The six stages of sex chromosome evolution obtained from Ming et al. (30) and adapted to ZW system. Stage 1: Unisexual mutation with complementary dominance. Stage 2: Recombination is suppressed between the two sex determination loci and WW genotype is viable. Stage 3: Suppression of recombination extends to neighboring regions and form a small female-specific region of the W chromosome (FSW). Stage 4: The FSW region expands in size, and the Z and W chromosomes become heteromorphic. Stage 5: Severe dejeneration of the W chromosome causes deletion of nonfunctional DNA sequences, and results in reduction of W-chromosome size. Stage 6: Suppression of recombination spreads to the entire W chromosome, W chromosome is totally lost, and Z-to-autosome ratio sex determination system has evolved.