polytene chromosomes

Sex Dosage Compensation in Drosophilia

In mammals, the X and Y chromosomes differ greatly in gene content, as the X chromosome is quite large and contains roughly over a thousand genes (with the Y chromosome containing less than a hundred); so to equalize the dose of gene product (the ratio of autosome to X chrmosome), life has evolved a dosage compensation mechanism to keep the ratio between males and females equal. 

In Drosophilia, dosage compensation occurs by doubling the transcription rate (hypertranscription) of X-linked genes in males.

Shown above are giant polytene chromosomes from the salivary glands, with the male-specfic lethal (MSL) complex labelled with specific antibodies in red and the rest of the DNA stained with Hoechst. There’s precise localization to the X-chromosome, where it significantly catalyzes (alongside the roX RNA the MSL contains) histone modification for gene regulation (specifically acetylation).

In our own species, females regulate dosage compensation by X-inactivation, and therefore every somatic cell has one random X-chromosome silenced (in what’s called a Barr body), with every cell descending from it sharing the same silenced X. 

This is what’s responsible for only female calico cats (males that are XXY are sterlie) having a tortoiseshell coat.