Speaker
Description
The transition from single cells to multicellularity, as with all other evolutionary transitions, requires acquisition of Darwinian properties at the new-higher level of organization. Primordial groups arise readily via mutations that cause cells to clump. However, cell collectives, although comprised of cells that are themselves Darwinian, are themselves, non-Darwinian. Darwinian properties at the collective level are first imposed by exogenous conditions in a process called ecological scaffolding. Once natural selection can act on the new-higher level entity a critical next step is the endogenization of the externally imposed Darwinian properties. This can happen through the evolution of a developmentally regulated life cycle, which has a soma phase (the collective) and a germ-line phase (propagules). A recent experiment imposing group selection to populations of the bacterium Pseudomonas fluorescens SBW25 via a life cycle alternating between mat formation at the air-liquid interface (collective) and a requirement to swim a certain distance in a semi-solid agar plate (propagules) resulted in the evolution of lineages that regulate the transition between the two phenotypes. These lineages have different developmental programmes, two of which were chosen and propagated for further nine life cycle generations. We found that although phenotypically similar in the short term, over the long term, lineages with different developmental programmes differed in their capacity to avoid extinction. Lineages carrying a mutation in awsR, which differentially regulate intracellular ci-di-GMP depending on environment, experienced little death. In contrast, lineages with a single mutation in bifA, which show minimal regulation of c-di-GMP, showed high variability in extinction rate, primarily during the collective phase. Much remains to be discovered concerning the complex dynamics observed, but what it is clear is that when lineages are the focus of selection, multiple strategies can evolve, but only those with both short- and long-term positive effects are likely to allow future evolutionary endogenization of externally imposed Darwinian properties.
