Figure 7.

Model for the evolution of elements 6-10 in the male meiotic sex chromosome chains of platypus and echidna. Three events are proposed to have led to the observed arrangement of platypus and echidna sex chromosomes. An initial X-autosome translocation captured the MHC-containing chromosome into the sex chromosome system. In a second step, a sex chromosome-autosome translocation occurred that disrupted the synteny of the MHC. In the echidna lineage, a further translocation between two sex chromosomes took place (see Discussion for details).