''Rigifila'' () is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of free-living

single-celled A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell (biology), cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic ...

eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

s, or

protist A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...

s, containing the sole

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

''Rigifila ramosa'' (). It is classified within the monotypic family Rigifilidae. Along with Micronucleariidae, it is a member of

Rigifilida Rigifilida is a clade of non-ciliate phagotrophic eukaryotes. It consists of two genera: '' Micronuclearia'' and '' Rigifila''. Characteristics Cells of rigifilids are covered with either a single or a double-layered submembrane pellicular lam ...

, an order of basal eukaryotes within the

CRuMs CRuMs is a clade of microbial eukaryotes, whose name is an acronym of the following constituent groups: i) Diphylleids, ii) rigifilids and iii) mantamonads as sister of the Amorphea. A new CRuMs order Glissandrida was proposed in 2025 to pla ...

clade. It differs from ''Micronuclearia'' by having two proteic layers surrounding their

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

instead of a single one, and having more irregular mitochondrial cristae, among other morphological differences.

Systematics

Etymology

The name of this genus is derived , referring to the rigid the

cell Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a de ...

body due to their submembrane proteic layers, and ''filum'' 'thread', referring to the

filopodia Filopodia (: filopodium) are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. Within the lamellipodium, actin ribs are known as ''microspikes'', and when they extend beyond the lamellipod ...

(thin thread-like

pseudopodia A pseudopod or pseudopodium (: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filaments and ...

). The species

epithet An epithet (, ), also a byname, is a descriptive term (word or phrase) commonly accompanying or occurring in place of the name of a real or fictitious person, place, or thing. It is usually literally descriptive, as in Alfred the Great, Suleima ...

is derived , referring to the branched filopodia exhibited by the organism.

Classification

The species ''Rigifila ramosa'' was described in 2012 by biologists Akinori Yabuki, Ken-Ichiro Ishida and

Thomas Cavalier-Smith Thomas (Tom) Cavalier-Smith, FRS, FRSC, NERC Professorial Fellow (21 October 1942 – 19 March 2021), was a professor of evolutionary biology in the Department of Zoology, at the University of Oxford. His research has led to discov ...

from freshwater amoebae isolated in an

India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...

n paddy field water sample. Although they presented a similar structure to '' Micronuclearia'', unique morphological differences and phylogenetic position of these newly discovered amoebae led to the creation of a separate genus, ''Rigifila'', and a separate family, Rigifilidae. In addition, a new order

was described to group the two similar organisms, ''Rigifila'' and ''Micronuclearia''. This order was placed within

Hilomonadea Rigifilida is a clade of non-ciliate phagotrophic eukaryotes. It consists of two genera: '' Micronuclearia'' and '' Rigifila''. Characteristics Cells of rigifilids are covered with either a single or a double-layered submembrane pellicular lamin ...

, a class which previously included Planomonadida but was emended to only include Rigifilida. Hilomonadea is currently considered one of the three clades of

, a basal eukaryotic clade closely related to

Amorphea Amorphea is a taxonomic supergroup that includes the basal Amoebozoa and Obazoa. That latter contains the Opisthokonta, which includes the fungi, animals and the choanoflagellates. The taxonomic affinities of the members of this clade were ...

, the clade containing

Amoebozoa Amoebozoa is a major Taxonomy (biology), taxonomic group containing about 2,400 described species of Amoeba, amoeboid protists, often possessing blunt, fingerlike, Pseudopod#Morphology, lobose pseudopods and tubular mitochondrial cristae. In trad ...

and Opisthokonta.

Biology

Cell structure

''Rigifila'' are

unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...

protozoa Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically ...

. As all members of

, they lack

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

, but generate slender branching

that can attach to the substrate. These filopodia lack any

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

s or

extrusome Extrusomes are membrane-bound organelles found in eukaryotic cells that are capable of discharging material contained within to the exterior of the cell. Due to the diversity in structure and function, it is unlikely that different types of extruso ...

s, and arise from the ventral side of the cell through a common filopodial stem, which is surrounded by an aperture. Under the cell membrane, they present a double-layered

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

aceous lamina (or pellicle) that covers the whole cell body, making it rigid except for the ventral aperture. They have one

nucleus Nucleus (: nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucleu ...

on the dorsal side of the cell. ''Rigifila'', and by extension all of Rigifilidae, differs from Micronucleariidae by having two pellicular dense layers, as opposed to a single-layered pellicle. In addition, they exhibit several broad microtubular bands, of around 20 microtubules each, underneath the dorsal and lateral pellicle, while '' Micronuclearia'' lost all cytoplasmic microtubules. Mitochondrial cristae are completely flat in ''Micronuclearia'', but partially flat and partially inflated in ''Rigifila''.

Feeding

''Rigifila'' are free-living

phagotrophic Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ca ...

organisms. They feed on

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

, which they collect with their

and ingest at the base of a ventral collar or aperture that surrounds the base of the filopodial stem.

References

{{Taxonbar, from1=Q16934875, from2=Q21214940 Eukaryote genera Podiata