Research article

Dosage compensation is less effective in birds than in mammals

Yuichiro Itoh^{* 1} , Esther Melamed^{* 1} , 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

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

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

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

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

⁵Rosetta Inpharmatics, Seattle, WA 98034, USA

author email corresponding author email* Contributed equally

Journal of Biology 2007, 6:2doi: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.