Independent Evolution of Transcriptional Inactivation on Sex Chromosomes in Birds and Mammals
Alexandra Livernoisa b✉, Shafagh A. Watersc, Janine E. Deakinb d, Jennifer A. Marshall Gravesd e, Paul D. Watersc✉
- aEvolution, Ecology and Genetics, Research School of Biology, The Australian National University, Canberra, Australian Capital Territory, Australia. alexandra.livernois@anu.edu.au
- bEvolution, Ecology, and Genetics, Research School of Biology, The Australian National University, Canberra, Australian Capital Territory, Australia
- cSchool of Biotechnology & Biomolecular Sciences, Faculty of Science, University of New South Wales, Sydney, New South Wales, Australia
- dEvolution, Ecology and Genetics, Research School of Biology, The Australian National University, Canberra, Australian Capital Territory, Australia
- eLa Trobe Institute of Molecular Sciences, La Trobe University, Melbourne, Victoria, Australia
- ✉Corresponding author
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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.
TL;DRResults 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, highlighting the universal importance that (at least partial) silencing plays in the evolution on amniote dosage compensation and, therefore, the differentiation of sex chromosomes. (Semantic Scholar)