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Here, for the first time, we provide evidence for inactivation of genes on the chicken Z chromosome in ZZ males, and on all five Xs in female platypus. In contrast, genes on the five platypus (a monotreme mammal) X chromosomes and the chicken Z chromosome (which are orthologous but independently evolved) are expressed globally at a higher level in female platypus and male chicken respectively, indicating partial dosage compensation. The carry-through of overexpression to females would result in functional tetraploidy, so there is subsequent inactivation of one X chromosome in the somatic cells of females, leaving males (XY) and females (XX) with a single upregulated X. In placental and marsupial mammals this is achieved by upregulating the single X in males. This regulatory mechanism must have been exapted independently to the non-homologous sex chromosomes in birds and mammals in response to an over-expressed Z or X in the homogametic sex, highlighting the universal importance that (at least partial) silencing plays in the evolution on amniote dosage compensation and, therefore, the differentiation of sex chromosomes.ĭosage compensation is a mechanism that restores the expression of X chromosome genes back to their original level when Y homologues lose function.

Our results indicate that the probability of inactivation is controlled on a gene-by-gene basis (or small domains) on the chicken Z and platypus X chromosomes. Each gene displayed a reproducible frequency of 1Z/1X-active and 2Z/2X-active cells in the homogametic sex.

We used RNA-fluorescent in situ hybridization (RNA-FISH) to demonstrate, on individual fibroblast cells, inactivation of 11 genes on the chicken Z and 28 genes on the X chromosomes of platypus. upregulation of the single Z in females, down regulation of one or both Zs in males, or a combination). The bird ZW sex chromosome system represents a third independently evolved amniote sex chromosome system with dosage compensation, albeit partial and gene-specific, via an unknown mechanism (i.e.

However, many X genes escape inactivation in humans, inactivation of the X in marsupials is partial, and the unrelated sex chromosomes of monotreme mammals have incomplete and gene-specific inactivation of X-linked genes. X chromosome inactivation in eutherian mammals has been thought to be tightly controlled, as expected from a mechanism that compensates for the different dosage of X-borne genes in XX females and XY males.