Allele-Specific Expression Analysis Does Not Support Sex Chromosome Inactivation on the Chicken Z Chromosome

Heterogametic sex chromosomes have evolved many times independently, and in some cases, are accompanied by chromosome-wide transcriptional mechanisms to correct for dosage differences between males and females. Dosage compensation in birds is incomplete, with greater expression of Z-linked genes in males (ZZ) than females (ZW). However, recent reports have shown that the Z chromosome is expressed less in males than would be expected from two copies of the chromosome, and recent data from cell-based approaches on a limited number of loci have suggested that one Z chromosome is partially inactivated in males, in a mechanism homologous to that in therian mammals. In the present study, we use controlled crosses and allele-specific expression in three tissues to test for the presence of Z inactivation in males, which would be expected to bias transcription to the active gene copy. We show that for the vast majority of genes on the Z chromosome, males express both parental alleles at statistically similar levels, indicating no Z chromosome inactivation in male chickens. For those Z chromosome loci with detectable ASE in males, we show that the most likely cause is cis-regulatory variation, rather than Z chromosome inactivation. Taken together, our results indicate that unlike the X chromosome in mammals, Z inactivation does not affect an appreciable number of loci in chicken.