This post first appeared on August 5, 2011
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.
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.
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, I highly recommend the book Endless Forms Most Beautiful by Sean Carroll.
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!)
(The zebra photos in this post are from Wikimedia Commons.)