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 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 making hybridization impossible, the abnormal white sturgeon must be autopolyploids.  Triploidization is not a management technique in sturgeon husbandry and therefore any autopolyploidy occurring must be spontaneous.

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

Daphne 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 white sturgeon at the caviar farm and also the Kootenai Tribe of Idaho white sturgeon conservation hatchery.  We’ve discovered that ~15% of the sturgeon at the caviar farm and ~10% of the offspring produced in the conservation hatchery have abnormal ploidy. The majority of the abnormal fish are 12n, which represents genome size increase of 1.5x.  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. Can blood smears be used to quickly and inexpensively assay ploidy in captive white 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.

Can blood smears be used to quickly and inexpensively assay ploidy in captive white sturgeon?

Slide1_cropped

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

As polyploid cells contain more DNA than normal cells, their nuclei tend to be larger.  Therefore measuring the sizes of cell nuclei can detect of individuals with abnormally large ploidy.  Fish have nucleated blood cells so we can obtain genome size data from a simple blood smear, or a sample of blood that is fixed on a slide and stained. We are currently validating a blood smear technique for detecting spontaneous autopolyploidy in white sturgeon. This technique is less labor intensive, uses fewer chemicals, and is relatively inexpensive compared to flow cytometry, making it more accessible for small caviar farms and conservation hatcheries.

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, 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)