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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

  • † 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



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.


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.


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.