title: Independent evolution of transcriptional inactivation on sex chromosomes in birds and mammals.
abstract: 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. 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. 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. upregulation of the single Z in females, down regulation of one or both Zs in males, or a combination). 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. Each gene displayed a reproducible frequency of 1Z/1X-active and 2Z/2X-active cells in the homogametic sex. 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. 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.
authors.affiliation: Evolution, Ecology and Genetics, Research School of Biology, The Australian National University, Canberra, Australian Capital Territory, Australia. alexandra.livernois@anu.edu.au
authors.forename: Alexandra M
authors.initials: AM
authors.lastname: Livernois
authors.forename: Shafagh A
authors.initials: SA
authors.lastname: Waters
authors.forename: Janine E
authors.initials: JE
authors.lastname: Deakin
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: Marshall Graves
authors.forename: Paul D
authors.initials: PD
authors.lastname: Waters
mesh_terms: D000818:Animals; D005075:Biological Evolution; D002645:Chickens / Q000235:genetics* / Q000502:physiology; D004303:Dosage Compensation, Genetic; D005260:Female; D050172:Genes, X-Linked; D006801:Humans; D008297:Male; D010987:Platypus / Q000235:genetics* / Q000502:physiology; D012730:Sex Chromosomes / Q000235:genetics*; D014158:Transcription, Genetic; D049951:X Chromosome Inactivation / Q000235:genetics*