Helkesida

Helkesida (formerly known as Sainouroidea)^[1] is a group of microscopic protists belonging to the supergroup Rhizaria, both discovered through molecular phylogenetic analyses. It contains amoeboid flagellates with two flagella. They are either free-living, mostly on fecal matter, or live inside the gut of animals. Among these amoebae, one lineage has independently evolved aggregative multicellularity similarly to slime moulds.^[3]

Helkesida
Scientific classification
Domain:	Eukaryota
Clade:	Sar
Clade:	Rhizaria
Phylum:	Cercozoa
Superclass:	Eoglissa
Class:	Helkesea Cavalier-Smith 2018
Order:	Helkesida Cavalier-Smith 2018[1]
Families[1][2]
Sainouroidea Sainouridae Helkesimastigoidea Helkesimastigidae Guttulinopsidae
Diversity
24 species

Biology

The organisms classified as Helkesida commonly have a gliding motility in which the cells glide on their posterior flagellum. They are ancestrally amoeboid bi-flagellates without scales or theca.^[2] Unlike most Cercozoa which have tubular mitochondrial cristae, they can also present flat cristae or discoid cristae. They are the only group within Rhizaria that present discoid mitochondrial cristae.^[4]

These organisms have an amorphous apical centrosome attached to the nucleus by a rhizoplast. The kinetid arises from 2–4 very short centrioles with dense fibrous roots that attach them to each other and to the nucleus. Their anterior flagellum is reduced to a stub without its 9+2 axoneme. The centrosome also generates numerous microtubules in larger cells. The Golgi apparatus is seen attached to the nuclear envelope and the anterior rhizoplast. They have a microbody attached to the posterior end of the nucleus.^[5]

One helkesid genus, Guttulinopsis, represents an independent lineage in which aggregative multicellularity has evolved to generate "fungi-like" fruiting bodies called sorocarps, similarly to slime moulds such as Dictyostelium.^[4]

Ecology

The helkesid amoebae are bacterivores that can be free-living, mostly associated to fecal environments, or endozoic, associated to animals.^[6] They thrive in aerobic conditions and the microaerophilic gut environment of animals. Rosculus can thrive in anaerobic culture. It is unknown if their preferred habitat is free-living or endozoic.^[3]

Some host species can harbor different helkesid genera and species. One animal can be infected by multiple species simultaneously, and one species can also infect different animal hosts. More sampling of hosts, amoebae and molecular data is needed to better understand the life history and ecology of these protists.^[3]

Evolution and systematics

History

Helkesida is a group initially named Sainouroidea. It was discovered in 2009 as a highly divergent clade within Cercozoa through phylogenetic analyses that used the sequencing of 18S ribosomal RNA from Cholamonas cytrodiopsidis, Sainouron acronematica and Helkesimastix marina. It is a molecularly diverse clade that branches within a group of ancestrally amoeboid bi-flagellates that usually lack an outer cell coat, known as Monadofilosa.^[5] A 2016 study revealed a previously unknown wide diversity of Sainouroidea in fecal environments. Previous environmental samplings excluded sequences from Sainouroidea due to their highly divergent 18S rDNA sequences.^[6] A 2018 study described several new genera and species.^[3]

The initial name for this group, Sainouroidea, had the -oidea suffix for superfamilies, but it was not assigned to any existing classes or orders due to the uncertainty of its phylogenetic position.^[5] In a 2018 revision, the class Helkesea and order Helkesida were created as a substitute for this name. Sainouroidea was then modified to only include one of the three helkesid families, Sainouridae. A second superfamily, Helkesimastigoidea, was created to host the remaining two families, Helkesimastigidae and Guttulinopsidae.^[1]

Classification

Phylogeny of Helkesida

Cholamonas Sainouron Acantholus Homocognata Helkesimastix Puppisaman Olivorum multicellularity Guttulinopsis Rosculus

Cladogram of Helkesida, based on a 18S rDNA phylogenetic analysis within a 2018 study.[3] The study names this group "Sainouroidea" due to being published prior to the taxonomic change to Helkesida.[1]

Currently, Helkesida contains 24 species distributed in 9 genera, 3 families^[2] and 2 superfamilies.^[1] Additionally, many OTUs found through environmental sequencing may represent undescribed clades.^[4][6]

• Superfamily Sainouroidea Cavalier-Smith et al. 2009 emend. 2018
  • Family Sainouridae Cavalier-Smith 2008
    • Acantholus Schuler & Brown 2018^[3]
      • Acantholus ambigus Schuler & Brown 2018
    • Cholamonas Flavin, O'Kelly, Nerad & Wilkinson 2000^[7]
      • Cholamonas cytrodiopsidis
    • Homocognata Schuler, Silberman & Brown 2018^[3]
      • Homocognata vulgaris Schuler, Silberman & Brown 2018
    • Sainouron Sandon 1924^[8]
      • Sainouron acronematica Cavalier-Smith et al. 2008^[9]
      • Sainouron mikroteron Sandon 1924
• Superfamily Helkesimastigoidea Cavalier-Smith 2018
  • Family Helkesimastigidae Cavalier-Smith 2008
    • Helkesimastix Woodcock & Lapage 1915^[10]
      • Helkesimastix faecicola Woodcock & Lapage 1915
      • Helkesimastix major Woodcock 1921
      • Helkesimastix marina Cavalier-Smith 2009^[5]
  • Family Guttulinopsidae Olive 1970
    • Guttulinopsis Olive 1901^[11]
      • Guttulinopsis clavata Olive 1901
      • Guttulinopsis erdosi Schuler, Silberman & Brown 2018
      • Guttulinopsis rogosa Schuler, Tice, Silberman & Brown 2018
      • Guttulinopsis stipitata Olive 1901
      • Guttulinopsis vulgaris Olive 1901
    • Olivorum Schuler, Tice, Silberman & Brown 2018^[3]
      • Olivorum cimiterus Schuler, Tice, Silberman & Brown 2018
    • Puppisaman Schuler & Brown 2018^[3]
      • Puppisaman gallanis Schuler & Brown 2018
    • Rosculus Hawes 1963^[12]
      • Rosculus hawesi Schuler, Tice & Brown 2018
      • Rosculus incognitus Schuler & Brown 2018
      • Rosculus ithacus Hawes 1963
      • Rosculus liberus Schuler & Brown 2018
      • Rosculus macrobrachii Aravindan, Kalavati & Sheeja 2002
      • Rosculus philanguis Schuler, Silberman & Brown 2018
      • Rosculus piscicus Schuler, Silberman & Brown 2018
      • Rosculus terrestris Jousset, Bass & Geisen 2016
      • Rosculus vulgaris Schuler, Silberman & Brown 2018