Evolution of Mammalian Sex Chromosomes

To understand the peculiarities of sex chromosomes and know how they work, we must understand how they evolved. We now have the detailed information on sex chromosomes of a range of animals; closely related mammals to spot recent events, more distantly related mammals to explore the early events in our sex chromosome histories, even non-mammal vertebrates and beyond to discover the deepest secrets that unite sex chromosomes of all animals. Comparing human sex chromosomes with those of other mammals reveals deep conservatism, but those of marsupials reveal that a large autosomal segment was added to a smaller ancient mammal XY. The bizarre multiple XY system of the platypus has no homology to the XY of other mammals, nor to the sex chromosomes of reptiles, frogs and fish, implying that our sex chromosomes derived from ancient autosomes relatively recently. The patterns of homology between sex chromosomes and different autosomes show that several autosomes have become sex chromosomes in one or other vertebrate lineage. Thus, sex chromosome turnover has occurred many times in vertebrate history, and may be involved in speciation.