Spontaneous Autopolyploidy in Sturgeon Aquaculture

For more information contact Andrea Schreier: amdrauch@ucdavis.edu

Background and Significance of Study

White sturgeon, like all sturgeon species, are ancient polyploids.  A normal white sturgeon is an evolutionary octoploid (8n), possessing eight genome copies.  While studying inheritance of microsatellite markers in white sturgeon families from a caviar farm, we noticed that one of the female parents had a greater number of alleles than would be expected from an octoploid organism.  Analysis showed that this female’s offspring also possessed more alleles than progeny from other families.  We returned to the caviar farm and conducted flow cytometry on six randomly sampled individuals and found that two of these individuals had abnormally large genomes. Since white sturgeon are the only species on the farm and therefore making hybridization was impossible, the abnormal white sturgeon must be autopolyploids. Triploidization is not a management technique used in sturgeon husbandry and therefore any autopolyploidy occurring must be spontaneous. The spontaneous autopolyploids have a genome size 1.5x that of normal white sturgeon, making them 12N.

Daphne draws blood from a Kootenai River white sturgeon wild female broodstock

Daphne Gille draws blood for flow cytometry from a Kootenai River white sturgeon wild female broodstock

We’ve expanded our study of spontaneous autopolyploidy in white sturgeon and now have measured genome sizes of hundreds of individuals from multiple captive populations. The incidence of spontaneous autopolyploidy varies widely among families. Some families contain no or very few 12N progeny while in other families the majority of progeny are 12N.  We’ve also discovered that 12N fish are fertile and when crossed with 8N adults, they produce viable offspring with an intermediate genome size of 10N.

The next step was to determine the mechanism behind spontaneous autopolyploidy in captively bred white sturgeon.  Daphne worked with Tom Famula to determine that the maternal genome was duplicated and therefore the proximate cause of white sturgeon spontaneous autopolyploidy is an egg’s inability to eject the 2nd polar body during meiosis II.  What is yet unknown is why sturgeon eggs can’t always complete meiosis II during artificial spawning procedures in captivity.  Another unknown is whether spontaneous autopolyploidy affects individual performance or fitness.  Preliminary data suggest that 10N sturgeon have abnormal reproductive development. Caviar farmers are concerned that inadvertent production of 10N individuals can reduce production and conservation biologists worry that unknowingly releasing 12N fish into the wild could harm wild population persistence.

We have received funding from the Western Regional Aquaculture Center to answer the following questions about white sturgeon spontaneous autopolyploidy:

1. What aspect of sturgeon culture leads to 12N production?

2. What is the best method to detect spontaneous autopolyploid sturgeon?

3. Do 10N sturgeon exhibit reproductive abnormalities?

4. How does spontaneous autopolyploidy affect sex ratios, growth, and stress physiology?

Results

What aspect of sturgeon culture leads to 12n production?

We hypothesize that oocyte aging during the time lapse between ovulation and fertilization in captive breeding may be causing causing 2nd polar body retention.  As fish eggs age, the cytoskeletal components necessary for ejecting the 2nd polar body successfully begin to break down. We are working with two caviar farms to do time trial experiments, intentionally aging eggs after ovulation and comparing rates of spontaneous autopolyploidy between aged eggs and controls.

Which is the best method to determine ploidy of captive white sturgeon?

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Blood smear of 8n (L) and 12n (R) white sturgeon erythrocytes stained with Wright Giesma.

We are comparing three methods used to determine sturgeon ploidy: flow cytometry, coulter counter analysis, and blood smear analysis. Flow cytometry involves staining of DNA and therefore provides a direct measure of DNA content and ploidy. Coulter counter analysis measures the volume of cell nuclei, which is increased in polyploid individuals. Because polyploid cells have increased size overall, measuring cell size parameters in blood smears should detect individuals of abnormal ploidy. Our study will compare the accuracy and precision of each method and provide recommendations for producers looking to cost effectively screen their stock for spontaneous autopolyploids.

Do 10N sturgeon exhibit reproductive abnormalities?

We are studying the reproductive development of 200 10N juvenile white sturgeon.  We will also be measuring ploidy of “non-reproductive” white sturgeon from a caviar farm.  Females classed as “non-reproductive” are those that have surpassed the age of normal sexual maturation and still have not produced caviar.

How does spontaneous autopolyploidy affect sex ratios, growth, and stress physiology?

We are conducting experiments using full sibling families that contain both 8N and 12N individuals to determine whether having an extra genome copy changes an individual’s growth rate, condition, and response to stress.  We also have preliminary evidence that 12N white sturgeon are more likely to be female.  This is not unexpected given that the female is the sex-determining gender in white sturgeon and therefore offspring with multiple copies of the female genome are more likely to inherit the female sex determining element.  We be comparing the sex ratio of 8N and 12N white sturgeon looking for deviations from the expected 50:50 ratio.

Collaborators

Tom Famula, Dept of Animal Science, UC Davis

Joel Van Eenennaam, Dept of Animal Science, UC Davis

Anne Todgham, Dept of Animal Science, UC Davis

Fred Conte, Dept of Animal Science, UC Davis

Molly Webb, US Fish and Wildlife Service

Shawn Young and Cara Holem-Bell, Kootenai Tribe of Idaho

Publications

Gille, D., F.R. Famula, B.P. May, and A.D. Schreier. 2015. Evidence for a maternal origin of spontaneous autopolyploidy in cultured white sturgeon (Acipenser transmontanus). Aquaculture 435:467-474. (pdf)

Schreier, A. D., B. May, and D. A. Gille. 2013. Incidence of spontaneous autopolyploidy in cultured populations of white sturgeon, Acipenser transmontanus. Aquaculture 416-417:141-145. (pdf)

Schreier, A.D., D. Gille, B. Mahardja, and B. May. 2011. Neutral markers confirm the octoploid origin and reveal spontaneous polyploidy in white sturgeon,Acipenser transmontanus. J. Appl. Ichthiol. 27:24-33 (pdf)