''Olisthodiscus'' is a genus of
heterokont
Heterokonts are a group of protists (formally referred to as Heterokonta, Heterokontae or Heterokontophyta). The group is a major line of eukaryotes. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which a ...
algae. It is the only genus in the family Olisthodiscaceae, the order Olisthodiscales, and the class Olisthodiscophyceae. After a long history of controversial classifications, in 2021 it was recognized as a phylogenetically distinct lineage from the rest of
ochrophyte classes.
Taxonomy
History of classification
After its description in 1937, ''Olisthodiscus'' was placed in
Xanthophyceae
Yellow-green algae or the Xanthophyceae (xanthophytes) are an important group of heterokont algae. Most live in fresh water, but some are found in marine and soil habitats. They vary from single-celled flagellates to simple colonial and filamen ...
.
In 1985, a study of the ultrastructure of a Japanese strain of ''Olisthodiscus'' concluded that it would be more appropriately placed in the
Raphidophyceae.
However, publications from 1980 to 1992 noted the differences between ''Olisthodiscus'' and other raphidophytes, since it lacked ejectile
organelles and had a yellowish colour;
additionally, it was observed that the flagellar root system was more similar to
chrysophyte
The Chrysophyceae, usually called chrysophytes, chrysomonads, golden-brown algae or golden algae are a large group of algae, found mostly in freshwater. Golden algae is also commonly used to refer to a single species, '' Prymnesium parvum'', whic ...
s and
brown algae
Brown algae (singular: alga), comprising the class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the temperate and p ...
than to raphidophytes.
Despite the differences, a revision in 1992 did not accept these arguments and preferred to maintain ''Olisthodiscus'' in Raphidophyceae.
A thesis from 1999 proposed Olisthodiscophyceae as a separate class for the first time, but it was never published in a
peer-reviewed journal.
With the use of
molecular phylogenetic analyses, ''Olisthodiscus'' appeared as a branch outside Raphidophyceae or any other described ochrophyte class. Because of this, in 2013 it was assigned to a new subclass
Sulcophycidae as part of class
Hypogyristea, along with
pelagophytes and
dictyophytes, although with low support for the
monophyly
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...
of this class. It was proposed that ''
Sulcochrysis'' belonged to this class along with ''Olisthodiscus'', but due to lack of
DNA sequences
A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. By convention, sequences are us ...
from this organism it could not be confirmed.
Present classification
In 2021 a study investigated ''Olisthodiscus'' and proved it is a separate phylogenetic lineage. It was then placed in its own class, Olisthodiscophyceae.
Inner classification
Apart from the type species described in 1937, ''
Olisthodiscus luteus'', two other species were added in the 20th century: ''
O. carterae'' and ''
O. magnus'', which were later recognized as conspecific with ''
Heterosigma akashiwo'' and ''
Chattonella marina'' respectively. This rendered ''Olisthodiscus''
monotypic
In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispe ...
, with ''
O. luteus'' as its only species, until 2021, when a new species was described: ''
O. tomasii''.
*''
Olisthodiscus luteus''
*''
Olisthodiscus tomasii''
References
External links
{{taxonbar, from1=Q25367492, from2=Q111210857, from3=Q111210858, from4=Q111210859
Ochrophyte genera
Taxa described in 1937