Introduction to Sex Determination in Drosophila

Sex determination in Drosophila does not rely on the presence of the Y chromosome as it does in mammals. Instead, it depends on a genic balance system in which the ratio of X chromosomes to sets of autosomes (X:A) determines sexual fate. Every cell in the embryo independently reads this ratio and triggers sex-specific developmental programs accordingly.

Key Mechanism: The X:A Ratio

• The X:A ratio is the proportion of the number of X chromosomes to the number of sets of autosomes present in the cell.

• Normal Drosophila has two sets of autosomes (diploid), so the ratio is calculated as:

  X:A ratio=Number of X chromosomesNumber of sets of autosomesX:A ratio=Number of sets of autosomesNumber of X chromosomes

Interpretation of X:A Ratio

• 1.0 (e.g., XX : 2 sets of autosomes): The fly develops as a female.

• 0.5 (e.g., XY or XO : 2 sets of autosomes): The fly develops as a male.

• Ratios greater than 1: Leads to an intersex or metafemale phenotype.

• Ratios between 0.5 and 1: Can cause intersex characteristics.

Role of the Y Chromosome

• The Y chromosome does not determine male sex in Drosophila. It mainly carries genes required for fertility (spermatogenesis), so XY flies are fertile males, while XO flies are sterile males.

• The sex determination decision is solely based on the X:A ratio, making it fundamentally different from humans, where the Y chromosome carries the primary male-determining gene (SRY).

Molecular Basis

• The X:A ratio influences the activation of sex-lethal (Sxl) gene, which controls a cascade of sex-specific splicing of downstream genes such as transformer (tra) and doublesex (dsx).

• These genes regulate development of sex-specific traits throughout the fly’s body by turning on female or male gene expression patterns.

Summary

Sex determination in Drosophila is governed by the ratio of the number of X chromosomes to the number of sets of autosomes (X:A ratio), not by the presence or absence of the Y chromosome. This ratio controls the activation of sex-specific genetic pathways that determine the fly’s sexual phenotype.

Final Answer:
(2) Sets of autosomes