The predicted ancestral therian chromosome containing segments C10, C11 and C12 and the derivation of opossum and wallaby chromosomes. (A) The predicted therian ancestral chromosome aligned against chicken chromosomes containing C10, C11 and C12 genes. An inversion and the addition of genes corresponding to part of human chromosomes 1 and 19 to the distal end of this chromosome and two more inversion events result in a putative marsupial ancestral chromosome consisting of all three segments in the order of C10, C12 and C11. Opossum (MDO) chromosomes 4 and 7 are derived from a fission event taking place in segment C12. (B) Wallaby (MEU) chromosomes 5 and 6 are derived from the predicted marsupial ancestor via inversions, a fission between C10 and C12 and a further inversion within C11.