From endosymbiont to organelle: Mechanisms of cellular integration between Paulinella and its chromatophores

The endosymbiotic integration of cyanobacteria within Eukaryotes was one of the pivotal evolutionary transitions.

This transition, however, is difficult to study because modern plastids represent highly derived end-states and are not transitionary; we know, therefore, very little about the mechanisms involved.

Primary plastid endosymbiosis has occurred only twice: once in the ancestor of Archaeplastida and once independently in the amoeba Paulinella.

Each Paulinella cell houses two chromatophores, which are photosynthetic units that resemble plastids yet are more closely related to cyanobacteria.

Genetic studies of Paulinella and its cyanobacterial-derived chromatophores revealed it is an intermediary in the evolution of a photosynthetic organelle. Studying Paulinella, therefore, offers unique insight into the mechanistic processes of plastid integration.

Despite our growing genetic knowledge, little is known about the integration of the chromatophores in terms of their cellular physiology.

In this proposal, I aim to address this by exploring this unique association at the metabolomic, transcriptomic and proteomic levels.

First, I plan to characterise the metabolites exchanged between the cell and chromatophores using an isotope-labelling metabolic experiment (objective 1).

Second, I will study the coordinated light response of the nucleus and chromatophore at the transcriptional, metabolic, and protein level (objective 2).

Finally, I will perform a long-term evolution experiment to expose Paulinella chromatophora to high and fluctuating light regimes to test whether its light regulation can evolve in response to different light environments (objective 3).

The outcomes will provide insight into the molecular mechanisms that facilitate the integration of the chromatophores within Paulinella.

This in turn will aid our understanding of the acquisition and integration of plastids, and the evolutionary trajectory from endosymbiont to organelle.