Transfection of the free-living alga <i>Chromera velia</i> enables direct comparisons with its parasitic apicomplexan relative, <i>Toxoplasma gondii</i>

Chromera velia is a photosynthetic, free-living alga closely related to the apicomplexan parasites, a phylum of intracellular pathogens responsible for many devastating diseases, including malaria, cryptosporidiosis and toxoplasmosis. With molecular and cellular landmarks clearly related to but distinguishable from those found in apicomplexan parasites, Chromera provides an opportunity to investigate the evolutionary origin of the structures and processes needed for intracellular parasitism. However, tools for defining localization and functions of gene products do not exist for Chromera, which creates a major bottleneck for exploring its biology. Here we report two major advances in exploring the cell biology of this free-living relative of a large group of intracellular parasites: (1) successful cell transformation and (2) the implementation of expansion microscopy. The initial analysis enabled by these tools generated new insights into subcellular organization in different life stages of Chromera. These new developments boost the potential of Chromera as a model system for understanding the evolution of parasitism in apicomplexans.