The researchers used the fruit fly Drosophila as a genetic model system to study stochastic events like color sensitivity in photoreceptor cells. The eye of the fly contains some 800 optical units, called ommatidia. Each ommatidium contains six outer and two inner photoreceptors (R7 and R8); the inner receptors detect color, like cone cells in human eyes. Approximately 30 percent of the ommatidia are named "pale," with sensitivity to blue light, and about 70 percent are called "yellow," with sensitivity to green light. This ratio of 30 to 70 appears in a large number of diverse species of flies. However, these receptors are stochastically (or randomly) distributed.
Desplan, the corresponding author of the article, said, "The key question we explored was how each individual R7 and R8 receptor 'decides' to be either 'pale' or 'yellow,' and how this 'decision' contributes to the stochastic distribution of each type in the mosaic of color photoreceptors in the fruit flies' eyes."
In this study, the researchers identified a transcription factor gene nicknamed spineless as the key regulator for establishing the retinal mosaic when expressed during the fruit flies' mid-pupation stage. For fruit flies in which the spineless gene could not be expressed, all of the R7 photoreceptors and most R8 receptors were "pale"; for flies for which there was an overexpression of spineless, all photoreceptors were "yellow." Moreover, the researchers found that the receptor cells made their "decision" autonomously (i.e., without the influence, as in the case of some similar biological processes, of other, nearby cells).