Background on Sxl Gene Function

The Sex-lethal (Sxl) gene encodes an RNA-binding protein vital in Drosophila sex determination and dosage compensation. The gene has sex-specific splicing patterns:

• Female-specific splicing produces functional Sxl protein necessary for female development.

• Default (male) splicing produces a truncated, nonfunctional protein, leading to male development.

Consequences of Default Sxl Splicing Mutation

• A mutant embryo whose Sxl gene undergoes only default (male) splicing lacks functional Sxl protein.

• Without functional Sxl, the developmental cascade leading to female sexual characteristics does not initiate, so the embryo develops as a male fly.

Regulatory Cascade: Sxl, Transformer (tra), and Double-sex (dsx)

• Sxl does not directly regulate the splicing of the double-sex (dsx) RNA.

• Instead, Sxl regulates sex-specific alternative splicing of transformer (tra) pre-mRNA, generating a functional TRA protein in females.

• TRA, together with TRA2, controls sex-specific splicing of dsx mRNA, which is critical for sexual differentiation of somatic tissues.

• Therefore, Sxl’s influence on dsx is indirect, mediated through tra.

Statement Evaluation

• A. The embryo will develop into a male fly: True, since default splicing produces nonfunctional Sxl resulting in male phenotype.

• B. The embryo will develop into a female fly: False, no functional female-specific Sxl here.

• C. Sxl gene product directly regulates sex-specific alternative splicing of dsx RNA: False, Sxl regulates dsx indirectly via tra.

• D. Sxl gene product regulates sex-specific splicing of tra RNA which in turn regulates splicing of dsx RNA: True.

Correct Combination

The statements that accurately explain the sex determination of the mutant embryo are:

(2) A and D

Final Answer:
(2) A and D