An aerobic eukaryotic parasite with functional mitochondria that likely lacks a mitochondrial genome

Abstract

Dinoflagellates are microbial eukaryotes that have exceptionally large nuclear genomes; however, their organelle genomes are small and fragmented and contain fewer genes than those of other eukaryotes. The genus <i>Amoebophrya</i> (Syndiniales) comprises endoparasites with high genetic diversity that can infect other dinoflagellates, such as those forming harmful algal blooms (e.g., <i>Alexandrium</i>). We sequenced the genome (~100 Mb) of <i>Amoebophrya ceratii</i> to investigate the early evolution of genomic characters in dinoflagellates. The <i>A. ceratii</i> genome encodes almost all essential biosynthetic pathways for self-sustaining cellular metabolism, suggesting a limited dependency on its host. Although dinoflagellates are thought to have descended from a photosynthetic ancestor, <i>A. ceratii</i> appears to have completely lost its plastid and nearly all genes of plastid origin. Functional mitochondria persist in all life stages of <i>A. ceratii</i>, but we found no evidence for the presence of a mitochondrial genome. Instead, all mitochondrial proteins appear to be lost or encoded in the <i>A. ceratii</i> nucleus.