title: Regional differences in dosage compensation on the chicken Z chromosome.
abstract: BACKGROUND
  Most Z chromosome genes in birds are expressed at a higher level in ZZ males than in ZW females, and thus are relatively ineffectively dosage compensated. Some Z genes are compensated, however, by an unknown mechanism. Previous studies identified a non-coding RNA in the male hypermethylated (MHM) region, associated with sex-specific histone acetylation, which has been proposed to be involved in dosage compensation.
  
  
  RESULTS
  Using microarray mRNA expression analysis, we find that dosage compensated and non-compensated genes occur across the Z chromosome, but a cluster of compensated genes are found in the MHM region of chicken chromosome Zp, whereas Zq is enriched in non-compensated genes. The degree of dosage compensation among Z genes is predicted better by the level of expression of Z genes in males than in females, probably because of better compensation of genes with lower levels of expression. Compensated genes have different functional properties than non-compensated genes, suggesting that dosage compensation has evolved gene-by-gene according to selective pressures on each gene. The group of genes comprising the MHM region also resides on a primitive mammalian (platypus) sex chromosome and, thus, may represent an ancestral precursor to avian ZZ/ZW and monotreme XX/XY sex chromosome systems.
  
  
  CONCLUSIONS
  The aggregation of dosage compensated genes near the MHM locus may reflect a local sex- and chromosome-specific mechanism of dosage compensation, perhaps mediated by the MHM non-coding RNA.
authors: Department of Physiological Science, and Laboratory of Neuroendocrinology of the Brain Research Institute, University of California, Los Angeles, CA 90095-1606, USA.
authors: Esther
authors: E
authors: Melamed
authors: Arthur P
authors: AP
authors: Arnold
mesh_terms: D000818:Animals; D002645:Chickens; D002874:Chromosome Mapping; D004303:Dosage Compensation, Genetic; D005260:Female; D005786:Gene Expression Regulation; D008297:Male; D008957:Models, Genetic; D020411:Oligonucleotide Array Sequence Analysis; D012333:RNA, Messenger / Q000378:metabolism; D022661:RNA, Untranslated; D012730:Sex Chromosomes / Q000648:ultrastructure*; D014018:Tissue Distribution
chemical_list: D012333:RNA, Messenger; D022661:RNA, Untranslated