It’s all in the timing

Burchell

(The zebra photos in this post all come from the Wikipedia site.)

In the previous post I mentioned the curious fact that zebra hybrids had more stripes than their zebra parent. So how does an animal with some stripes, bred to one without any at all, produce offspring more extensively striped?

According to the theory offered by developmental biologist Jonathan Bard, it’s all about the timing. The amount of striping depends on when the pigmentation initiates during embryonic development. The interval of striping is the same in each species – he postulates every 20 cells – but starting earlier means there are fewer cells. Alternating colors every twenty cells won’t give you quite so many stripes. That is why the Burchell’s Zebra has such sparse, but broad, striping. It is estimated that striping here began 21 days into development.

MtZebra

If you wait a little longer, when the developing fetus has more cells, that same 20 cell interval will give more stripes. This is a Mountain Zebra, with stripes estimated to start at 28 days.

Grevys

And finally there is the heavily striped Grevy’s Zebra, with striping initiated at 35 days. That late in development, when the fetus was made up of many more cells, the twenty-cell interval created a lot more stripes.

This theory could explain why a hybrid might have more stripes than the parent. It wouldn’t need a genetic mechanism to tell it to make more stripes; it just needs the mechanism already there to be delayed a little. That is the part about zebras and their striping that has implications for horse color. If this can work for striping, it could work for other forms of patterning. It might not be necessary for a horse to have some genetic component that said “make more spots”. All that might be needed is something that set the stage for those spots to start later in development. Certainly this situation calls to mind the kind of changes in spot size and frequency seen in horses with some types of sabino patterning.

For anyone interested in a more detailed explanation of Jonathan Bard’s theory, this post has a detailed but still easy-to-understand explanation. I would also highly recommend the book that first alerted me to it.

For those interested in animal color, the chapter “Paint it Black” is great reading. But mostly about how advances in genetics and embryonic development have shed new light on the theory of evolution. I found it fascinating and very readable, even if he did talk too much about bugs for me. (I am horribly bug phobic!)

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2 Responses to It’s all in the timing

  1. Diane Knoth August 5, 2011 at 10:01 pm #

    Well, you just made me go to the library website and place this book on hold; this is all rather fascinating to me. They also had a DVD called “What Darwin Never Knew” that was listed as based on this book so I put a hold on that too. As I’m also a bug phobic individual I do hope they show mostly butterflies and maybe an ant or two. :-)

    I have to say, I always look forward to your blog posts!

  2. Brooke August 12, 2011 at 8:16 pm #

    This is unrelated to this topic, but I have a shetland pony stallion that we raised that is a bay roan, but he has unusual faint striping that is visible on his sides and neck. It is best seen when he is body clipped http://www.encoressecret.com/DSC_6634_edited.JPG
    or here are some more photos of him http://www.encoressecret.com/Roan%20Ranger.htm
    Any idea what it might be related to? His sire and one of his half-siblings also has it, they are blue roans. I do not have a good picture on my website of the striping on his sire yet. Our blue roan stallion is prone to siring sabino marked offspring on solid mares and has sired a couple of solid white offspring even, his dam was a sabino herself, but our stallion only exibits the pattern on his legs and bottom lip. As Ranger is getting older he is developing more of a starburst pattern on his forehead and one of his back legs is developing a white roaned sock. Any opinions would be greatly appreciated.