(or maybe to one of the papers this paper refers to as false?)
It finds that platypuses have five sex chromosomes which arrange themselves in a fixed order, that these five chromosomes do not share homologous material with the ancestral therian X chromosome, and that several of them (possibly all?) do share homologous material with the (single) chicken Z chromosome.
It says that earlier findings indicated (erroneously) that one end of the platypus sex chromosome sequence shared homology with the therian X, and the other end shared homology with the bird Z, and that this was taken as evidence contradicting the established view that the bird and mammal sex determination systems evolved independently of each other. But it goes on to contradict those earlier findings, which would seem to leave the even earlier view of independent evolution of sex determination systems in place.
It is not clear why this would make the platypus a missing link between mammals and birds. The focus seems to be pretty squarely on the evolution of the standard mammalian X chromosome, not on mammals generally.
(or maybe to one of the papers this paper refers to as false?)
It finds that platypuses have five sex chromosomes which arrange themselves in a fixed order, that these five chromosomes do not share homologous material with the ancestral therian X chromosome, and that several of them (possibly all?) do share homologous material with the (single) chicken Z chromosome.
It says that earlier findings indicated (erroneously) that one end of the platypus sex chromosome sequence shared homology with the therian X, and the other end shared homology with the bird Z, and that this was taken as evidence contradicting the established view that the bird and mammal sex determination systems evolved independently of each other. But it goes on to contradict those earlier findings, which would seem to leave the even earlier view of independent evolution of sex determination systems in place.
It is not clear why this would make the platypus a missing link between mammals and birds. The focus seems to be pretty squarely on the evolution of the standard mammalian X chromosome, not on mammals generally.