Nuclearia

Abstract: A new species of amphizoic amoeba, Nuclearia pattersoni sp. n., isolated from gills of Rutilus rutilus L. is described. It is characterised by elongate flattened trophozoites of irregular shape. The longer dimension of their bodies is 13.2 (11.0-15.7) µm. Filopodia radiating mostly from the poles are 2 to 2.5 times longer than the body. The diameter of less frequently observed spherical trophozoites is 8.2-10.8 µm; their filopodia radiate to all directions. Cyst-like stages have shorter pseudopodia that arise from one pole only. The surface of locomotive forms from agar plate cultures has a thin amorphous glycocalyx, while most cells are covered by two layers of extracellular matrix. Mitochondria have flattened cristae, dictyosomes are located in the perinuclear zone. A conspicuous ultrastructural feature of the morphologically similar N. simplex, perinuclear striated band, is not present. Light microscopic and ultrastructural data are completed with the sequence of SSU rRNA gene and phylogenetic analysis including sequences of related taxa. The bacterial endosymbiont found in N. pattersoni type strain RR2G2 is assigned to the genus Rickettsia.

Abstract: The ultrastructural organization of Nuclearia moebiusi is described. The organism has filose pseudopodia without supportive microtubules; it has mitochondria with flattened cristae, lacks extrusomes, microtubule-organizing centers, and cytoplasmic microtubules. During cell division, microtubules appear only within the nucleus, and reorganization of the nuclear envelope occurs late in the nuclear division cycle. Ultrastructural studies reveal that Nuclearia spp. have a pattern of cellular organization distinct from that of other amoebae. Comparison of ultrastructural features suggests that this organism is only distantly related to other rhizopod amoebae (including other filose amoebae) and to the heliozoa.

Abstract: Nucleariid amoebae (Opisthokonta) have been known since the nineteenth century but their diversity and evolutionary history remain poorly understood. To overcome this limitation, we have obtained genomic and transcriptomic data from three Nuclearia, two Pompholyxophrys and one Lithocolla species using traditional culturing and single-cell genome (SCG) and single-cell transcriptome amplification methods. The phylogeny of the complete 18S rRNA sequences of Pompholyxophrys and Lithocolla confirmed their suggested evolutionary relatedness to nucleariid amoebae, although with moderate support for internal splits. SCG amplification techniques also led to the identification of probable bacterial endosymbionts belonging to Chlamydiales and Rickettsiales in Pompholyxophrys. To improve the phylogenetic framework of nucleariids, we carried out phylogenomic analyses based on two datasets of, respectively, 264 conserved proteins and 74 single-copy protein domains. We obtained full support for the monophyly of the nucleariid amoebae, which comprise two major clades: (i) Parvularia-Fonticula and (ii) Nuclearia with the scaled genera Pompholyxophrys and Lithocolla. Based on these findings, the evolution of some traits of the earliest-diverging lineage of Holomycota can be inferred. Our results suggest that the last common ancestor of nucleariids was a freshwater, bacterivorous, non-flagellated filose and mucilaginous amoeba. From the ancestor, two groups evolved to reach smaller (Parvularia-Fonticula) and larger (Nuclearia and related scaled genera) cell sizes, leading to different ecological specialization. The Lithocolla + Pompholyxophrys clade developed exogenous or endogenous cell coverings from a Nuclearia-like ancestor. This article is part of a discussion meeting issue 'Single cell ecology'.