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Open Access Highly Accessed Research article

Dosage compensation is less effective in birds than in mammals

Yuichiro Itoh1, Esther Melamed1, Xia Yang2, Kathy Kampf1, Susanna Wang2, Nadir Yehya2, Atila Van Nas2, Kirstin Replogle3, Mark R Band4, David F Clayton3, Eric E Schadt5, Aldons J Lusis2 and Arthur P Arnold1*

  • * Corresponding author: Arthur P Arnold arnold@ucla.edu

  • † Equal contributors

Author Affiliations

1 Department of Physiological Science, University of California, Los Angeles, CA 90095, USA

2 Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA

3 Department of Cell and Developmental Biology, University of Illinois, Urbana, IL 61801, USA

4 W.M. Keck Center for Comparative and Functional Genomics, University of Illinois, Urbana, IL 61801, USA

5 Rosetta Inpharmatics, Seattle, WA 98034, USA

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Journal of Biology 2007, 6:2  doi:10.1186/jbiol53

Published: 22 March 2007

Abstract

Background

In animals with heteromorphic sex chromosomes, dosage compensation of sex-chromosome genes is thought to be critical for species survival. Diverse molecular mechanisms have evolved to effectively balance the expressed dose of X-linked genes between XX and XY animals, and to balance expression of X and autosomal genes. Dosage compensation is not understood in birds, in which females (ZW) and males (ZZ) differ in the number of Z chromosomes.

Results

Using microarray analysis, we compared the male:female ratio of expression of sets of Z-linked and autosomal genes in two bird species, zebra finch and chicken, and in two mammalian species, mouse and human. Male:female ratios of expression were significantly higher for Z genes than for autosomal genes in several finch and chicken tissues. In contrast, in mouse and human the male:female ratio of expression of X-linked genes is quite similar to that of autosomal genes, indicating effective dosage compensation even in humans, in which a significant percentage of genes escape X-inactivation.

Conclusion

Birds represent an unprecedented case in which genes on one sex chromosome are expressed on average at constitutively higher levels in one sex compared with the other. Sex-chromosome dosage compensation is surprisingly ineffective in birds, suggesting that some genomes can do without effective sex-specific sex-chromosome dosage compensation mechanisms.