Faced with inequality: chicken do not have a general dosage compensation of sex-linked genes

TLDR

Sex chromosome dosage differences can create large gene‑expression imbalances, and many organisms have evolved dosage compensation mechanisms such as X‑inactivation in mammals or X‑regulation in flies and worms. The study aims to investigate whether birds exhibit dosage compensation by comparing Z‑linked gene expression in male and female chicken embryos using microarrays. The authors performed microarray profiling of male and female chicken embryos to quantify Z‑linked gene expression in somatic tissues and gonads. Microarray data reveal that male chicken embryos exhibit a mean 1.4–1.6 fold higher Z‑linked gene expression than females, indicating a lack of global dosage compensation, while females show lower Z‑linked than autosomal expression, a pattern that may underlie avian sexual dimorphism and represents the first known organism without global dosage compensation.

Abstract

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.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.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.