Use of Polysomic Genetic Markers to Address Critical Uncertainties in White Sturgeon Biology and Management
The application of genetic markers to investigate evolutionary and ecological questions about white sturgeon, Acipenser transmontanus, has been limited due to the species’ highly duplicated nuclear genome. Here, polysomic microsatellite markers were used to 1) examine the ancestral level of genome duplication in white sturgeon, 2) examine genetic diversity and patterns of population structure within and among drainages across the species’ range, and 3) provide genetic monitoring for a conservation aquaculture program sustaining an endangered white sturgeon population. In the first chapter, we followed the inheritance of eight microsatellite markers in 15 families of white sturgeon from a commercial caviar farm to determine whether white sturgeon (~250 chromosomes) should be classified as tetraploid or octoploid. The eight microsatellite loci were detected predominantly in four or eight copies, with one locus observed in >8 copies. Numbers of alleles per locus, patterns of allele transmission, and inference of gene copy number in parents suggested that white sturgeon should be considered ancient octoploids. The discovery of dodecaploid parents and their decaploid offspring in the farm population, confirmed by flow cytometry analysis, indicated that some aspect of sturgeon aquaculture was inducing spontaneous autopolyploidy in white sturgeon.
Next, microsatellite markers were applied to examine white sturgeon population structure across the species’ range. Population assignment testing was used to determine the origin of white sturgeon sampled in non-natal estuaries, or those not containing a spawning population, to evaluate marine dispersal behavior. The Sacramento-San Joaquin River system was found to contain a single white sturgeon population while the8Fraser River exhibited a hierarchical pattern of population structure. Strong levels of genetic divergence were detected above and below a natural barrier, Hells Gate, and fine-scale population substructure was identified above Hells Gate. Population structure in the Columbia River drainage (including the mainstem Columbia and Snake Rivers) was complex and suggested a pattern of isolation by distance. Net downstream gene flow also may have contributed to this pattern, with individuals migrating downstream through impoundments and over barriers with little upstream movement possible. There was no support for the current practice of managing each impounded reach on the Columbia or Snake Rivers as a separate population. Lack of population structure within historically continuous river habitat found across the species’ range suggested spawning site fidelity in white sturgeon may occur on a regional scale, with local gene flow among geographically proximate spawning sites. Population assignment of samples collected from non-natal estuaries indicated that all populations with ocean access make marine migrations, and individuals did not necessarily originate from the nearest spawning population.
Finally, microsatellites were used to conduct genetic monitoring of the Kootenai Tribe of Idaho’s conservation aquaculture program (CAP) for the endangered Kootenai River white sturgeon population. Continuous recruitment failure in this population has left it entirely dependent on the CAP for reproduction. A genetic profile database of wild broodstock used in the CAP was created to monitor hatchery-induced genetic changes in the Kootenai River population. Broodstock genotypes also were used to evaluate the accuracy of parentage assignment in the Kootenai River population, as hatchery managers soon will depend on this analysis to prevent inbreeding when most sexually mature adults available for captive breeding will be derived from hatchery production. Numbers of alleles and numbers of alleles per individual per locus were calculated to monitor the amount of wild type genetic diversity captured in broodstock utilized by the CAP. Parentage analysis with 18 microsatellite markers was validated in known hatchery families from the 2010 year class. Genetic diversity in the Kootenai River population was very low relative to other populations examined, likely due to founder effects and genetic drift after isolation from the mainstem Columbia c. 10,000 YBP. In less than one sturgeon generation, 96% of Kootenai River genetic diversity has been captured in broodstock that contributed offspring that survived to release in the Kootenai River and further propagation will likely preserve additional genetic variation. The 18 microsatellite panel improved parentage assignment accuracy and allowed a greater number of assignments relative to the previous panel used for parentage analysis, suggesting that this technique may become a useful tool in the management of this vulnerable population.