Dosage compensation in birds.
The Z and W sex chromosomes of birds have evolved independently from the mammalian X and Y chromosomes [1]. Unlike mammals, female birds are heterogametic (ZW), while males are homogametic (ZZ). Therefore male birds, like female mammals, carry a double dose of sex-linked genes relative to the other sex. Other animals with nonhomologous sex chromosomes possess "dosage compensation" systems to equalize the expression of sex-linked genes. Dosage compensation occurs in animals as diverse as mammals, insects, and nematodes, although the mechanisms involved differ profoundly [2]. In birds, however, it is widely accepted that dosage compensation does not occur [3-5], and the differential expression of Z-linked genes has been suggested to underlie the avian sex-determination mechanism [6]. Here we show equivalent expression of at least six of nine Z chromosome genes in male and female chick embryos by using real-time quantitative PCR [7]. Only the Z-linked ScII gene, whose ortholog in Caenorhabditis elegans plays a crucial role in dosage compensation [8], escapes compensation by this assay. Our results imply that the majority of Z-linked genes in the chicken are dosage compensated.
Institute of Cell and Molecular Biology, The University of Edinburgh, EH9 3JR, Scotland, United Kingdom.
H A
HA
McQueen
D
D
McBride
G
G
Miele
A P
AP
Bird
M
M
Clinton
D000154:Aconitate Hydratase / Q000235:genetics; D000199:Actins / Q000235:genetics; D000818:Animals; D030161:Avian Proteins; D001717:Birds / Q000235:genetics*; D018797:Cell Cycle Proteins; D002642:Chick Embryo; D004268:DNA-Binding Proteins / Q000235:genetics; D004303:Dosage Compensation, Genetic; D005260:Female; D038681:Follistatin; D006023:Glycoproteins / Q000235:genetics; D013006:Growth Hormone / Q000235:genetics; D008297:Male; D009687:Nuclear Proteins / Q000235:genetics; D012730:Sex Chromosomes; D014157:Transcription Factors / Q000235:genetics
D000199:Actins; D030161:Avian Proteins; D018797:Cell Cycle Proteins; C110547:DMRT1 protein; D004268:DNA-Binding Proteins; D038681:Follistatin; D006023:Glycoproteins; D009687:Nuclear Proteins; C089784:ScII protein, Gallus gallus; D014157:Transcription Factors; D013006:Growth Hormone; D000154:Aconitate Hydratase