Inheritance Studies Implicate a Genetic Mechanism for Apparent Sex Reversal in Chinook Salmon

Abstract

AbstractThe apparent increase in altered sexual differentiation in Pacific salmon Oncorhynchus spp. is a growing concern. Previous studies suggest that incongruence between genetic and phenotypic sex in Chinook salmon O. tshawytscha may be a result of altered sexual differentiation through exposure to endocrine‐disrupting chemicals (EDCs). Artificial crosses between genotypically normal Chinook salmon, or between genotypically normal males and apparent sex‐reversed males (XY females), were performed to test the validity of OtY1 and growth hormone pseudogene (GHΨ) genetic markers as indicators of phenotypic sex in fall‐run Chinook salmon. The offspring produced were genotyped with the Y‐chromosome‐specific markers and were dissected to observe gonad morphology. The results of the breeding experiments indicate that approximately half of the phenotypic female offspring of XY females have a male genotype according to both Y‐chromosome markers. These results refute an earlier hypothesis that phenotypic female Chinook salmon with a male genotype (XY females) are the result of altered sexual differentiation caused by EDC exposure. Instead, either the OtY1 and GHΨ markers have recombined between the Y and X chromosomes or an autosome, or a mutation has inactivated the sex‐determining region of the Y chromosome. In none of the 2,384 Chinook salmon evaluated did the genetic markers contradict one another in a single individual. These results present evidence that both OtY1 and the GHΨ genetic markers appear to not be diagnostic for sex in fall‐run Chinook salmon in the Central Valley of California.