The role and mechanism of estrogen/estrogen receptor α in sexual identity determination of chicken somatic cells

Unlike mammals, where systemic sex hormones determine somatic sexual identity, chickens primarily employ a cell-autonomous mechanism, known as the cell autonomous sexual identity (CASI) hypothesis. However, tissue sensitivity to sex hormones varies significantly. This study aimed to elucidate the specific role of the estrogen/estrogen receptor α (ERα) system in this context. We assessed protein levels of key sex hormone receptors, including ERα, estrogen receptor β (ERβ), and androgen receptor (AR), in the tissues of 6-week-old and adult chickens. Results showed that ERα protein was markedly abundant in hormone-sensitive tissues (comb and wattles) but low in insensitive ones (muscle and liver); this pattern was not observed for ERβ or AR. Functionally, in the absence of estrogen, ERα overexpression in ERα-deficient primary muscle cells affected immunity but not sexual differentiation-related pathways. In contrast, analysis on the transcriptomes of ERα-rich wattle tissues indicated that the genes differentially expressed between estrogen-treated versus untreated groups were enriched in the pathways critical for development, neuroactive ligand-receptor interaction, and glycerolipid metabolism to establish sexual phenotype. In conclusion, estrogen regulates somatic sex identity via ERα in a tissue-specific manner, with regulatory intensity dictated by local ERα abundance. This provides a crucial supplement to the CASI hypothesis.