype. One
interesting consequence of the assumptions on which the model is based is that the level of
Sxl+ product in the cell, when examined as a function of increasing repressor concentration,
first goes up and then decreases, yielding a bell-shaped curve. This feature of the model
provides an explanation for some of the remarkable interactions among mutants at the Sxl, da
and mle loci and leads to several predictions. The proposed mechanism may also have
relevance to certain other problems, such as size regulation during development, which seem
to involve measurement of ratios at the cellular level.
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Keywords
X-Chromosome transcription, sex-lethal mutations, maternal effect, RNA polymerase, size regulation
Citation
Gadagkar Raghavendra, Nanjundiah Vidyanand, Joshi N V, Chandra H Sharat. Dosage compensation and sex determination in Drosophila: mechanism of measurement of the X/A ratio. Journal of Biosciences. 1982 Sept; 4(3): 377-390.