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Dosage compensation is less effective in birds than in mammals

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Author(s): Itoh Yuichiro | Melamed Esther | Yang Xia | Kampf Kathy | Wang Susanna | Yehya Nadir | Van Nas Atila | Replogle Kirstin | Band Mark R | Clayton David F | Schadt Eric E | Lusis Aldons J | Arnold Arthur P

Journal: Journal of Biology
ISSN 1478-5854

Volume: 6;
Issue: 1;
Start page: 2;
Date: 2007;
Original page

ABSTRACT
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.
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