Dosage compensation: do birds do it as well?

In birds males carry ZZ and females ZW sex chromosomes, and it has been proposed that there is no dosage compensation in the expression of sex-linked genes. However, recent data suggest the opposite, indicating that male and female birds might demonstrate similar levels of expression of Z-linked genes. If they do, the equalization between the sexes is probably not achieved by inactivation of one of the male Z chromosomes. Other possible mechanisms include the transcription of Z-Iinked genes being upregulated in females or downregulated in males, or equalization at the translation stage in either sex. A recently identified hypermethylated region on the Z chromosome, with similarities to the X inactivation centre on the mammalian X chromosome, might play a part in this process or have a role in avian sex determination.