Australia: The Land Where Time Began

A biography of the Australian continent 

Anatomy Based Relationship - Sex chromosomes

In mammals, the sex chromosomes are a single pair of  chromosomes carrying genes determining the sexual differentiation. The X chromosome, usually of normal size, also contains many genes not directly connected with sexual differentiation. The Y chromosome is usually very small and contains few genes not connected with sex determination. In most marsupials the X chromosomes are small, comprising less than 3 % of the total DNA, and even smaller Y chromosomes that don't pair with the corresponding X chromosome. In placentals, the X chromosome has more than 5 % of the total DNA, and the Y chromosome has some genes that are also present on the X chromosome, and pairs with the X chromosome during meiosis. It has been suggested that the marsupial pattern is the original, primitive condition, additional sections of an autosome becoming attached to the X and Y chromosomes at some early stage in the evolution of placentals (Jennifer Graves, 1996). 4 marsupial species have a large X chromosome, so presumably a similar process occurred in them independently. The female potoroo (Potorous tredactylus), has 10 autosomes and 2 large X chromosomes. Males of this species have 10 autosomes, 1 large X chromosome, as well as 1 large Y and 1 small Y chromosome. The large Y chromosome is the remaining half of the autosome that fused with the original X chromosome, pairing at meiosis with 1 arm of the Large X chromosome, the small Y chromosome pairing with the other arm of the X chromosome.

It is believed a similar process occurred in the swamp wallaby, with 8 autosomes, 1 large X chromosome, 1 large Y and 1 small Y chromosome. The sequence of events involved has been determined by the use of chromosome painting, the addition of antibodies to the chromosomes of tammar wallabies. The long arm of the X chromosome and the whole of the large Y chromosome are homologous with chromosomes 2 and 7 of the tammar, while only the short arm of the the X is homologous with the X chromosome of the tammar (Toder et al., 1997. Closely related species have the normal XX/XY arrangement of sex chromosomes, indicating that the unusual arrangements don't appear to have a noticeable effect on the aberrant species.

1. Chris Johnson, Australia's Mammal Extinctions, a 50,000 year history, Cambridge University Press, 2006
2. M.Archer, S.J. Hand & H. Godthelp in Hill, Robert S., (ed.), 1994, History of the Australian Vegetation, Cambridge University Press.
3. Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.