ABSTRACT The greater rhea ( Rhea americana ), the largest bird species in South America, has significant economic value due to its meat, eggs, leather, feathers and oil. However, its lack of external sexual dimorphism poses challenges for reproductive management and breeding programmes. Key factors involved in avian gonadal differentiation include the cytochrome P450c17 (CYP17A1), P450 aromatase and the oestrogen receptor alpha (ERα). This study investigated the spatiotemporal immunolocalization of these proteins in germ and somatic cells of gonadal crests and developing gonads in R. americana embryos from Day 9 to Day 24 of incubation. Immunohistochemistry revealed consistent expression of all three markers throughout gonadogenesis, with distinct intensity patterns between sexes and cell types. P450c17 has been detected in both germ and somatic cells in testes and ovaries, with increasing expression from the onset of sexual differentiation. Aromatase and ERα were strongly expressed in the ovarian cortex and lacunar regions, and moderately present in testicular cords and interstitium. These findings suggest coordinated endocrine signalling between germ and somatic cells during embryonic sexual differentiation in R. americana and provide foundational insight for future research on reproductive development and sex control in ratites.
Cristofoli et al. (2026) studied this question.