A bikont ("two flagella") is any of the
eukaryotic organisms classified in the group Bikonta. Many single-celled members of the group, and the presumed ancestor, have two
flagella.
Enzymes
Another shared trait of bikonts is the fusion of two
genes into a single unit: the genes for
thymidylate synthase
Thymidylate synthase (TS) () is an enzyme that catalyzes the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP). Thymidine is one of the nucleotides in DNA. With inhibition of TS, an imbalance of deoxynucle ...
(TS) and
dihydrofolate reductase (DHFR) encode a single
protein with two functions.
The genes are separately translated in
unikont
Amorphea are members of a taxonomic supergroup that includes the basal Amoebozoa and Obazoa. That latter contains the Opisthokonta, which includes the Fungi, Animals and the Choanomonada, or Choanoflagellates. The taxonomic affinities of the mem ...
s.
Relationships
Some research suggests that a unikont (a eukaryotic cell with a single flagellum) was the ancestor of
opisthokonts (Animals, Fungi, and related forms) and
Amoebozoa
Amoebozoa is a major taxonomic group containing about 2,400 described species of amoeboid protists, often possessing blunt, fingerlike, lobose pseudopods and tubular mitochondrial cristae. In traditional and currently no longer supported classi ...
, and a bikont was the ancestor of
Archaeplastida
The Archaeplastida (or kingdom Plantae ''sensu lato'' "in a broad sense"; pronounced /ɑːrkɪ'plastɪdə/) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group ...
(Plants and relatives),
Excavata
Excavata is a major supergroup of unicellular organisms belonging to the domain Eukaryota. It was first suggested by Simpson and Patterson in 1999 and introduced by Thomas Cavalier-Smith in 2002 as a formal taxon. It contains a variety of free- ...
,
Rhizaria
The Rhizaria are an ill-defined but species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthethic, but many foramin ...
, and
Chromalveolata
Chromalveolata was a eukaryote supergroup present in a major classification of 2005, then regarded as one of the six major groups within the eukaryotes.
It was a refinement of the kingdom Chromista, first proposed by Thomas Cavalier-Smith in ...
.
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 ...
has suggested that
Apusozoa
The Apusozoa are an Obazoa phylum comprising several genera of flagellate eukaryotes. They are usually around 5–20 μm in size, and occur in soils and aquatic habitats, where they feed on bacteria. They are grouped together based on the prese ...
, which are typically considered ''
incertae sedis
' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertaint ...
'', are in fact bikonts.
Relationships within the bikonts are not yet clear. Cavalier-Smith has grouped the Excavata and Rhizaria into the
Cabozoa
In the classification of eukaryotes (living organisms with a cell nucleus), Cabozoa was a taxon proposed by Cavalier-Smith.
It was a putative clade comprising the Rhizaria and Excavata. More recent research tends to place the Rhizaria with th ...
and the
Archaeplastida
The Archaeplastida (or kingdom Plantae ''sensu lato'' "in a broad sense"; pronounced /ɑːrkɪ'plastɪdə/) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group ...
and
Chromalveolata
Chromalveolata was a eukaryote supergroup present in a major classification of 2005, then regarded as one of the six major groups within the eukaryotes.
It was a refinement of the kingdom Chromista, first proposed by Thomas Cavalier-Smith in ...
into the
Corticata
Corticata ("one with a cortex"), in the classification of eukaryotes (living organisms with a cell nucleus), is a clade suggested by Thomas Cavalier-Smith
to encompass the eukaryote supergroups of the following two groups:
* Plantae, or Archaep ...
, but at least one other study has suggested that the
Rhizaria
The Rhizaria are an ill-defined but species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthethic, but many foramin ...
and
Chromalveolata
Chromalveolata was a eukaryote supergroup present in a major classification of 2005, then regarded as one of the six major groups within the eukaryotes.
It was a refinement of the kingdom Chromista, first proposed by Thomas Cavalier-Smith in ...
form a
clade.
An alternative to the Unikont–Bikont division was suggested by Derelle ''et al.'' in 2015,
where they proposed the
acronym
An acronym is a word or name formed from the initial components of a longer name or phrase. Acronyms are usually formed from the initial letters of words, as in ''NATO'' (''North Atlantic Treaty Organization''), but sometimes use syllables, as ...
s
Opimoda–Diphoda respectively, as substitutes to the older terms. The name Diphoda is formed from the letters of
DIscoba and
diaPHOretickes (shown in capitals).
Cladogram
A "classical" cladogram (data from 2012, 2015) is:
However, a cladogram (data from 2015, 2016) with the root in Excavata is
The
corticates correspond roughly to the bikonts. While
Haptophyta
The haptophytes, classified either as the Haptophyta, Haptophytina or Prymnesiophyta (named for '' Prymnesium''), are a clade of algae.
The names Haptophyceae or Prymnesiophyceae are sometimes used instead. This ending implies classification at ...
,
Cryptophyta
The cryptophyceae are a class of algae, most of which have plastids. About 220 species are known, and they are common in freshwater, and also occur in marine and brackish habitats. Each cell is around 10–50 μm in size and flattened in shape, ...
,
Glaucophyta,
Rhodophyta, the
SAR supergroup and
viridiplantae are usually considered
monophyletic,
Archaeplastida
The Archaeplastida (or kingdom Plantae ''sensu lato'' "in a broad sense"; pronounced /ɑːrkɪ'plastɪdə/) are a major group of eukaryotes, comprising the photoautotrophic red algae (Rhodophyta), green algae, land plants, and the minor group ...
may be paraphyletic, and the mutual relationships between these phyla are still to be fully resolved.
Recent reconstructions placed Archaeplastida and Hacrobia together in an "HA supergroup" or "AH supergroup", which was a sister clade to the SAR supergroup within the
SAR/HA supergroup. However, this seems to have fallen out of favor as the monophyly of hacrobia has come under dispute.
See also
*
Corticata
Corticata ("one with a cortex"), in the classification of eukaryotes (living organisms with a cell nucleus), is a clade suggested by Thomas Cavalier-Smith
to encompass the eukaryote supergroups of the following two groups:
* Plantae, or Archaep ...
References
Further reading
*
*
*
Tree of Life Eukaryotes
{{Taxonbar, from=Q860203
Eukaryote unranked clades
Taxa named by Thomas Cavalier-Smith