Genetic evidence against a role for W-linked histidine triad nucleotide binding protein (HINTW) in avian sex determination

Abstract

Birds have a ZZ/ZW sex chromosome system, but the mechanism of sex determination remains unknown. The heterogametic sex is female (ZW) and one hypothesis holds that the W chromosome carries a dominant-acting ovary-determining gene. The strongest candidate ovary-determinant on the W chromosome is HINTW, which encodes an aberrant nucleotide hydrolase enzyme. HINTW is conserved amongst all carinate (flying) birds and it is strongly expressed in the gonads and other tissues of female chicken embryos. This and other lines of circumstantial evidence support the proposal that HINTW is the female-determining gene in birds. However, in vivo gain-of-function or loss-of-function studies have not hitherto been reported to test this hypothesis. We tested the potential role of HINTW by mis-expressing it in genetically male (ZZ) embryos, using the RCASBP avian retroviral vector. Strong, widespread expression was delivered throughout the embryo, including the urogenital system, as assessed by whole mount in situ hybridisation. This expression pattern mimicked that seen in normal ZW females, in which HINTW is widely expressed. Strong mis-expression was observed throughout the gonads of genetic male (ZZ) embryos. However, despite strong HINTW expression, ZZ gonads developed normally as bilateral testes. In tissue sections of ZZ urogenital systems transgenic for HINTW, normal testicular histology was observed. Female (ZW) gonads over-expressing HINTW also developed normally, with normal ovarian structure and left/right asymmetry. These results provide genetic evidence against a dominant role for HINTW in avian sex determination.