BNMC Seminar:  Partial penetrance facilitates the evolution of discrete morphological changes

Avidgor Eldar
Division of Biology, Caltech

Development normally occurs similarly in all individuals within an isogenic population, but mutations often affect the fate of individual organisms differently. This phenomenon, known as partial penetrance, has been observed in diverse developmental systems. However, it remains unclear how the underlying genetic network specifies the set of possible alternative fates and how the relative frequencies of these fates evolve. Using B. subtilis sporulation as a model system, we identify a stochastic cell fate determination process in mutants, where multiple discrete cell fates are exposed, including a novel, potentially adaptive fate, where two spores are made by a single cell (twin sporulation). We experimentally analyze the underlying genetic network and show how fate discreteness and frequency depend on competition between the underlying cellular processes of differentiation, replication and division. Specifically, we show that the accumulation of consecutive mutations can gradually increase the penetrance of the new sporulation phenotype. Finally, we demonstrate the relevance of our mutational analysis to real evolutionary changes by analyzing wild species that show the alternative sporulation phenotype. Together our results suggest that noise can facilitate developmental evolution by enabling the initial expression of discrete morphological traits at low penetrance, and allowing their stabilization by gradual adjustment of genetic parameters.