Understanding the evolution of spherical cell shape in bacteria: An experimental evolution approach
Dr Heather Hendrickson, Molecular Biosciences, Massey University, Auckland
To be brutally honest, few people care that bacteria have different shapes. Which is a shame, because the bacteria seem to care very much. – Kevin D Young
Cell shape in bacteria determines the physical limitations of essential processes like cell division, DNA segregation, nutrient uptake, toxin elimination and motility. Cell shape is faithfully maintained and carefully modulated.
Phylogenetic analyses suggest that the last common ancestor of all bacteria was a rod-like cell but spherical cell shape has evolved in many independent cases. In most instances, the transition between rod-like and spherical cell shape involves the loss of the ancient actin homologue, the mreB gene.
In order to understand how spherical cell shape evolves Dr Hendrickson and colleagues deleted mreB in a model rod-like organism, Pseudomonas fluorescens SBW25, and allowed this strain to evolve independently in 10 parallel lineages for 1,000 generations. After an initial decrease in fitness, each linage regains fitness whilst remaining spherical in shape.
Complete genome sequencing reveals the simple mutations responsible for this remarkable adaptation to spherical cell shape and the resulting phenotypes suggest new models for understanding how cell shape evolves. Dr Hendrickson will discuss models for how this evolutionary transition takes place, as well as preliminary results that suggest why bacteria might evolve to become spherical in the first place.