Nucleariid

Nucleariids are single-celled amoebae that lack flagella and have radiating filopodia (i.e., thread-like pseudopodia). They have a spherical or sometimes flattened cell bodies The mucous coat itself—sometimes called glycocalyx—is enigmatic, as it can be present or absent in the same organism depending on the conditions. It appears to be made of one or two layers fibrous material running parallel to the cell membrane, and it often houses bacterial ectosymbionts. Surrounding the cell periphery, the characteristic hyaline (i.e., transparent) filopodia are found, originating from any point of the cell surface, sometimes branching or tapering but are never stiff or anastomosing (fusing with one another). Unlike Heliozoa, these filopodia are not supported by microtubules and do not contain extrusomes. Most species develop a resting cyst during their life cycle consisting of a smooth spherical cell covered by one or more thick layers of a translucent material. == Ecology ==

Nucleariids thrive in water bodies worldwide. Most live in a variety of freshwater environments, including hot spring waters of around 30 °C. Others are found in marine environments (e.g., Lithocolla), and others inhabit faeces (Fonticula). Small-celled species like those of Parvularia and Fonticula feed on small bacteria, while larger cells such as Nuclearia, Pompholyxophrys and Lithocolla can also feed on detritus and unicellular eukaryotic algae (e.g., diatoms). All of them are slow-paced grazers that probably grow in response to the availability of their food sources, such as after algal blooms. == Evolution ==

'' developing into sorocarps. Scale bar: 1 mm. Nucleariids are among the closest relatives of fungi, together forming the clade Holomycota. This clade is, in turn, closely related to Holozoa, the clade containing animals and their closest protist relatives. Together, they form the clade Opisthokonta. After animals and fungi, nucleariids include the third known occurrence of multicellularity among opisthokonts: the species Fonticula alba, a type of slime mold, capable of aggregative multicellular fruiting that develops sorocarps (stalks with masses of spores) for dispersal. The existence of Fonticula alba suggests that opisthokonts have a great propensity toward multicellularity. == Classification ==

History The history of the classification of nucleariids is full of incongruence between morphology and molecular phylogeny. Toward the end of the 19th century, most nucleariid species had already been described, and were classified with other naked or scaled filose amoebae. Protozoologists John P. Cann and Frederick C. Page established the family Nucleariidae to include the naked genera Nuclearia, Gobiella and Nucleosphaerium (later synonymized with Nuclearia). Through studies of their fine cellular ultrastructure via transmission electron microscopy, the order Cristidiscoidida was established within the class Filosea, to accommodate both families Nucleariidae and the silica-scaled Pompholyxophryidae (e.g., Pompholyxophrys, Pinaciophora and Rhabdiophrys), because they all shared disc-shaped mitochondrial cristae as a common characteristic (i.e., they were discicristate). or a slime mold, until 1993, when protozoologist Thomas Cavalier-Smith created the subclass Cristidiscoidia to house two orders: Nucleariida (with Nucleariidae and Pompholyxophryidae) and Fonticulida (with Fonticulidae). but this is no longer accepted. and Micronuclearia in this order, which now are known to belong to Amoebozoa and CRuMs, respectively. Mycologists have proposed a separate kingdom Nucleariae with lower ranks for Cavalier-Smith's two orders (phyla Nuclearida and Fonticulida, classes Nuclearidea and Fonticulidea), but without specifying their taxonomic composition. Many genera remain as incertae sedis due to the lack of molecular data, as it is difficult to confirm their evolutionary position other than by morphological similarity. The following is a list of nucleariid genera with definite placement: • Pompholyxophrys (=Hyalolampe • Pinaciophora (=Pinacocystis ; Pinaciocystis ; Potamodiscus ) • Vampyrellidium ==Notes==