29-615: Corallochytrium belongs to the class of Corallochytrea within Teretosporea and is a sister group to Ichthyosporea. Corallochytrium limacisporum is the only species of Corallochytrium known so far. It was first discovered and named in the Arabian Sea ’s coral lagoons by Kaghu-Kumar in 1987. It was first thought to be a member of the fungi -like thraustochytrids , however, this was later disproven due to Corallochytriums lack of cilia and sagenogenetosome. Little research has been done on
58-468: A paper published in the journal Current Biology . The study used complete mitochondrial genomes of a choanoflagellate ( Monosiga brevicollis ) and an ichthyosporean ( Amoebidium parasiticum ) to firmly resolve the position of Ichthyosporea as the sister group to Choanoflagellata+Metazoa. This clade was named Holozoa (from Ancient Greek ὅλος (holos) 'whole' and ζῷον (zoion) 'animal'), meaning 'whole animal', referencing
87-400: A choanoflagellate) and no sagenogenetosome (Cavalier-Smith, 2001). It has a thin wall of unknown composition, that does not resemble that of fungi (Cavalier-Smith, 2001). Like fungi and choanoflagellates, Corallochytrium has flat mitochondrial cristae (Cavalier-Smith, 2001). Little is known about Corallochytrium' s feeding and ecology, however its feeding regime has strong implications for
116-521: A clade that contains fungi and their protist relatives ( Holomycota ), are part of the larger supergroup of eukaryotes known as Opisthokonta . Holozoa diverged from their opisthokont ancestor around 1070 million years ago (Mya). The choanoflagellates, animals and filastereans group together as the clade Filozoa . Within Filozoa, the choanoflagellates and animals group together as the clade Choanozoa . Based on phylogenetic and phylogenomic analyses,
145-404: A crucial role in understanding the evolutionary steps leading to the emergence of multicellular animals from single-celled ancestors. Recent genomic studies have shed light on the evolutionary relationships between the various holozoan lineages , revealing insights into the origins of multicellularity . Some fossils of possible metazoans have been reinterpreted as holozoan protists. Holozoa
174-550: A significant discovery: many genetic characteristics previously thought as unique to animals can also be found in these unicellular relatives. This suggests that the origin of multicellular animals did not happen solely because of the appearance of new genes (i.e. innovation), but because of pre-existing genes that were adapted or utilized in new ways (i.e. co-option). For example: Additionally, many biological processes seen in animals are already present in their unicellular relatives, such as sexual reproduction and gametogenesis in
203-454: Is a clade that includes animals and their closest relatives, as well as their common ancestor , but excludes fungi . It is defined on a branch-based approach as the clade encompassing all relatives of Homo sapiens (an animal), but not Neurospora crassa (a fungus ). Holozoa, besides animals, primarily comprises unicellular protist lineages of varied morphologies such as choanoflagellates , filastereans , ichthyosporeans , and
232-578: Is a potential new holozoan lineage. It has been tentatively named MASHOL (for 'marine small Holozoa'). The quest to elucidate the evolutionary origins of animals from a unicellular ancestor requires an examination of the transition to multicellularity . In the absence of a fossil record documenting this evolution, insights into the unicellular ancestor of animals are obtained from the analysis of shared genes and genetic pathways between animals and their closest living unicellular relatives. The genetic content of these single-celled holozoans has revealed
261-475: Is an evolutionarily conserved enzyme in the fungi (Sumathi at al., 2006). The presence of α-AAR in Corallochytrium suggests it is a sister clade to fungi. Like fungi, Corallochytrium has also been found to be saprotrophic (Cavalier-Smith & Allsopp, 1996). Coralochytrium also possess the sterol C-14 reductase gene involved in animal and fungi sterol pathways (Sumathi at al., 2006). Little documentation of
290-424: Is comparable to the germination stage of non-animal holozoans. They possibly represent an evolutionary grade in which palintomic cleavage (i.e. rapid cell divisions without cytoplasmic growth in between, a characteristic of animal embryonic cleavage ) was the method of dispersal and propagation . Prior to 2002, a relationship between Choanoflagellata , Ichthyosporea and the animal - fungi divergence
319-463: The Corallochytrium life cycle exists. However it is known that Corallochytrium produces colonies by binary, palintomic cell division (Raghu-kumar, 1987). Completion of the Corallochytrium life cycle involves the release of limax-shaped spores (Raghu-kumar, 1987). Teretosporea Teretosporea are a proposed basal Holozoa clade in which Ichthyosporea and Corallochytrium emerged with
