(this presentation first appeared at http://emergent.brynmawr.edu/eprg/?page=ModulesInGeneticNetworks)

Modules in Genetic Networks

A presentation for the Bryn Mawr College Working Group on Emergent Things

Ted Wong, Oct. 28, 2003

Von Dassow et al. 2000. The segment polarity network is a robust developmental module. Nature 406:188-192
Context:

How does the Drosophila melanogaster larva go from having a continuous gradient of morphogens to having discretely bounded segments?

Gap and pair-rule genes can be active after the initial pattern is established, so expression of the segment polarity genes must occur in the same location despite varying expression levels of the pair-rule genes.

Claim: The segment polarity genes must be regulated such that their expression patterns are robust against different inpuits from the pair-rule gene products.

Question:Given what is known about how the segment polarity genes regulate each other, do they form such a robust module?

The desired pattern and its development:

Problem: The computer simulation requires almost 50 parameters, and for most of these we don't have any idea what would be realistic parameters. So: are there any parameter sets that give the desired robustness?

Surprise: yes, and in fact there are many parameter sets that work.

How robust is the robustness?

Von Dassow et al:

"Why should the segment polarity mechanism be so robust? Varying parameter values is proxy for mutations of small effect, and variation in initial conditions mimics one aspect of developmental 'noise'. We are exploring how much developmental noise embryos experience, which may explain why gene networks need buffering. Alternatively, in the evolution of segmentation there may have been pressure to neutralize mutations of small effect."

So, two suggested reasons for the robustness to evolve: