What is the molecular mechanism for stripes in zebras?
During vertebrate development, a group of cells called neural crest cells gives rise to the brain and migrates down the axis of the animal to develop into the spinal cord. Some of these neural crest cells along the spinal cord differentiate into melanocytes, or cells that produce colored pigment. These melanocytes then migrate perpendicular to the spinal cord, developing into pigmented skin. The exact pattern of pigment in different animals depends on the genetic regulation of melanocyte differentiation and migration.
By looking at the stripe patterns of the three living species of zebra, we can begin to understand the developmental origins of the stripes. Among the three species, the common (or plains) zebra has 26 stripes per side, the mountain zebra has 43 stripes per side, and the Grevy's zebra has 80 stripes per side. To account for these patterns, Jonathan Bard of the University of Edinburgh has proposed that the original pattern of melanocyte differentiation is the same in all three species, but the differentiation occurs at different times in development.
Differentiation is predicted to occur the earliest in the common zebra—around the third week of development. If the melanocytes are produced at this early stage, they will proliferate more (creating fewer, broader stripes) and will be pulled into a pattern parallel to the spinal cord axis in the rear as the rump continues to grow throughout development. In the Grevy's zebra, however, differentiation is not predicted to occur until the fifth week of development. The original spacing of the differentiating melanocytes is equal to that in the common zebra at week 3. But since the differentiation occurs at a later stage, when the embryo is larger and more developed, there are more melanocytes (creating more stripes) that do not proliferate as much (so the stripes are thinner). The larger size of the embryo also allows most of the melanocytes to migrate in a perpendicular direction rather than being pulled parallel by asymmetric growth. The timing of melanocyte differentiation in the mountain zebra is predicted to be at four weeks, in between that of the common and Grevy's zebras. The fact that no two individuals within a species have exactly the same pattern of stripes is due to the differential growth of developing individuals.
By accounting for the size of the embryos during development and the number of stripes in the adult forms of the three species, Bard calculated that the original spacing of melanocytes at differentiation should be 0.4 mm. How does this spacing of 0.4 mm develop? Or rather, in the differentiating neural crest cells, what determines which cells produce pigment and which remain white? While a number of genes influencing pigmentation have been identified in a number of species, the exact molecular pathway underlying zebra pigmentation is not known. Scientists predict that this process requires an "activator" that produces pigment and an "inhibitor" that suppresses the activator.
A mathematical model known as the Turing reaction-diffusion system shows that different patterns of animal coloration can be derived by manipulating the variables controlling the chemical reaction between the activator and the inhibitor, the diffusion properties of the two substances, and the boundaries of the reaction area. Such a model is consistent with a developmental mechanism in which the genes controlling pigment production are the same in all three species of zebra, but the regulation of when these genes are expressed is different. Future research may determine the identities of these genes.
For more information, visit these websites:
http://www.devbio.com/article.php?ch=1&id=5 (discusses Bard's theory of the development of stripes, with diagrams of embryos)
http://grace.evergreen.edu/artofcomp/examples/zebra/Zebra.html (a simulation illustrates how a random process mimicking a reaction-diffusion system can produce an ordered pattern like zebra stripes)