Introduction

• Chromosomal sex determination in Drosophila is different from mammals, as the Y chromosome is not involved in determining sex.
• Instead, the X chromosome contains genes that activate the Sex-lethal gene, which is the key regulator of sex determination in Drosophila.
• The Sex-lethal gene targets the pre-RNA of transformer and transformer-2, which are the next genes in the cascade.
• The transformer and transformer-2 proteins regulate the splicing of the doublesex gene, which is the final gene in the cascade that produces the male-specific and female-specific transcription factors.

X Chromosome Number and Sex Determination

• The number of X chromosomes in each cell determines the sex of the fruit fly.
  • If there is only one X chromosome in a diploid cell, the fly is male (XO).
  • If there are two X chromosomes in a diploid cell, the fly is female (XX).
  • If there are two X chromosomes and three sets of autosomes, the fly is a mosaic (XX/XO), where some of the cells are male and some are female.
• The X chromosome contains four protein factors that activate the early promoter of Sex-lethal (SisA, Scute, Runt, and Unpaired).
  • If these factors accumulate above a certain threshold, the Sex-lethal gene is activated early in XX embryos, during the syncytial blastoderm stage.
  • If these factors do not reach the threshold, the Sex-lethal gene is not activated early in XY embryos.

Sex-lethal Gene and Splicing Regulation

• The Sex-lethal gene encodes an RNA splicing factor that initiates a cascade of RNA processing events that lead to male-specific and female-specific transcription factors.
• The Sex-lethal gene has two promoters: an early promoter and a late promoter.
  • The early promoter is active only in XX cells, producing functional Sxl protein that binds to its own pre-RNA and splices it in a female-specific manner.
  • The late promoter is active in both XX and XY cells, producing nonfunctional Sxl protein that does not bind to its own pre-RNA and splices it in a male-specific manner.
• The Sxl protein has three major RNA targets: Sxl itself, msl2 (which controls dosage compensation), and transformer (tra).

Transformer Gene and Splicing Regulation

• The transformer gene encodes a splicing factor that regulates the splicing of its own pre-RNA and other genes involved in sex determination.
• The transformer gene has two transcripts: a nonspecific transcript and a female-specific transcript.
  • The nonspecific transcript contains an early termination codon that renders the protein nonfunctional. It is produced in both males and females.
  • The female-specific transcript does not contain the termination codon and produces functional Tra protein. It is produced only in females, as Sxl protein binds to its pre-RNA and splices it in a female-specific manner.
• The Tra protein works with another splicing factor called transformer-2 (tra2) to splice the doublesex gene in a female-specific manner.

Doublesex Gene and Transcription Regulation

• The doublesex gene encodes a transcription factor that activates or inhibits genes involved in producing either the male or female phenotype.
• The doublesex gene has two transcripts: a male-specific transcript and a female-specific transcript.
  • The male-specific transcript produces DsxM protein, which inhibits female traits and promotes male traits. It is produced in males, as Tra and Tra2 proteins do not bind to its pre-RNA and splice it in a default manner.
  • The female-specific transcript produces DsxF protein, which inhibits male traits and promotes female traits. It is produced in females, as Tra and Tra2 proteins bind to its pre-RNA and splice it in a female-specific manner.
• The DsxM and DsxF proteins regulate various aspects of sexual development, such as:
  • gonad cell fate
  • genital disc growth
  • accessory organ formation
  • yolk protein synthesis
  • sperm storage duct development
  • pigmentation pattern

Summary

• Chromosomal sex determination in Drosophila is regulated by the number of X chromosomes, which activate or not activate the early expression of Sex-lethal.
• Sex-lethal encodes an RNA splicing factor that regulates its own splicing and other genes involved in sex determination, such as transformer and msl2.
• Transformer encodes a splicing factor that regulates its own splicing and the splicing of doublesex, the final gene in the cascade.
• Doublesex encodes a transcription factor that activates or inhibits genes that produce the male or female phenotype.