On the quest to study polyploidy in cyanobacteria

This thesis investigates the nuanced interplay between polyploidy and evolvability in prokaryotes, with a focus on Synechocystis PCC 6803. Traditionally thought to be rare in prokaryotes, polyploidy has been identified across diverse bacterial and archaeal species, challenging conventional perspectives.
To investigate the influence of polyploidy on evolvability, it is crucial to identify experimental conditions allowing the control of ploidy levels. Chapter 2 explores the use of phosphate limitation in continuous cultures for this purpose. Unexpectedly, the WT strain from our lab failed to thrive under these conditions, contrary to literature findings. After prolonged cultivation, a mutant strain, SynPLSS+, capable of growth in such conditions, was isolated. Chapter 3 characterizes this strain, revealing a mutation in the WT strain compared to the closest reference genome. Remarkably, this mutation, responsible for the failure to respond to phosphate limitation, was reverted in SynPLSS+.
Chapter 4 employs simulations and laboratory evolution experiments to compare the evolvability of populations with different ploidy levels. Simulations reveal that the impact of polyploidy can be diverse, ranging from beneficial to detrimental, contingent on mutation likelihood and dominance. Experimental findings in this chapter underscore the positive effect of polyploidy on evolvability.
Chapter 5 describes a collaborative effort to improve reproducibility in the cyanobacterial field by means of an interlaboratory study performed in eight European laboratories.
This thesis provides insights into the dynamic relationship between polyploidy and evolvability, emphasizing experimental challenges and the need for standardization in cyanobacterial research.