title: Dosage compensation is less effective in birds than in mammals.
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.
  
  
  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.
authors.affiliation: Department of Physiological Science, University of California, Los Angeles, CA 90095, USA.
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Itoh
authors.affiliation: Department of Physiological Science, University of California, Los Angeles, CA 90095, USA.
authors.forename: Esther
authors.initials: E
authors.lastname: Melamed
authors.affiliation: Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
authors.forename: Xia
authors.initials: X
authors.lastname: Yang
authors.affiliation: Department of Physiological Science, University of California, Los Angeles, CA 90095, USA.
authors.forename: Kathy
authors.initials: K
authors.lastname: Kampf
authors.affiliation: Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
authors.forename: Susanna
authors.initials: S
authors.lastname: Wang
authors.affiliation: Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
authors.forename: Nadir
authors.initials: N
authors.lastname: Yehya
authors.affiliation: Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
authors.forename: Atila
authors.initials: A
authors.lastname: Van Nas
authors.affiliation: Department of Cell and Developmental Biology, University of Illinois, Urbana, IL 61801, USA.
authors.forename: Kirstin
authors.initials: K
authors.lastname: Replogle
authors.affiliation: W.M. Keck Center for Comparative and Functional Genomics, University of Illinois, Urbana, IL 61801, USA.
authors.forename: Mark R
authors.initials: MR
authors.lastname: Band
authors.affiliation: Department of Cell and Developmental Biology, University of Illinois, Urbana, IL 61801, USA.
authors.forename: David F
authors.initials: DF
authors.lastname: Clayton
authors.affiliation: Rosetta Inpharmatics, Seattle, WA 98034, USA.
authors.forename: Eric E
authors.initials: EE
authors.lastname: Schadt
authors.affiliation: Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
authors.forename: Aldons J
authors.initials: AJ
authors.lastname: Lusis
authors.affiliation: Department of Physiological Science, University of California, Los Angeles, CA 90095, USA.
authors.forename: Arthur P
authors.initials: AP
authors.lastname: Arnold
mesh_terms: 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