Tag Archives | zebras

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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Thinking zebras… and horses


When you hear hoofbeats, think horses, not zebras

I have been thinking about zebras lately. Part of the reason is probably best left for another post, since it’s a different tangent than this one. As readers of the studio blog know, I tend to wander off on tangents a lot. I do eventually get back to where I was, though it often takes a while. Oddly enough, this will bring us back to Dominant Whites, though a bit indirectly.

The other reason is that I recently ran across my copy of the Penycuik Experiments by Professor James Ewart. The Penycuik Experiments were conducted in the late nineteenth century. I originally found the book when looking for information on the Highland Ponies of Rhum, which are interesting because they are associated with the silver gene as well as the “tiger eye” trait. The text proved to be a dead-end for that, but the experiments described were really cool. I thought it might be fun to share them here, in part because the hybrids are interesting and in part because the experiment itself is a wonderful illustration of just how far we have come in our understanding of genetics in the last 100 years.

Professor Ewart was interested in disproving the theory of telegony, which was the belief that offspring from a cross could be influenced by the traits of the mother’s previous mates. While this might seem quite silly now, at the time the idea was almost universally accepted. Darwin mentions it in The Variation of Animals and Plants under Domestication, citing a case where a mare was crossed on a quagga and later produced horses with striping. The Penycuik Experiment was an attempt to recreate that situation to see if the theory of telegony held. The experiments are particularly interesting in that they predate the re-discovery of Mendel’s work by a few years.

The last quagga died in captivity in 1883, so Ewart used a Burchell Zebra stallion, Matopo. He crossed the Matopo, who is pictured at the top of the post, with a wide variety of mares. Among the first of the hybrid foals was Romulus, from the black Highland mare, Mulatto.

Most of the other hybrids looked much like Romulus – reddish brown ground color with an overlay of black stripes. Ewart also includes photos of the zebra hybrid bred by Lady Meux. In that case a Burchell’s zebra mare was crossed with a “Highland or Shetland Pony” with wall eyes.

He was said to be “light bay”, but in the photos above he looks chestnut. Unlike the other hybrid foals, his daughter does not have particularly visible striping.

She also looks like she might be chestnut, though it is hard to tell from an old black and white photograph. Another zebra hybrid, Birgus, was said to have grown up to be chestnut with black stripes. He was by Matopo and out of a chestnut polo pony mare. Photos of modern zorses suggest that in addition to black striping, the chestnuts also have black lower legs much like a wild bay.

What is interesting is that none of the hybrids in the Penycuik study had white markings of any kind. In addition to the wall-eyed pony stallion, one of the mares used by Ewart was a Clydesdale mare. White patterns can trump the zebra striping, which many have seen with the well-known tobiano zorse Eclypse.

The interesting thing about Romulus, and indeed all the other zebra hybrids, is that they had more stripes than their zebra parent. Ewart counted 43 stripes on Romulus, compared to the five between the shoulder stripe and hindquarter for Matopo. That seems counter-intuitive, that crossing an unstriped animal with a striped one might give the resulting offspring more stripes. That brings me to the other tangent I mentioned earlier. Tomorrow I’ll post about embryonic development and spot frequency, because that’s more really cool stuff.

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