Tiger stripes
Alright, friends, so Hanny and I figured out the perfect recipe for making copies of the DNA in our samples. Our struggle resulted in beautiful and delicious metaphorical baked goods. What now?
The specific region of DNA we were working with is called a microsatellite. It's a part of the DNA that doesn't code for proteins, and in fact, nobody really knows what microsatellites are for. Still, we can make use of them. Microsatellites have short sequences (something like ATA) repeated over and over again, and the number of repeats varies. In our case, the number of repeats varies between different species of corals in the genus Acropora, so we can use the microsatellites to tell us if we accidentally collected multiple species instead of just one.
Check out the image below. This is an electrophoresis gel. I've shown you images of them before, but basically, you load a sub-sample of the DNA into a gel, run an electrical current through it, and then fragments of DNA with different lengths migrate different distances. Small fragments (i.e. small numbers of tandem repeats) travel farther, while larger fragments (greater numbers of repeats) stay close to where they were loaded. In this case, larger fragments are at the top of the image, and smaller fragments are at the bottom.
The specific region of DNA we were working with is called a microsatellite. It's a part of the DNA that doesn't code for proteins, and in fact, nobody really knows what microsatellites are for. Still, we can make use of them. Microsatellites have short sequences (something like ATA) repeated over and over again, and the number of repeats varies. In our case, the number of repeats varies between different species of corals in the genus Acropora, so we can use the microsatellites to tell us if we accidentally collected multiple species instead of just one.
Check out the image below. This is an electrophoresis gel. I've shown you images of them before, but basically, you load a sub-sample of the DNA into a gel, run an electrical current through it, and then fragments of DNA with different lengths migrate different distances. Small fragments (i.e. small numbers of tandem repeats) travel farther, while larger fragments (greater numbers of repeats) stay close to where they were loaded. In this case, larger fragments are at the top of the image, and smaller fragments are at the bottom.
You'll notice each of the samples has a strong band, like a tiger stripe, at the bottom of the column - all except four. Samples 7, 11, 12, and 13 are missing the bottom band (noted with the white arrows). In addition, samples 11 and 12 have stripes higher up (white box). Based on the patterns in the bands, it appears that samples 1-6, 8, and 9 are all the same species, while samples 7 and 11-13 are different. The tiger-stripe patterns show us that some individuals are different.
With this new genetic information, Hanny and I went back to the photos we had taken in Palau. We were careful to photograph every individual we sampled, so now we could see if the tiger-stripe patterns correspond to any physical difference in the corals. See for yourself:
On the left is sample 1, and on the right is sample 7. They look pretty different! We had collected both these species in the field, knowing that they were different but not knowing which one we would find at our other sites (so we took them both while we could). Now, with the genetic information to add to the picture, we can tell that our eyes were trustworthy. There are strong genetic differences between corals that look really different, not between corals that look deceptively similar. We collected many more samples of the species on the left, so we'll proceed with the analysis of those specimens, being confident that they all belong to the same species. I love it when things line up!
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