Drosophila Melanogaster - Sex Determination

Sex Determination

Drosophila have both X and Y chromosomes as well as autosomes. Unlike humans, the Y chromosome does not confer maleness, rather it encodes genes necessary for making sperm. Sex is instead determined by the ratio of autosomes to X chromosomes. Further, each cell "decides" whether to be male or female independently of the rest of the organism resulting in the occasional occurrence of gynandromorphs.

3 major genes are involved in determination of Drosophila sex. These are Sex-lethal, Sisterless and Deadpan. Deadpan is an autosomal gene which inhibits sex-lethal while sisterless is carried on the X chromosome and inhibits the action of deadpan. An AAX cell has twice as much deadpan as sisterless and so sex-lethal will be inhibited creating a male. On the other hand an AAXX cell will produce enough sisterless to inhibit the action of deadpan allowing the sex-lethal gene to be transcribed creating a female.

Later control by deadpan and sisterless disappears and what becomes important is the form of the sex-lethal gene. A secondary promoter causes transcription in both males and females. Analysis of the cDNA has shown that different forms are expressed in males and females. Sex-lethal has been shown to affect the splicing of its own mRNA. In males the 3rd exon is included which encodes a stop codon causing a truncated form to be produced. In the female version, the presence of sex-lethal causes this exon to be missed out the other 7 amino acids are produced as a full peptide chain, again giving us a difference between males and females.

Presence or absence of functional Sex-lethal proteins now go on to affect the transcription of another protein known as Doublesex. In the absence of sex-lethal, Doublesex will have the 4th exon removed and be translated up to and including exon 6 (DSX-M), while in its presence the 4 exon which encodes a stop codon will produce a truncated version of the protein (DSX-F). DSX-F causes transcription of Yolk proteins 1 and 2 in somatic cells which will be pumped into the oocyte on its production.