Cultured deep-sea PVC bacteria shed light on eukaryogenesis

Evolutionary relationship between prokaryotes and eukaryotes continues to fascinate biologists. Accumulated studies suggest a eukaryogenesis model based on the PVC (Planctomycetes-Verrucomicrobia-Chlamydiae) bacteria. However, a decisive PVC-based eukaryogenesis scenario has not yet been reported. Here, we isolated PVC bacteria, unique for dividing by budding and for possessing developed endomembrane systems, from the deep sea. In cultured PVC bacterial strains, we detected typical eukaryotic organelle-like structures including endoplasmic reticulum, Golgi apparatus, vesicles, vacuoles and actin/tubulin-based microfilaments. Strikingly, we observed a nucleolus-containing nucleus in a Verrucomicrobia strain, which divides by mitosis. Transcriptomic results further demonstrated abundant presence of genes associated with eukaryotic cellular processes including membrane fusion. We propose that the prominent capability of membrane fusion drives eukaryogenesis by enabling PVC bacteria to evolve eukaryotic cellular features.