  While most processes in biology are highly deterministic, stochastic
mechanisms are sometimes used to increase cellular diversity, such as in the
specification of sensory receptors. In the human and Drosophila eye,
photoreceptors sensitive to various wavelengths of light are distributed
randomly across the retina. Mechanisms that underlie stochastic cell fate
specification have been analysed in detail in the Drosophila retina. In
contrast, the retinas of another group of dipteran flies exhibit highly ordered
patterns. Species in the Dolichopodidae, the "long-legged" flies, have regular
alternating columns of two types of ommatidia (unit eyes), each producing
corneal lenses of different colours. Individual flies sometimes exhibit
perturbations of this orderly pattern, with "mistakes" producing changes in
pattern that can propagate across the entire eye, suggesting that the
underlying developmental mechanisms follow local, cellular-automaton-like
rules. We hypothesize that the regulatory circuitry patterning the eye is
largely conserved among flies such that the difference between the Drosophila
and Dolichopodidae eyes should be explicable in terms of relative interaction
strengths, rather than requiring a rewiring of the regulatory network. We
present a simple stochastic model which, among its other predictions, is
capable of explaining both the random Drosophila eye and the ordered, striped
pattern of Dolichopodidae.
