// Twitter Cards // Prexisting Head The Biologist Is In: Chromosome Painting

Monday, February 12, 2018

Chromosome Painting

The figure at left shows the metaphase chromosomes of a pepper root-tip, in all their squiggly false-color glory. In it you can count the number of chromosomes and (with some little background research) determine the overall ploidy of the source plant. (It has 24 chromosomes, so is a diploid.)

The original image had all the same information, but it was much harder to look at and learn from. This is a fundamental lesson of, and reason for, data visualization.

Step 0.
The original image comes from Twitter user @ChaoticGenetics. They're studying chile genetics and routinely post cool photos derived from their work. The question paired with this image was, "How many chromosomes does everyone see?" I figured I'd take a stab at it.

Lets dive into the details of how I made my figure. I use GIMP for essentially all my image editing needs. With each step figure I'll include the menu options for each command I use in brackets, so others can repeat the procedure.

0) Load the image with GIMP. Open "Tool Options" [Control-B] and "Layers" [Control-L] windows.

Step 1.
Step 3.
1) Select a rectangular region around the interesting looking chromosomes, then crop [Image > Crop to Selection] the image.

Step 2.
2) Select the "Eraser Tool" and erase all the background color and spots that don't appear as chromosomes.

3) Right-click on the image in the layer window. Select, "Add Alpha Channel". Discard the color information in the image [Colors > Desaturate]. Remove the background color [Colors > Color to Alpha (Set "From:" color to white.)].

Step 4.
4) From the layer window, make a new image layer filled in white. Move this layer beneath the image layer. Select the image layer.

Step 5.
5) Using the "Free Select Tool", draw around a visually distinct chromosome. Invert the color of the selection [Colors > Invert]. Change the color of the selection [Colors > Components > Channel Mixer... (red=50,0,0; green=0,0,0; blue=0,0,50)].

Step 6.
6) Many of the chromosomes in this example are adjacent or overlapping with another. For these, we have to use some knowledge about chromosomes and some artistry. Lets have a look at the cluster here highlighted in green.

Step 7.
7) At this scale, chromosomes are essentially linear structures. They don't branch and they don't loop. From this we can tell the green feature in step 6 is actually three chromosomes. I cut each chromosome out of the image and pasted into a new layer. From there I could clean up their shape a little before changing the colors and recombining them.
Step 8.

8) Going progressively through the image, isolating and coloring the most apparent chromosomes at each stage, we come to 16 chromosomes that we can be confident about. (So, our cell isn't a haploid with 12 chromosomes.)

We're left with the region at left I've highlighted in pink. This region would need to account for a further 8 chromosomes to reach the expected diploid count of 14 in total. Though there are probably a few chromosomes in this region that we can confidently separate, much of it is down to guesswork.

It is possible for this specific pepper plant to have fewer chromosomes. Though it is unlikely for a chromosome pair to be lost, since each has been conserved over a long time period and likely contains critical genes, it is common enough evolutionarily for chromosomes to fuse. That pink mess could hypothetically be 6 or 4 chromosomes, though this one image isn't sufficient evidence to make me think it is likely. If the same pattern is shown in a few more images from the same plant, especially if the chromosomes are better spread, then I'd start to consider that as increasingly likely.

For now, the balance of the evidence leads me to think there are 24 chromosomes and they're just not perfectly isolated. So, I divided the uncertain pile of chromosomes into the number that I expect are remaining. Any figure you make will invariably include your assumptions. The key is to try and make those assumptions reasonable or at least apparent to the reader (though this may require some nice caption-writing).

Interestingly, there's a protocol which can experimentally produce the sorts of painted chromosomes we're simulating here. Fluorescent In-Situ Hybridization (FISH) relies on making DNA probes which are stained a unique color for each chromosome. When the probes are applied to a chromosome spread, the result helps visualize chromosome crossovers, deletions, and other large scale alterations that can be important in diagnosing cancer and other disorders. The setup work for this is pretty intense, so it's probably not going to be used for the simple task of seeing how many chromosomes a plant has.

While I was in grad school, I routinely modified figures from papers I was reviewing for in-class (or in-lab) presentations. Usually highlighting different components of the figure in different colors (like here), to make them stand out more when displayed. I was doing the hard work of figuring out the important parts of the figures so students watching my presentation didn't have to. My goal was for them to focus on what I was saying about the figures and see what wanted them to see at a glance.

Using colors to present different partitions of a larger dataset ended up being central to my last large graduate project (YMAP) as well as an important part of my current [non-academic] job. While using colors for data presentation, it is important to keep in mind that not everyone has the same ability to see color. The most common forms of color-blindness are often called Red-Green-colorblindness. From this, it is a good idea to try and avoid the commonly used Red-Green color scheme seen so often in biology research figures. (Blue-Yellow is a good alternative, but there are subtleties I'll have to go into later.) Being conscious of the issues means they will inform your decisions, even if you're not fully aware of the topic.

This post was inspired by a conversation over on Twitter. (You can follow me there as @thebiologistisn.)

The original picture of the chromosome spread was made by @ChaoticGenetics, who gave permission for me to use it in this post.