by Andrea Schreier
Sometimes scientific discoveries occur by pure chance – a scientist stumbles upon a new phenomenon without even trying. The discovery of spontaneous autopolyploidy, or abnormally increased genome size, in white sturgeon provides a perfect example.
When I began my PhD work in 2006, I was tasked with helping a senior graduate student develop new microsatellite markers for green and white sturgeon. Once polymorphic markers were identified, my first dissertation chapter focused on how these markers were inherited in white sturgeon. This may sound like a very pedestrian study but white sturgeon are ancient octoploids, meaning each individual possesses eight genome copies. It was unknown at the time how their genome was structured and I needed to confirm that microsatellite markers were transmitted in a Mendelian fashion before I could use them to answer genetic questions in that species. To accomplish this, I examined transmission of microsatellite alleles from parents to offspring in 15 half-sibling families produced at a local caviar farm. When examining the genotype data, I noticed right away that one of the female parents (Y192) had more alleles than she should have (>8) at several microsatellite loci. Interestingly, several of her offspring also had >8 alleles at one or more loci. While a normal octoploid should transmit only four gene copies to its offspring, Y192 transmitted >4 gene copies per locus. What could be going on?
The first thought that came to mind was potential contamination. Perhaps my DNA extracts or PCR reactions for Y192 contained DNA from more than one individual. The fact that the progeny of Y192 had >8 alleles suggested that I was observing a legitimate biological phenomenon rather than an error made at the lab bench. To be certain, I re-extracted DNA from Y192 and re-genotyped her at eight microsatellite loci using new reagents. As expected, I got the same results: >8 alleles at several loci. The only possible explanation for this result was that Y192 wasn’t octoploid but had a higher ploidy level. But how could I test this hypothesis?
It was around that time that I first met Daphne Gille, a PhD student from another lab who was looking to collaborate with the GVL. It just so happened that Daphne was an expert in flow cytometry, a technique that uses fluorescent dye to measure the DNA content of living cells. Now all we needed to test the hypothesis that Y192 had abnormal ploidy was a sample of her blood and access to a flow cytometer. We learned that UC Davis had a flow cytometry core facility. We were ready to go!
It wasn’t possible initially to collect blood from Y192 but Daphne and I decided to randomly sample a small number of white sturgeon at the farm just to validate our flow cytometry protocol and collect some preliminary data. Out of the six individuals we happened to sample, two had elevated ploidy! These abnormal individuals had a genome size 1.25x the normal octoploid (8n) genome size suggesting they where decaploid (10n). Subsequent sampling events at the caviar farm identified individuals that were dodecaploid (12n).
We inferred that Y192 was 12n and when crossed with an 8n male, produced 10n offspring that naturally possessed >8 alleles at each locus. Because the increase in genome size from 8n to 12n occurs spontaneously (no human intervention), we refer to this phenomenon as spontaneous autopolyploidy. The spurious discovery spontaneous autopolyploidy lead to a whole new line of research for me that continues to this day. Stay tuned for future posts about our investigations into the labile genome of sturgeon…