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

Sex determination is still poorly understood in birds and no key determinants have so far been identified. In contrast to most other species, dosage compensation of bird sex chromosomal genes appears rather ineffective. By comparing microarrays of 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 shows that among the male-biased genes found on the Z chromosome, more than 20 genes have a role in sex differentiation. Primitive streak tissues were dissected out of individual Hamburger and Hamilton (HH) stage 4 chicken embryos and extracted for total RNA. Total RNA was amplified 2-cycles, biotin-labeled and hybridized to Affymetrix chicken GeneChip. Gene expression profiles of female and male samples were analyzed for sex-dimorphic expression.