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

Abstract

Background: 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.

Evaluating dosage compensation on the chicken Z chromosome: should effective dosage compensation eliminate sexual bias?

“…It was surprising, therefore, when the lack of a chromosome-wide mechanism of DC was recently reported in the ZZ male/ZW female system of birds (Itoh et al, 2007), where Z gene expression is constitutively higher in males. This finding has now been confirmed in a second study in chickens (Ellegren et al, 2007) and detected in another ZZ/ZW system, the silkworm Bombyx mori (Zha et al, 2009).…”

All dosage compensation is local: Gene-by-gene regulation of sex-biased expression on the chicken Z chromosome

“…This would mean that critical genes would be individually regulated to a similar expression level in both sexes, and would result in a highly variable local sex-bias pattern across the Z chromosome that would vary somewhat among tissues. There is some support for this, with global transcription studies of sex-biased expression in birds recovering an overall male bias along the Z chromosome, with a wide variance in sex bias locally (Ellegren et al, 2007;Itoh et al, 2007;Melamed and Arnold, 2007). It is also possible that sex chromosome dosage occurs at critical developmental time points (Wilkins, 2002), or shows some other temporal dynamic such as that has been recently noted in mice (Lin et al, 2007).…”

“…In birds, the female is the heterogametic sex, with one Z and one W chromosome, and the male is homogametic with two Z chromosomes. The lack of dosage compensation of the chicken Z chromosome produces a pronounced overall pattern of male bias for most genes along the Z, which is in marked contrast to the autosomes, which, when averaged over all loci, have a roughly equal expression level between males and females (Ellegren et al, 2007;Itoh et al, 2007). The chicken Z chromosome is large, and contains more than 700 known or predicted protein coding genes, and is greatly diverged from the W in terms of gene content (www.ensembl.org).…”

“…It was therefore surprising when recent studies showed evidence that birds seem to lack an overall sex chromosome dosage compensating mechanism (Ellegren et al, 2007;Itoh et al, 2007). In birds, the female is the heterogametic sex, with one Z and one W chromosome, and the male is homogametic with two Z chromosomes.…”

Disruption of FEM1C-W gene in zebra finch: evolutionary insights on avian ZW genes

“…The ratio of FEM1C to GAPDH expression is not obviously different in testis vs. ovary, but the use of a loading control such as GAPDH is problematic because of the fundamental differences in cell types in the two tissues. It is not clear if the higher expression in testis suggests testis-specific function, particularly because most Z genes are not dosage-compensated and are thus expected to be expressed higher in males than females (Itoh et al 2007;Ellegren et al 2007;Arnold et al 2008). The northern blot offers no strong evidence for alternative splicing in testis as occurs in mouse and human (Ventura-Holman et al 2003).…”