Figure 1. The Drosophila sex hierarchy.

(A) The sex determination hierarchy in Drosophila regulates all sexually dimorphic aspects of development. The ratio of X chromosomes to autosomes determines the RNA splicing activity of SXL, which leads to sex-specific alternative splicing of dsx and fru transcripts at the bottom of the hierarchy. The resulting male- and female-specific DSX and FRU isoforms confer sexual identity to the cells in which they are produced. (Adapted from [2].) (B) The dsx^GAL4 gene. Targeted insertion of GAL4 coding sequence (orange box) after the translational start codon (arrow) of dsx in exon 2 allows expression of GAL4 wherever dsx is expressed. Chromosomal sequences are color-coded as follows: 5′ UTR sequences common to the mRNAs of both sexes (light gray), N-terminal coding sequence common to mRNAs of both sexes (dark gray), coding and 3′ UTR sequences specific to females (magenta and pink, respectively), coding and 3′ UTR sequences specific to males (dark and light blue, respectively), and intronic sequences (black line). Male-specific and female-specific splicing patterns are depicted above or below the chromosome, respectively. Slash marks in the second intron represent ∼24 kb of DNA.