Bio 580, Developmental Biology.  Friday, May 9, 1997
Sex Determination  

• Chapter 21 of Gilbert.
• Extra references for: Vertebrates and Insects
• Chromosomal Sex determination
  • Sex chromosomes and genes (primary sex determination)
    • Mammals
      - XX female, XY male
      - zygote is by default female, Y chromosome carries TDF (1959)
      - Y chromosome small arm encodes a testis determining factor (TDF)
      
      • Figures 21.5-21.7
      - mouse gene SRY (sex-determining-region of Y) may encode the TDF
      
      • Figures 21.8-21.10
      • SRY gene product may recognize regulatory elements in the aromatase gene promoter
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    • Insects
      - XX female, XY or XO male
      - X to autosome ratio determines sex (X:A)
      
  • Autosomal chromosomes and genes (secondary sex determination)
    • Mammals
      - autosomal testis determining gene Tda-1 of mice
      
      • some mouse strains TDFs will only cooperate with own Tda-1
      - aromatase (responsible for degradation of testosterone)
    • Insects
      - X:A ratio 1:1 activates Sxl via daughterless, sis-a, sis-b
      - Sxl activates tra, tra-2 to modify dsx transcript to female type - Sxl and tra, tra-2 remain unactive in male (X:A = 1:2)
      - default sex is male in Drosophila
      

      
      

      

      
      

      
      

      
      

      
      

      - the female cascade of genes is activated by the tra/tra-2 activated pathways
      

      
      

      

      
      

      - at least one additional branch of the sex determination pathway in females has been identified involving behavioral sex dimorphism
      

      
      

      

      
      

      - fruitless (fru) regulates MOL, a muscle pattern that shows sexual dimorphism is seen to depend upon the sex of the innervating nerve
      

      
      

      - dissatisfaction regulates sexual receptivity behavior in female