Crystal-eyes. The complex patterns of a fly's eyes form much like a crystal does.

Flies Have Crystal Vision

LOS ANGELES--A fly's eye contains a striking hexagonal pattern of individual lenses and photoreceptors. Yet this complex array forms through a simple process that does not require any sophisticated instructions for assembly, scientists say. The trick may be another tool nature uses to create exquisite biological structures.

From the graceful spirals in the faces of sun flowers to the intricate segmentation of insect bodies, nature abounds with complex patterns. Yet, explaining precisely how these patterns form remains a challenge. Genes cannot specify every detail, so at least in part, the forms must emerge spontaneously through some physical mechanism. According to one leading theory, a pair of substances, an "activator" and an "inhibitor," diffuse into each other to form a pattern of spots and stripes that then influences development. In this so-called reaction-diffusion scheme, a few characteristics of the chemicals, such as how far they diffuse, predetermine the pattern in all its details.

But the eye pattern of the fruit fly results from an even simpler, step-by-step process that mimics the growth of a crystal, report David Lubensky of the Free University of Amsterdam in the Netherlands and Boris Shraiman of the Kavli Institute of Theoretical Physics at the University of California, Santa Barbara. The layered pattern of atoms in a crystal emerges when additional atoms nestle into the dimples between atoms in the previous layer. That simple process yields a repeating structure with no need for a blueprint.

In the same way, the hexagonal pattern of a larval fly's eye emerges as each new photoreceptor bud positions itself near the gap between neighboring buds in the previous row, Lubensky told a meeting of the American Physical Society here on Monday. The process is controlled by a transcription factor, atonal, a wave of which moves across the undifferentiated eye disk. Using experimentally measured properties of atonal and proteins with which it interacts, Lubensky and Shraiman modeled the process and found that, instead of encoding a specific pattern, the chemical interactions merely ensured that each row patterned itself after the previous one.

The findings support the idea that short-range interactions between cells can drive pattern formation, says Albrecht Ott, a physicist from the University of Bayreuth, Germany. But Herbert Levine, a theoretical physicist at the University of California, San Diego, says the validity of the model hinges on whether it can predict, for example, effects that might be seen in experiments with altered or mutant flies.

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