Dosage compensation is less effective in birds than in mammals.
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.


CONCLUSIONS
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.
Itoh
Yuichiro
Y
Department of Physiological Science, University of California, Los Angeles, CA 90095, USA.
Melamed
Esther
E
Department of Physiological Science, University of California, Los Angeles, CA 90095, USA.
Yang
Xia
X
Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
Kampf
Kathy
K
Department of Physiological Science, University of California, Los Angeles, CA 90095, USA.
Wang
Susanna
S
Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
Yehya
Nadir
N
Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
Van Nas
Atila
A
Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
Replogle
Kirstin
K
Department of Cell and Developmental Biology, University of Illinois, Urbana, IL 61801, USA.
Band
Mark R
MR
W.M. Keck Center for Comparative and Functional Genomics, University of Illinois, Urbana, IL 61801, USA.
Clayton
David F
DF
Department of Cell and Developmental Biology, University of Illinois, Urbana, IL 61801, USA.
Schadt
Eric E
EE
Rosetta Inpharmatics, Seattle, WA 98034, USA.
Lusis
Aldons J
AJ
Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
Arnold
Arthur P
AP
Department of Physiological Science, University of California, Los Angeles, CA 90095, USA.
D000818:Animals; D002642:Chick Embryo; D002645:Chickens / Q000235:genetics*; D004303:Dosage Compensation, Genetic; D005260:Female; D046369:Finches / Q000235:genetics*; D018628:Gene Dosage; D020869:Gene Expression Profiling; D006801:Humans; D008297:Male; D051379:Mice; D020411:Oligonucleotide Array Sequence Analysis; D012730:Sex Chromosomes; D012744:Sex Ratio