Question

describe how the Sxl gene in Drosophila influences the differentiation between male and female. How does Sxl get activated? How does the active Sxl enzyme changed the product of the Dsx gene?

Answer 1

Sexual differentiation in Drosophila is controlled by the regulated expression of a cascade of three genes. The differential expression of these three genes in male and female embryos depends on the regulated splicing of their hnRNAs. The first of these genes to be expressed, is called sex-lethal (Sxl), is transcribed from an early promoter (PE) that is active only in very early female embryos. Sxl protein is a sequence specific RNA binding protein which is not produced in early male embryos. As development of the Drosophila embryo proceeds transcription of Sxl gene from PE (early promoter) is repressed, while transcription from an upstream late promoter (PL) is induced in both males and females. The presence of early Sxl protein (an RNA binding protein) in female embryos directs splicing of the late sxl transcript so that a functional Sxl protein is produced. In contrast, male embryos produce a non-functional Sxl protein which lacks the RNA binding motif.

            The second gene to be expressed in the regulatory cascade leading to sexual differentiation in Drosophila is called transformer (tra). Again, Sxl protein controls splicing of tra pre-mRNA in female embryos so that they express functional Tra protein. Due to lack of functional Sxl protein males cannot produce functional Tra. The Tra protein can form a complex with another protein called Tra-2. The Tra-2 has an RNA binding domain. In female embryos splicing of the pre-mRNA, synthesized from the third gene in the cascade – double sex (dsx) is controlled by the Tra-Tra-2 complex. As a result the females express a female specific dsx mRNA and Dsx protein, which repress transcription of genes required for male sexual development. Due to the absence of functional Tra protein male embryos produce an alternate male specific version of Dsx protein, which repress female development genes.