Figure 5.

Schematic drawing of step-by-step evolution of the sex chromosome constitution in female silkmoth of Samia cynthia subspecies, based on comparative mapping of orthologues of Bombyx mori genes. In two Japanese subspecies, the neo-sex chromosomes evolved in two steps by fusions of the ancestral sex chromosomes with autosomes. In the first step, the W and Z chromosomes each fused with an autosome of the A₁A₁ pair, corresponding to chromosome 12 of B. mori; this resulted in the neo-Wneo-Z constitution of S. c. walkeri female silkmoth. In the second step, only the neo-W chromosome fused with an autosome (A₂), partly corresponding to chromosome 11 of B. mori; this resulted in a neo-WZ₁Z₂ constitution of S. cynthia subsp. indet. female silkmoth. On the contrary, loss of the W chromosome in the putative S. cynthia ancestor generated a Z0 constitution of S. c. ricini. A segment corresponding to another part of the B. mori chromosome 11 containing the NOR plus a segment corresponding to a part of B. mori chromosome 24 formed the only NOR chromosome in all three S. cynthia subspecies. B. mori chromosomes are represented by grey vertical bars drawn to relative scale in Mb taken from KAIKObase.