Introduction

Sex determination in Drosophila hinges on the function of the Sex-lethal (Sxl) gene, which serves as the master switch controlling sexual identity and dosage compensation. The molecular mechanism behind Sxl regulation is fundamental in developmental biology and genetics.

Sxl Regulation of Gene Expression

• Sxl encodes an RNA-binding protein that exerts regulatory effects primarily at the post-transcriptional level.

• This includes alternative splicing of pre-mRNAs and translational repression, thereby controlling the production of key proteins involved in sex determination pathways.

Key Molecular Actions of Sxl

• Alternative Splicing: Sxl controls the splicing of its own transcript and other downstream genes such as the transformer (tra) gene. This ensures female-specific isoforms that promote female development. Without Sxl, male-specific splicing takes place leading to male development.

• Translational Regulation: Sxl represses translation of the msl-2 gene, a key component of dosage compensation machinery that balances X chromosome gene expression between males and females. By inhibiting msl-2 translation in females, Sxl prevents inappropriate dosage compensation.

• Autoregulation: Sxl maintains its expression in females through an autoregulatory splicing loop, ensuring sustained female-specific gene expression throughout development.

Levels of Regulation Excluded

• Sxl regulation is not primarily at transcriptional level, meaning it does not control the initiation of mRNA synthesis directly.

• It also does not regulate target proteins by post-translational modifications; its control occurs before the protein is made.

Summary

The Sex-lethal (Sxl) gene regulates expression predominantly at the post-transcriptional level via alternative splicing and translational repression to control sexual differentiation and dosage compensation in Drosophila.

Final Answer:
(2) Post transcriptional level