The X chromosome of monotremes shares a highly conserved region with the eutherian and marsupial X chromosomes despite the absence of X chromosome inactivation.
Eight genes, located on the long arm of the human X chromosome and present on the marsupial X chromosome, were mapped by in situ hybridization to the chromosomes of the platypus Ornithorhynchus anatinus, one of the three species of monotreme mammals. All were located on the X chromosome. We conclude that the long arm of the human X chromosome represents a highly conserved region that formed part of the X chromosome in a mammalian ancestor at least 150 million years ago. Since three of these genes are located on the long arm of the platypus X chromosome, which is G-band homologous to the Y chromosome and apparently exempt from X chromosome inactivation, the conservation of this region has evidently not depended on isolation by X-Y chromosome differentiation and X chromosome inactivation.
Department of Genetics and Human Variation, La Trobe University, Bundoora, Victoria, Australia.
J M
JM
Watson
J A
JA
Spencer
A D
AD
Riggs
J A
JA
Graves
D000818:Animals; D005075:Biological Evolution; D015139:Blotting, Southern; D002874:Chromosome Mapping; D004247:DNA / Q000235:genetics; D015342:DNA Probes; D006801:Humans; D007621:Karyotyping; D008394:Marsupialia / Q000235:genetics*; D008970:Molecular Weight; D009007:Monotremata / Q000235:genetics*; D010987:Platypus / Q000235:genetics*; D013045:Species Specificity; D014960:X Chromosome
D015342:DNA Probes; D004247:DNA