What do a tigers stripes, your fingers and and a crocodiles evenly spaced teeth have in common?
In my last blog post, I talked about the evolutionary explanations for the different coat patterns of wild cat species, which focused more on why these patterns arose rather than how. How these patterns are actually formed in a growing embryo is a question which requires us to look to developmental biology for answers.
We must first turn to a mathematician in the 1950s, who had just published a set of equations which he proposed could explain morphogenesis, the development of regular repeating patterns seen in biological systems. This includes everything from the stripes on a tigers fur, to the whorls on plant leaves, to the formation of digits on a human hand. This mathematician was Alan Turing, most famous for cracking the Enigma code during the Second World War.
He proposed that these patterns were formed by a reaction-diffusion system. The system involves the reaction between chemicals, and the movement of these chemicals through a system, much how a drop of food colouring will spread through water. One important thing to note is that the chemicals would move through the system at different rates. The chemicals responsible for the formation of patterns in biological systems are called morphogens. Turing thought that interactions between a pair of morphogens are responsible for pattern formation. One morphogen would result in the activation of genes needed for pigmentation in a cell, whereas the other would inhibit this process.
One of the defining features of the reaction-diffusion system is that it is self-organising and self-regulating. The only thing needed to generate a pattern is some activator morphogen. This is because the activator morphogen stimulates further production of itself as well as stimulating the production of the inhibitor morphogen. As I mentioned earlier, the chemicals do not spread through a system at the same rate. The inhibitor morphogen spreads quicker.
Mathematical biologist James Murray explained Turing’s idea in this way:
“Imagine a field of dry grass dotted with grasshoppers. If the grass were set on fire at several random points and no moisture were present to inhibit the flames, the fires would char the entire field. If this scenario played out like a Turing mechanism, however, the heat from the encroaching flames would cause some of the fleeing grasshoppers to sweat, dampening the grass around them and thereby creating periodic unburned spots in the otherwise burnt field.”
Evidence for the self-organising characteristic of Turing pattern formation is the observation that each animal has a unique pattern, with offspring of patterned animals not having the same pattern of their parents.
Therefore the formation of a stripe can occur as follows:
- As the activator spreads through a system it causes pigmentation, the production of more activator and the production of inhibitor.
- The activator has a head start at first, but as the inhibitor moves faster through the system it will catch up with the activator in space
- This will cause the activator to stop inducing pigmentation resulting in the end of the stripe
A whole array of different patterns, from spots to stripes to swirls can be produced just by changing certain characteristics of the activator and inhibitor morphogens, such as the rate at which the activator spreads through a system.
The main problem with Turing’s theory was the elusive morphogens involved in pattern formation. Up until recently, there had been no chemicals which had been identified as morphogens. However in 2012 a study on the formation of the ridges on the human palate (press your tongue against the roof of your mouth and you can feel them) finally identified the morphogens that were responsible for their formation. This is the first time that there has been experimental evidence for Turing’s reaction-diffusion system. Below you can watch Dr Jeremy Green from Kings College London talking about the research which led to this evidence.
Thanks for reading!!
Photo credit tiger: <a href=”http://www.flickr.com/photos/94178903@N03/24527170969″>LOOK</a> via <a href=”http://photopin.com”>photopin</a> <a href=”https://creativecommons.org/licenses/by-sa/2.0/”>(license)</a>
Photo credit tiger cub and mother: <a href=”http://www.flickr.com/photos/8070463@N03/6812337407″>Luva confronting her mother</a> via <a href=”http://photopin.com”>photopin</a> <a href=”https://creativecommons.org/licenses/by-nd/2.0/”>(license)</a>
Photo credit zebra fur:
<a href=”http://www.flickr.com/photos/20428799@N06/4253949714″>Black&White</a> via <a href=”http://photopin.com”>photopin</a> <a href=”https://creativecommons.org/licenses/by/2.0/”>(license)</a>
Photo credit leopard fur:
<a href=”http://www.flickr.com/photos/8070463@N03/8402942475″>Leopard rosettes</a> via <a href=”http://photopin.com”>photopin</a> <a href=”https://creativecommons.org/licenses/by-nd/2.0/”>(license)</a>
Photo credit leaf patterning:
Photo credit giraffe fur: