Sex chromosome homology and incomplete, tissue-specific X-inactivation suggest that monotremes represent an intermediate stage of mammalian sex chromosome evolution.
Female mammals have two X chromosomes and males have a single X and a smaller, male-determining Y chromosome. The dosage of X-linked gene products is equalized between the sexes by the genetic inactivation of one X chromosome in females. The characteristics of the mechanism of X-chromosome inactivation differ in eutherian and metatherian mammals, and it has been suggested that the metatherian system represents a more primitive stage. The present study of monotreme sex chromosomes and X-chromosome inactivation suggests that the prototherian mammals may represent an even more primitive stage. There is extensive G-band homology between the monotreme X and Y chromosomes, and differences in the patterns of replication of the two X chromosomes in females suggest that X inactivation is tissue specific and confined to the unpaired segment of the X. On the basis of these results, we propose a model for the differentiation of mammalian sex chromosomes and the evolution of the mechanism of X-chromosome inactivation. This model involves a gradual reduction of the Y chromosome and an accompanying gradual recruitment of (newly unpaired) X-linked loci under the control of a single inactivation center.
Wrigley
J M
JM
Department of Genetics and Human Variation, La Trobe University, Bundoora, Victoria, Australia.
Graves
J A
JA
D000818:Animals; D005075:Biological Evolution; D002871:Chromosome Banding; D005260:Female; D007621:Karyotyping; D008297:Male; D009007:Monotremata / Q000235:genetics*; D010987:Platypus / Q000235:genetics; D012867:Skin / Q000166:cytology; D004442:Tachyglossidae / Q000235:genetics; D014960:X Chromosome; D014998:Y Chromosome