Wave the Stripes on the Zebra
One of Kipling’s Just-So Stories is “How the Leopard Got His Spots.” In the fanciful tale for children, the sandy-colored leopard and the Ethiopian make an arrangement to share features so that they can camouflage themselves in the forest. Spots and stripes are widespread in the living world, but how do they come about? Surely science can come up with a better explanation than Kipling’s. Just so, a recent scientific paper suggests that understanding the process is still a long way off.
Since Science Daily mentioned zebras, we’ll start with them. The article suggested the answer is simple. In Kipling fashion, the headline read, “How the Zebra Gets Its Stripes: A Simple Genetic Circuit.” Referring to the paper in Science by Liu et al.,1 they implied that the genetic mechanisms behind stripes and spots on animals is finally coming to light. “Now a team of scientists has designed a simple genetic circuit that creates a striped pattern that they can control by tweaking a single gene,” the article said (actually just a self-promoting press release from UC San Diego, home of one of the researchers). The suggestion from the headline is that this goes a long way toward understanding zebra stripes and other patterns on animals and plants.
Strangely, though, the authors of the original paper (mostly from China) said nothing about zebras or any particular animal:
Periodic stripe patterns are ubiquitous in living organisms, yet the underlying developmental processes are complex and difficult to disentangle…. Living organisms display an amazing array of regular spatial patterns. Traditionally, elucidation of their developmental mechanisms has been pursued through forward or reverse genetics. However, essential components required for pattern formation and control are often buried in the overwhelmingly complex physiological context.
In addition, the authors did their research on colonies of bacteria – not zebras or any other multicellular animal. Their findings were quite modest. By tweaking a gene for switching E. coli flagella from swimming to tumbling mode, they got them to form waves of pile-ups, like regular traffic jams caused by poorly coordinated signal lights. As the bacteria tried to swim outward, they formed concentric rings. The researchers were able to model the patterns mathematically as phase transitions, then vary the stripe patterns by varying the motility of the bacteria. But does this really have anything to do with zebras? Perhaps, but only in a most preliminary sense:
Natural occurrences of well-coordinated spatial and/or temporal patterns are abundant in developmental systems and are believed to involve elaborate control mechanisms. Similarly, stripe formation in various bacterial systems has been attributed to complex effects involving chemotaxis, swarming, and differentiation. Using synthetic circuits, we demonstrate that precise and robust spatiotemporal patterns can be generated autonomously from a very simple interaction—motility control by density. A recurrent mechanism enables structures to form periodically and sequentially. Important features of the pattern such as the number of stripes can be manipulated by tuning components of the circuit, such as the basal expression level of a single gene.
1. Liu, Fu et al., “Sequential Establishment of Stripe Patterns in an Expanding Cell Population,” Science 14 October 2011:
Vol. 334 no. 6053 pp. 238-241, DOI: 10.1126/science.1209042.
Satisfied? This paper leaves a number of questions begging. If a zebra got its stripes this way, the model would predict uniform concentric rings– not what we observe on African safaris. Stripe formation in mammals may bear some resemblance to bacterial colony rings but only in the most obtuse sense, even granted that a mammal develops from a single cell. In evolutionary theory, hundreds of millions of years separate a bacterium and a zebra. What caused the “elaborate control mechanisms” to emerge by an unguided process that is blind to the resulting phenotype? If the zebra got its stripes this way, why didn’t all the animals in the same habitat get them? Tell us How the Flamingo Got Its Pink and How the Giraffe Got Its Spots by playing with bacteria. Tell us why cheetahs have spots and lions have manes from experiments on E. coli. That should be fun.
We do not wish to disparage the efforts of 15 scientists who put in time and put out a paper. Maybe it sheds some light on a process that is good to know about in E. coli bacteria. But whether it explains how a zebra got its stripes is another matter. In that regard, all it really showed was that the developmental mechanisms are enormously complex. Science reporters and university press departments should avoid the urge to exaggerate their findings. It might make the headline more like a dull donkey than a flashy zebra, but the public is better served by honesty.
Exercise: Make a list of striking patterns found on mammals, birds, plants, hummingbirds, butterflies and other organisms. Next to each one, indicate how well the new research “explains” the pattern.