Polyploidy and Sturgeon Evolution

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

Background and Significance of Study

Sturgeon belong to the family Acipenseridae, which is thought to be derived from an ancestor with 60 chromosomes. Within extant sturgeon species, various levels of ploidy have been inferred. Some authors propose that 120 chromosome species should be considered diploid, while others suggest they are ancient tetraploids. The same goes for the 250 chromosome group, with some scientists maintaining these species are tetraploid while others have inferred octoploidy. Inheritance data can be used to infer chromosome behavior at meiosis, from which ploidy level can be inferred. I sampled half- and full-sibling white sturgeon families from a caviar farm in central California. I genotyped eight parents and 48 offspring (N = 720) from each of 15 families of known parentage to determine mode of inheritance and gene copy number for eight microsatellite loci.

Inference of ploidy many sturgeons closeup image Inference of ploidy project image of many sturgeons in swimming

Results

My data suggest that the white sturgeon is an ancient octoploid. This is corroborated by previous GVL research on white sturgeon inheritance, which found that several microsatellites in this organism were present in more than four copies. We also discovered alternative ploidy states within the farmed population and future work will examine the prevalence, cause, and implications of spontaneous polyploidy in farmed white sturgeon.

Another impetus for doing this study was to ascertain whether these eight microsatellite loci were appropriate to use in molecular ecological studies of white sturgeon. Because of the high gene copy number in white sturgeon, it is impossible to apply standard population genetic tests for Hardy Weinberg expectations or linkage equilibrium. Microsatellite alleles must be scored as dominant present/absence loci, as dosage cannot be scored from electrophoretic data. By following the transmission of alleles from parents to offspring in 15 half-sibling families of known parentage, we were able to determine that with the exception of one locus tested, these newly developed microsatellite loci conformed to Mendelian expectations and were appropriate for use in population genetic analysis.

Future Work

I am very interested in studying the functional ploidy of sturgeon genomes.  White sturgeon may be evolutionary octoploids but are all eight copies of their genes expressed?  Or have some extra gene copies been lost or silenced because they are no longer needed? Recent research on polyploid plants has documented high levels of duplicate gene retention and subfunctionalization (different gene copies operating in different places or different times) but no one has investigated this in a vertebrate animal.  I’m seeking funding to examine the immediate effects of autopolyploid genome duplication in vertebrates, using white sturgeon as a model.

Publications

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).