title: Faced with inequality: chicken do not have a general dosage compensation of sex-linked genes.
abstract: BACKGROUND
  The contrasting dose of sex chromosomes in males and females potentially introduces a large-scale imbalance in levels of gene expression between sexes, and between sex chromosomes and autosomes. In many organisms, dosage compensation has thus evolved to equalize sex-linked gene expression in males and females. In mammals this is achieved by X chromosome inactivation and in flies and worms by up- or down-regulation of X-linked expression, respectively. While otherwise widespread in systems with heteromorphic sex chromosomes, the case of dosage compensation in birds (males ZZ, females ZW) remains an unsolved enigma.
  
  
  RESULTS
  Here, we use a microarray approach to show that male chicken embryos generally express higher levels of Z-linked genes than female birds, both in soma and in gonads. The distribution of male-to-female fold-change values for Z chromosome genes is wide and has a mean of 1.4-1.6, which is consistent with absence of dosage compensation and sex-specific feedback regulation of gene expression at individual loci. Intriguingly, without global dosage compensation, the female chicken has significantly lower expression levels of Z-linked compared to autosomal genes, which is not the case in male birds.
  
  
  CONCLUSIONS
  The pronounced sex difference in gene expression is likely to contribute to sexual dimorphism among birds, and potentially has implication to avian sex determination. Importantly, this report, together with a recent study of sex-biased expression in somatic tissue of chicken, demonstrates the first example of an organism with a lack of global dosage compensation, providing an unexpected case of a viable system with large-scale imbalance in gene expression between sexes.
authors: Department of Evolutionary Biology, Uppsala University, Norbyvägen 18D, SE-752 36 Uppsala, Sweden. Hans.Ellegren@ebc.uu.se
authors: Hans
authors: H
authors: Ellegren
authors: Lina
authors: L
authors: Hultin-Rosenberg
authors: Björn
authors: B
authors: Brunström
authors: Lennart
authors: L
authors: Dencker
authors: Kim
authors: K
authors: Kultima
authors: Birger
authors: B
authors: Scholz
mesh_terms: D000818:Animals; D002642:Chick Embryo; D002645:Chickens / Q000235:genetics*; D004303:Dosage Compensation, Genetic; D005260:Female; D015870:Gene Expression; D050172:Genes, X-Linked; D008297:Male; D020411:Oligonucleotide Array Sequence Analysis; D012727:Sex Characteristics