A Mitosome With Distinct Metabolism in the Uncultured Protist Parasite Paramikrocytos canceri (Rhizaria, Ascetosporea)

Ascetosporea are endoparasites of marine invertebrates that include economically important pathogens of aquaculture species. Owing to their often-minuscule cell sizes, strict intracellular lifestyle, lack of cultured representatives and minimal availability of molecular data, these unicellular parasites remain poorly studied. Here, we sequenced and assembled the genome and transcriptome of Paramikrocytos canceri, an endoparasite isolated from the European edible crab Cancer pagurus. Using bioinformatic predictions, we show that P. canceri likely possesses a mitochondrion-related organelle (MRO) with highly reduced metabolism, resembling the mitosomes of other parasites but with key differences. Like other mitosomes, this MRO is predicted to have reduced metabolic capacity and lack an organellar genome and function in iron–sulfur cluster (ISC) pathway-mediated Fe–S cluster biosynthesis. However, the MRO in P. canceri is uniquely predicted to produce ATP via a partial glycolytic pathway and synthesize phospholipids de novo through the CDP-DAG pathway. Heterologous gene expression confirmed that proteins from the ISC and CDP-DAG pathways retain mitochondrial targeting sequences that are recognized by yeast mitochondria. This represents a unique combination of metabolic pathways in an MRO, including the first reported case of a mitosome-like organelle able to synthesize phospholipids de novo. Some of these phospholipids, such as phosphatidylserine, are vital in other protist endoparasites that invade their host through apoptotic mimicry.