Meteora, “Protist X”, and what we can learn from culturing the diversity of eukaryotes Yana Eglit, Takashi Shiratori, Jon Jerlstöm-Hultqvist, Andrew J. Roger, Ken-Ichiro Ishida, Alastair G.B. Simpson

To a first approximation, eukaryote diversity is comprised of protists. After three centuries of exploring microscopic life, we are still finding protists that are dramatically different from any known forms, and often distantly related to well-studied eukaryotes. Here, we present two examples of newly cultivated organisms, whose place in the eukaryote tree of life was completely unclear from morphology. Meteora sporadica was described in 2002, but there were neither cultures nor molecular data available. It has a strikingly unusual cell organisation unlike either a ‘flagellate’ or an ‘amoeba’: the central cell body bears long thin anterior and posterior extensions (the ‘long axis’), plus (usually) two shorter lateral ‘arms’, which swing back and forth, collecting bacteria as the cell glides on its long axis. ‘Protist X’, an undescribed organism, is a protist-eating anaerobe with four flagella in a cruciform arrangement. Phylogenies constructed from >200 genes show, surprisingly, that both Meteora and ‘Protist X’ are placed outside any established “kingdom” or “supergroup” of eukaryotes, demonstrating how there are not only novel species still to discover, but entire new major groups of previously uncharacterised eukaryotes. Great strides have recently been made in uncovering this diversity with environmental molecular methods (including single cell genomics/transcriptomics and metagenomics), yet classical culturing approaches continue to yield a greater proportion of significant novel lineage discoveries, and remain crucially important for revealing the planet’s microbial biodiversity.