Sex-dependent gene expression in early brain development of chicken embryos

Using cDNA microarrays and real-time PCR we found gene expression differences between the male and female embryonic brain (or whole head) that may be independent of morphological differentiation of the gonads. Genes located on the sex chromosomes (ZZ in males and ZW in females) were common among the differentially expressed genes, several of which (WPKCI-8, HINT, MHM non-coding RNA) have previously been implicated in avian sex determination. A majority of the identified genes were more highly expressed in males. Three of these genes (CDK7, CCNH and BTF2-P44) encode subunits of the transcription factor IIH complex, indicating a role for this complex in neuronal differentiation.In conclusion, this study provides novel insights into sexually dimorphic gene expression in the embryonic chicken brain and its possible involvement in sex differentiation of the nervous system in birds.Sexual differences that arise in the brain during embryonic development underlie sex-specific reproductive behavior in adults of vertebrate species. Better knowledge of the mechanisms behind sexual differentiation of the nervous system can contribute to the understanding of the brain's functions and its susceptibility to disease [1], and is critically needed in the context of developmental neurotoxicity and chemically induced disruption of the neuroendocrine system [2]. For a long time the idea prevailed that somatic tissues in the embryo are gender neutral until the gonads become hormonally active following their sex-specific differentiation, a process presumed to be governed by cell-autonomous programs genetically determined by the sex chromosomes [3-5]. However, several recent studies indicate that sex-specific neuronal characteristics may be influenced by genetic mechanisms preceding or occurring in parallel with the effects produced by the gonadal hormones [6]. That the genetic sex of neurons contributes to the process of sexual differentiation is illustrated by studies of the neural song