Conservation in toto of the mammalian X-linkage group as a frozen accident

Abstract

It now seems that the X-linkage group has indeed been conserved in toto throughout evolution of not only placental mammals but also of marsupials as well (Ohno et al.,1964a). The unique dosage compensation mechanism that relies on inactivation of one or the other X in females (Lyon,1961) has often been invoked as the cause of this extreme evolutionary conservation. I see no merit in this widely held belief. There apparently exists no dosage compensation mechanism for avian Z-linked genes. In fact, avian species seem to expolit dosage effects of their Z-linked genes for the manifestation of sexual dimorphism. I had a personal experience with a flock of pigeons. A single dose of a particular Z-linked mutant gene in hemizygous ZW females merely diluted the plumage color to creamy yellow, whereas double dose of the same gene in homozygous males made them almost white. In fact, hemizyous females were phenotypic eqivalents of heterozygous males. Similarly in the barred Plymouth Rock breed of chickens, roosters appear considerably lighter in color than hens, for the double dose of Z-linked Barred caused white bands in individual feathers to be much wider than white bands of hemizygous females. Yet the Z-linkage group of birds has been conserved as dilligently as the mammalian X-linkage group (Ohno et al,1964b, Baverstock et al,1982).