12 March 2010
Arthur Lander
Director, Center for Complex Biological Systems
UC Irvine
The generation of pattern—from zebra stripes to body axes to body segments—is among the oldest questions in Developmental Biology. Lately there has been a surge of interest in the problem of pattern robustness, i.e. how pattern formation manages to be insensitive to commonplace perturbations, both environmental and intrinsic. Numerous mechanisms have recently been identified that, in principle, could render pattern formation systems—typically those driven by secreted morphogens—impressively insensitive to specific, individual perturbations (e.g. fluctuations in the rate of morphogen synthesis). Underlying this line of approach is the unstated assumption that, by learning how a system can resist individual disturbances, we will get a good general picture of how systems resist the wide range of disturbances they normally face. In fact, this assumption runs counter to a widely held tenet of engineering—variously known as "conservation of fragility", or "the no-free-lunch principle"—which states that decreased sensitivity to one type of disturbance is always purchased at the expense of increased sensitivity to another. Drawing on examples from the fruit fly wing disc, and the exploration of mathematical models of morphogen gradient formation and signaling, I will argue that the absence of "free lunch" indeed makes real world robustness much harder to achieve, and explore how these difficulties may account for much of the as-yet-unexplained mechanistic complexity observed in so many morphogen systems.