Sex determination in Drosophila melanogaster: Which molecular mechanism is a key step in specifying sexual fate?

Introduction / Context: Drosophila sex determination is a classic model showing how RNA processing controls developmental fate. Key regulators direct female- or male-specific splicing patterns that cascade into distinct transcriptional programs.

Given Data / Assumptions:

• Genes such as Sex-lethal (sxl) and transformer (tra) produce different splice isoforms.
• These isoforms direct downstream sex-specific gene expression (for example, doublesex).
• DNA methylation plays a minor role in flies compared to mammals.

Concept / Approach: Alternative splicing creates sex-specific mRNAs; female-specific splicing of sxl and tra generates functional proteins that in turn drive female splicing of downstream targets. Male pathways prevail when these splicing events do not occur.

Step-by-Step Solution: Identify dominant mechanism in flies → alternative splicing. Connect to canonical genes → sxl and tra set the sex-specific splicing program. Eliminate mechanisms not central in Drosophila (heavy DNA methylation, gene amplification). Choose the option explicitly naming alternative splicing.

Verification / Alternative check: Genetic and molecular studies show that mis-splicing of sxl/tra causes sex transformation, confirming splicing as causal.

Why Other Options Are Wrong: DNA methylation is limited in flies; large-scale gene amplification is not the primary determinant; “none of these” ignores established splicing mechanisms.

Common Pitfalls: Projecting mammalian epigenetic paradigms onto insects; overlooking RNA-level control.

Final Answer: Alternate (alternative) splicing of specific pre-mRNAs (for example, sxl and tra).