Content   Introduction
                 Sex-Chromosome 
                 Systems 
                 Sex-Chromosome 
                 Evolution
                 Sex-Chromosome 
                 Organization
                 Molecular evolution
                 Conclusions
                          Introduction
                          Sexual reproduction is common in 
                          multicellular organisms 
                          This includes fusion of gametes 
                          (isogamy vs anisogamy) from two 
                          individuals
                          Anisogamy can give rise to separate 
                          sexes (males and females)
                          The sex can be determined by 
                          environment or genetic factors 
                          Sex chromosomes have evolved 
                          independently several times in 
                          different linages 
  SEX-CHROMOSOME 
  SYSTEMS: 
                   Sex chromosomes from related 
                   Sex chromosomes from related 
                   linages are not necessarily 
                   linages are not necessarily 
                   homologous
                   homologous
                   Liability hypothesis: Spread 
  SEX              Liability hypothesis: Spread 
                   of new sex-determining loci 
  CHROMOSOMES      of new sex-determining loci 
                   linked to autosomal loci under 
  ARE LABILE       linked to autosomal loci under 
                   sexually antagonistic selection
                   sexually antagonistic selection
                   ‘Simple’ organization of XY 
                   ‘Simple’ organization of XY 
                   males and XX females does not 
                   males and XX females does not 
                   reflect the variety of sex-
                   reflect the variety of sex-
                   chromosome systems 
                   chromosome systems 
  SEX-                XY/XX        ZW/ZZ
  CHROMOSOME 
  SYSTEMS: 
                      X0/XX        Z0/ZZ
  VARIETY OF SEX-
  CHROMOSOME 
  SYSTEMS                           XY1Y2/XX
                    Z1Z2W/Z1Z1Z2Z2
                                   RINGS OR 
                                   CHAINS
  SEX-          XY/XX
  CHROMOSOME                       ZW/ZZ
  SYSTEMS: 
                      X0/XX        Z0/ZZ
  VARIETY OF SEX-
  CHROMOSOME 
  SYSTEMS                           XY1Y2/XX
                    Z1Z2W/Z1Z1Z2Z2
                                   RINGS OR 
                                   CHAINS