Sex Determination in Drosophila melanogaster

Sex Determination in Drosophila Melanogaster

• Sex determination in Drosophila melanogaster is based on the X:A ratio, where X represents the number of X chromosomes and A represents the number of sets of autosomes.
• Individuals with one X chromosome and two sets of autosomes are male (X:A=0.5), whereas individuals with an equal number of X and autosomal chromosomes are female (X:A=1).
• If X/A ratio is greater than 1, metafemales are formed, and if X/A ratio is less than 0.5, metamales are formed.
• If X/A ratio is between 0.5 and 1, intersexes are formed.

Genic Balance and Sex-Lethal Gene

• The primary signal for sex determination is the X:A ratio, which activates or inactivates the Sex-lethal (Sxl) gene.
• In females, the Sxl gene is activated, and the Sxl protein is produced, whereas in males, the Sxl gene is inactive, and no Sxl protein is produced.
• Inappropriate expression of the Sxl gene leads to embryonic death.

Single Gene Effects

• The Transformer (tra) gene: autosomal recessive, transforms females into sterile males.
• The Doublesex (dsx) gene: transforms normal males and females into flies with intersex phenotypes.

Environmental Sex Determination

• Temperature-dependent sex determination (TSD): e.g., in turtles and crocodiles, where low temperatures produce females and high temperatures produce males.
• Location-dependent sex determination: e.g., in the echiuroid worm Bonellia, where sex is determined by where the larva settles.

Chromosomes Theory of Inheritance and Sex Chromosomes

• Chromosomes theory of inheritance: states that genes are located on chromosomes, which serve as vehicles for gene segregation in meiosis.
• Sex chromosomes: determine the sex and differ between males and females.
• Autosomes: non-sex chromosomes, same for males and females.

Haplodiploidy and Sex Chromosomal Systems

• Haplodiploidy: exhibited by bees, where 2n embryos develop from fertilized eggs and become females (workers), and 1n embryos develop parthenogenetically and become males (drones).
• XX-XO system: e.g., if 2n=12, males are heterogametic sex, and females are homogametic sex.
• ZZ-ZW system: exhibited by birds, snakes, and butterflies, where females are heterogametic sex, and males are homogametic sex.
• XX-XY system: e.g., in humans, females are XX and homogametic sex, and males are XY and heterogametic sex.