Postgaardia is a proposed basal

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

of flagellate

Euglenozoa Euglenozoa are a large group of flagellate Discoba. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. Euglenozoa are represented by four major groups, ''i.e.,'' Kinetoplastea, ...

, following

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 ...

. , the

Interim Register of Marine and Nonmarine Genera The Interim Register of Marine and Nonmarine Genera (IRMNG) is a taxonomic database which attempts to cover published genus names for all domains of life (also including subgenera in zoology), from 1758 in zoology (1753 in botany) up to the pre ...

treats the group as a subphylum. A 2021 review of Euglenozoa places Cavalier-Smith's proposed members of Postgaardia in the class Symbiontida. As Euglenozoans may be basal

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, the Postgaardia may be key to studying the evolution of

eukaryotes The eukaryotes ( ) constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus. All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes. They constitute a major group of ...

, including the incorporation of eukaryotic traits such as the incorporation of

alphaproteobacteria ''Alphaproteobacteria'' or ''α-proteobacteria'', also called ''α-Purple bacteria'' in earlier literature, is a class of bacteria in the phylum '' Pseudomonadota'' (formerly "Proteobacteria"). The '' Magnetococcales'' and '' Mariprofundales'' ar ...

mitochondrial A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...

endosymbiont An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...

s. Euglenozoa are a large group of

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and the ...

Discoba Excavata is an obsolete, extensive and diverse Paraphyly, paraphyletic group of unicellular Eukaryote, Eukaryota. The group was first suggested by Simpson and Patterson in 1999 and the name latinized and assigned a rank by Thomas Cavalier-Smit ...

. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. Euglenozoa are represented by four major clades, ''i.e.,''

Kinetoplastea Kinetoplastida (or Kinetoplastea, as a class) is a group of flagellated protists belonging to the phylum Euglenozoa, and characterised by the presence of a distinctive organelle called the kinetoplast (hence the name), a granule containing a la ...

Diplonemea Diplonemidae is a family of biflagellated unicellular protists that may be among the more diverse and common groups of planktonic organisms in the ocean. Although this family is currently made up of three named genera; '' Diplonema'', '' Rhynchop ...

, Euglenida, and Symbiontida. Euglenozoa are unicellular, mostly around in size, although some euglenids get up to long.

Characteristics

Euglenozoa are characterized by the ultrastructure of the

flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

. In addition to the normal supporting microtubules or

axoneme In molecular biology, an axoneme, also called an axial filament, is the microtubule-based cytoskeletal structure that forms the core of a cilium or flagellum. Cilia and flagella are found on many cells, organisms, and microorganisms, to pr ...

, each contains a rod (called ''paraxonemal''), which has a tubular structure in one flagellum and a latticed structure in the other. Based on this, two smaller groups are included: the

diplonemid Diplonemidae is a family of biflagellated unicellular protists that may be among the more diverse and common groups of planktonic organisms in the ocean. Although this family is currently made up of three named genera; '' Diplonema'', '' Rhynchop ...

s and ''

Postgaardi ''Postgaardi mariagerensis'' is a species of single-celled eukaryote in the Euglenozoa Euglenozoa are a large group of flagellate Discoba. They include a variety of common free-living species, as well as a few important parasites, some of which ...

''. Postgaardea is a third deep-branching euglenozoan clade that may be a sister to Euglenoida but does not branch within them or Glycomonada on the evolutionary most realistic sequence trees presented in the next three sections, contrary to some poorly resolved earlier trees. They were placed in the new subphylum Postgaardia because they are radically different ultrastructurally from both euglenoids and glycomonads. Postgaardia are biciliate free-living anaerobes covered in epibiotic bacteria in longitudinal rows are the diagnosis. A highly contractile pellicle with multiple evenly spaced microtubules and no morphogenetic pairs that are specifically distinguished. Without cytostomal or reservoir encircling fibers, cemented jaw supports, or hard longitudinal straight cemented rods, the

cytopharynx A cytostome (from ''cyto-'', cell and ''stome-'', mouth) or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacuole ...

is simplified. Postgaardea is the lone included class in etymology. Reconstructions of FA ultrastructure in Postgaardi and Calkinsia confirmed that they were fundamentally similar and deserved to be classified together as a distinct order Postgaardida and class Postgaardea, as both genera share six finger-like projections.

Taxonomy

According to Cavalier-Smith (2017): * Infraphylum Postgaardia Cavalier-Smith 2016 stat. nov. Cavalier-Smith 2017 ** Class Postgaardea Cavalier-Smith 1998 s.s. ymbiontida Yubuki et al., 2009*** Order Bihospitida Cavalier-Smith 2016 **** Family Bihospitidae Cavalier-Smith 2016 *** Order Postgaardida Cavalier-Smith 2003 **** Family Calkinsiidae Cavalier-Smith 2016 **** Family Postgaardidae Cavalier-Smith 2016

References

External links

Tree of Life: Euglenozoa
Euglenozoa Eukaryote subphyla Taxa named by Thomas Cavalier-Smith {{Excavata-stub