Question

Explain how alternative splicing affects sex determination in Drosophila.

Short answer

Alternative splicing, governed by the Sxl gene in Drosophila, determines sex by directing the creation of sex-specific proteins. The Sxl protein in females changes the splicing pattern of the tra pre-mRNA, creating a functional Tra protein. This, with Tra2, influences the splicing of dsx pre-mRNA to create female-specific DsxF proteins. In males, where Sxl is inactive, tra pre-mRNA is spliced differently, resulting in a non-functional Tra protein and leading to the production of male-specific DsxM protein.

Step-by-step solution

Role of Sxl gene

The Sxl (Sex lethal) gene in Drosophila plays a crucial role in sex determination. It is activated in females due to the XX chromosome configuration but remains inactive in males (XY). The active Sxl gene in females directs the synthesis of the Sxl protein.

Alternative splicing of transformer pre-mRNA

In females, the Sxl protein changes the splicing pattern of the transformer (tra) pre-mRNA, including an otherwise skipped exon. This creates functional Tra protein. However, in the absence of Sxl protein in males, this exon is not included, resulting in a non-functional Tra protein.

Effect on Doublesex mRNA

Both males and females have the doublesex (dsx) gene which produces the dsx pre-mRNA. In females, the presence of functional Tra protein, along with another factor Tra2, leads to the production of female-specific Dsx protein (DsxF), while in males, the absence of Tra protein leads to a different splicing pattern resulting in male-specific Dsx protein (DsxM). These proteins govern the development of female and male characteristics respectively.