Sex-dimorphic gene expression and ineffective dosage compensation of Z-linked genes in gastrulating chicken embryos

Abstract

Background: Considerable progress has been made in our understanding of sex determination and dosage compensation mechanisms in model organisms such as C. elegans, Drosophila and M. musculus. Strikingly, the mechanism involved in sex determination and dosage compensation are very different among these three model organisms. Birds present yet another situation where the heterogametic sex is the female. Sex determination is still poorly understood in birds and few key determinants have so far been identified. In contrast to most other species, dosage compensation of bird sex chromosomal genes appears rather ineffective.

Results: By comparing microarrays from microdissected primitive streak from single chicken embryos, we identified a large number of genes differentially expressed between male and female embryos at a very early stage (Hamburger and Hamilton stage 4), long before any sexual differentiation occurs. Most of these genes are located on the Z chromosome, which indicates that dosage compensation is ineffective in early chicken embryos. Gene ontology analyses, using an enhanced annotation tool for Affymetrix probesets of the chicken genome developed in our laboratory (called Manteia), show that among these male-biased genes found on the Z chromosome, more than 20 genes play a role in sex differentiation.

Conclusions: These results corroborate previous studies demonstrating the rather inefficient dosage compensation for Z chromosome in birds and show that this sexual dimorphism in gene regulation is observed long before the onset of sexual differentiation. These data also suggest a potential role of non-compensated Z-linked genes in somatic sex differentiation in birds.

Figure 1

Chromosomal distribution of dimorphically expressed genes. (a) The distribution of male-biased genes with M:F ratios greater than 1.5. Total probeset number is 275. (b) The distribution of female-biased genes with F:M ratios greater than 1.5. Total probeset number is 27.

Figure 2

Comparison of M:F ratios and expression levels of Z-linked genes to those of autosomal genes. (a) The average values of M:F ratios for autosome chr1-chr28, along with that of Z chromosome. Values are represented on log2 scale. (b) The distribution of the expression level of genes on autosome chr1 to chr28 and on chrZ in female (blue) and male (red). Values are represented on log2 scale. For simplicity, the W chromosome is not included. Chromosome 32 is not annotated and thus not included in this analysis.

Figure 3

Distribution of M:F ratios and average expression levels of Z-linked genes. (a) The distribution of M:F ratio (Y axis) as a function of position on Z chromosome (X axis). Zp arm, Zq arm, centromere and MHM region of Z chromosome are indicated. (b) The percentile distribution (Y axis) of Z genes as a function of ratio values (X axis, log2 scale). (c) The trend of M:F ratio distribution is represented as running averages of 30 consecutive ratios positioned at the median position of the 30 ratios. (d) The average expression level (Y axis, log2 scale) of all expressed Z genes (Z), compensated Z genes (Compensated) and non-compensated Z genes along with autosomal genes in females and males.

Figure 4

Female-biased gene expression. (a) The sex of samples can be pre-identified by PKCIW-based PCR genotyping. A PCR product of 500 bp indicates female genotype. F, Female, M, Male. (b) and (c) PKCIW is expressed in the female but not in the male at HH4 (b) and HH15 (c). In the male embryo (c), the staining in neural tube cavity is non-specific.

Figure 5

Male-biased gene expression. (a-g) HH4, dimorphic expression of PKCIW (a), PELOTA (b), FANCG (c), HSD17B4 (d), PGTER4 (e) and StARD (f). (a) and (b) are the same female and male embryos hybridized by the two different probes. In each panel, the embryo on the left is female and on the right is male.