Using Kleptoplasty to illuminate the Evolution of Plastids and the Order of Steps during endosymbiosis

Plastids, the photosynthetic organelles of eukaryotes, arose via endosymbiosis of cyanobacteria by a eukaryotic host and were subsequently spread across eukaryotic diversity by additional endosymbioses.

These events greatly impacted the evolution of eukaryotes, introducing a novel lifestyle and shaping their diversity, yet we still know very little about the steps taking place during plastid endosymbiosis.

The traditional view considers this a straightforward uptake and retention of the symbiont that later becomes the plastid, followed by genetic integration.

Recently, profoundly different models proposing several, transient interactions with a variety of endosymbionts that transfer genes to the host before fixation of the final endosymbiont have gained traction.

Kleptoplastidic species, lineages that transiently retain and use plastids from their prey, are at the centre of this paradigm shift, providing the first insights into the order of steps during plastid endosymbiosis.

Kleptoplasty is rare, primarily found in a group of microbial eukaryotes, and the few kleptoplastidic taxa known have not been studied in the context of endosymbiosis.

I propose here to resolve the process of plastid endosymbiosis using kleptoplasty as a unique model by addressing these questions: 1) What is the hidden diversity of kleptoplastidic taxa? 2) What is the level of genetic integration in these taxa? 3) What is the composition of the plastid proteome in a kleptoplastidic lineage? 4) How does the kleptoplast and control of the kleptoplast change during integration?

To answer these questions, I will combine a novel imaging-based screen for kleptoplastidic lineages with high-throughput transcriptomics and phylogenetics and provide the first plastid proteome of a kleptoplastidic lineage.

This project will not only advance our shifting understanding of endosymbiosis, but the methods developed will provide a foundation for future studies on the evolution of microbial eukaryotes.