Sex Reversal in Central Valley Chinook Salmon: Occurrence and Population Genetic Consequences

EPA Grant Number: U916237
Title: Sex Reversal in Central Valley Chinook Salmon: Occurrence and Population Genetic Consequences
Investigators: Williamson, Kevin S.
Institution: University of California - Davis
EPA Project Officer: Packard, Benjamin H
Project Period: January 1, 2003 through January 1, 2006
Project Amount: $60,990
RFA: STAR Graduate Fellowships (2003) Recipients Lists
Research Category: Academic Fellowships , Ecological Indicators/Assessment/Restoration , Fellowship - Ecological Risk Assessment


We will determine the frequency and distribution of phenotypic female Fall-Run Chinook salmon in the Central Valley that test positive for a Y-chromosome specific genetic marker. Using protocols developed during a preliminary study, we will perform genetic analyses on fish from the Sacramento and San Joaquin River Basins. The objectives of this research project are to:

1. Screen Fall-Run Chinook salmon collected throughout the Central Valley for the Y-chromosome specific marker OtY1 and compare the results with observations of gonads in the same fish. This will allow us to rigorously evaluate the incidence of sex reversal in Fall-Run Chinook in the Central Valley by determining if the genotype of an individual fish matches its external secondary sexual characteristics and gonad phenotype.

2. Determine the relative distribution and frequency of sex-reversed individuals (female fish positive for the Y-chromosome marker OtY1) throughout both the Sacramento and San Joaquin River Basins. Locating where sex-reversed individuals home within each basin as well as within each tributary will allow one to determine if the phenomenon of sex-reversal is specific to particular tributaries.

3. Perform controlled crosses between sex-reversed and normal male Fall-Run Chinook from Coleman National Fish Hatchery and compare the resultant sex ratio of the progeny with that of a controlled cross between a normal male and female. This will permit validation of Mendelian inheritance of the OtY1 marker in both normal and sex-reversed scenarios and provide evidence of whether or not gametes from sex-reversed individuals were capable of being fertilized and producing viable offspring. We hope to provide management agencies with information regarding the impact sex-reversed fish have on reproduction, population genetics, and population persistence of Fall-Run Chinook.


Genetic, statistical, and histological methodologies will be used to address the three objectives of the research. Collections of fin-clip samples and observations of gonads from the same fish will be made during the 2002 and 2003 spawning runs. All fish will be genetically screened using two sex-linked markers, OtY1 and the Growth Hormone psuedogene. Significant differences in the frequency of sex-reversed Fall-Run Chinook between tributaries of the Sacramento and San Joaquin Rivers, and between natural spawning and hatchery-raised fish on tributaries that have hatcheries will be analyzed using a bootstrap technique. The difference in frequency of phenotypic females positive for each Y-chromosome marker will be calculated between the generated sets of data. Through comparison of the difference in frequency of sex reversal in the original data to the bootstrap-generated frequency distribution, the statistical significance of differences in the frequency of sex reversal between different localities may be evaluated.

Sexual genotype of offspring from the controlled crosses will be evaluated using two sex-linked markers, OtY1 and the Growth Hormone psuedogene. Comparisons of the progeny sex-ratio of crosses using eggs from normal (XX) females and sex-reversed males (XY females) will be made with a Chi squared "goodness of fit" test. Phenotypic sex of progeny will be verified by histological analysis of gonad tissue, thereby permitting validation of OtY1 as a biomarker of sex reversal in Fall-Run Chinook.

Supplemental Keywords:

fellowship, Fall-Run Chinook Salmon, population genetic consequences, Central Valley, sex reversal, population persistence, growth hormone, pseudogene.

Progress and Final Reports:

  • 2003
  • 2004
  • Final