Morphogenetic degeneracies in the actomyosin cortex.

One of the great challenges in biology is to understand the mechanisms by which morphogenetic processes arise from molecular activities. We investigated this problem in the context of actomyosin-based cortical flow in C. elegans zygotes, where large-scale flows emerge from the collective action of actomyosin filaments and actin binding proteins (ABPs). Large-scale flow dynamics can be captured by active gel theory by considering force balances and conservation laws in the actomyosin cortex. However, which molecular activities contribute to flow dynamics and large-scale physical properties such as viscosity and active torque is largely unknown. By performing a candidate RNAi screen of ABPs and actomyosin regulators we demonstrate that perturbing distinct molecular processes can lead to similar flow phenotypes. This is indicative for a 'morphogenetic degeneracy' where multiple molecular processes contribute to the same large-scale physical property. We speculate that morphogenetic degeneracies contribute to the robustness of bulk biological matter in development.

Systematic genetic interaction screens uncover cell polarity regulators and functional redundancy.

Although single-gene loss-of-function analyses can identify components of particular processes, important molecules are missed owing to the robustness of biological systems. Here we show that large-scale RNAi screening for suppression interactions with functionally related mutants greatly expands the repertoire of genes known to act in a shared process and reveals a new layer of functional relationships. We performed RNAi screens for 17 Caenorhabditis elegans cell polarity mutants, generating the most comprehensive polarity network in a metazoan, connecting 184 genes. Of these, 72% were not previously linked to cell polarity and 80% have human homologues. We experimentally confirmed functional roles predicted by the network and characterized through biophysical analyses eight myosin regulators. In addition, we discovered functional redundancy between two unknown polarity genes. Similar systematic genetic interaction screens for other biological processes will help uncover the inventory of relevant genes and their patterns of interactions.