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#1732883716201348-503: The Filozoa as sister clade. Since it is close to the divergence between the main lineages of Fungi and Animals , the study of Teretosporea can provide crucial information on the divergent lifestyles of these groups. Fungi Opisthosporidia Nucleariida Fonticulida Ichthyosporea Corallochytrium Filasterea Choanoflagellatea Animalia With the inclusion of Syssomonas there remained very little support for
377-533: The choanoflagellate Salpingoeca rosetta and several types of multi cellular differentiation . A billion -year-old freshwater microscopic fossil named Bicellum brasieri is possibly the earliest known holozoan. It shows two differentiated cell types or life cycle stages . It consists of a spherical ball of tightly packed cells (stereoblasts) enclosed in a single layer of elongated cells . There are also two populations of stereoblasts with mixed shapes, which have been interpreted as cellular migration to
406-401: The cladogram of Holozoa is shown below: Cristidiscoidea [REDACTED] Fungi [REDACTED] Dermocystida [REDACTED] Ichthyophonida [REDACTED] Corallochytrium [REDACTED] Syssomonas [REDACTED] Tunicaraptor Filasterea [REDACTED] Choanoflagellata [REDACTED] Metazoa [REDACTED] Uncertainty remains around
435-463: The protists Choanoflagellata , Filasterea , Pluriformea and Ichthyosporea . Along with fungi and some other groups, Holozoa is part of the Opisthokonta , a supergroup of eukaryotes . Choanofila was previously used as the name for a group similar in composition to Holozoa, but its usage is discouraged now because it excludes animals and is therefore paraphyletic . The holozoan protists play
464-742: The Teretosporea, and Corallochytrium appears to be a more derived Holozoa. The alternate phylogeny is the Pluriformea hypothesis. Holozoa Incertae sedis Holozoa (from Ancient Greek ὅλος (holos) 'whole' and ζῷον (zoion) 'animal') is a clade of organisms that includes animals and their closest single-celled relatives, but excludes fungi and all other organisms. Together they amount to more than 1.5 million species of purely heterotrophic organisms, including around 300 unicellular species. It consists of various subgroups, namely Metazoa (or animals) and
493-678: The cell. (Raghu-kumar, 1987). C. limacisporum was first discovered and named in 1987 in coral reef lagoons of three Lakshadweep islands in the Arabian sea; Agatti , Kavaratti and Bangaram (Raghu-kumar, 1987). The organism was initially thought to be a new thraustochytrid protist, a group of protists that closely resemble fungi and produce filaments from which they absorb nutrients (Raghu-kumar 1987). However, Cavalier-Smith & Allsopp (1996) explain that C. limancisporum had been wrongly classified as it lacks all defining characteristics of thraustochytrids. After phylogenetic analysis, Corallochytrium
522-437: The classification of eukaryotes. In classifications that use traditional taxonomic ranks (e.g. kingdom, phylum, class), all holozoan protists are classified as subphylum Choanofila (phylum Choanozoa , kingdom Protozoa ) while the animals are classified as a separate kingdom Metazoa or Animalia. This classification excludes animals, even though they descend from the same common ancestor as choanofilan protists, making it
551-499: The closest relatives to animals (Marshall, 2014) Corallochytrium has become an important species in unraveling the diversification of animals from fungi and opisthokont evolution. Corallochytrium and Ichthyosporea are thought to be the earliest branches of the holozoans (animals and all their ancestors, excluding fungi) (Torruella et al. 2015). However new phylogenetic trees placing Corallochytrium closer to fungi are emerging (Sumanthi et al. 2006). Phylogenetic trees based on
580-484: The distinct genera Corallochytrium , Syssomonas , and Tunicaraptor . The first sequenced unicellular holozoan genome was that of Monosiga brevicollis , a choanoflagellate . It measures around 41.6 mega–base-pairs (Mbp) and contains around 9200 coding genes , making it comparable in size to the genome of filamentous fungi . Animal genomes are usually larger (e.g. human genome , 2900 Mbp; fruit fly , 180 Mbp), with some exceptions. Holozoa, along with
609-466: The evolution of animals and fungi. Corallochytrium is a marine protist that inhabits coral reef lagoons in the Arabian Sea (Raghu-kumar, 1987). C. limacisporum is predatory and feeds on large eukaryotic prey (Hehenberger et al. 2017). Corallochytrium has the fungal signature α-aminoadipate reductase (α-AAR) which is involved in the α-aminoadipate (AAA) pathway that synthesises amino acids from inorganic nitrogen in fungi (Sumathi et al. 2006). α-AAR
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#1732883716201638-433: The life cycle or morphology. Most research concerning this genus has been done to uncover the evolution of animals and fungi, as Corallochytrium possess both animal and fungal enzymatic trademarks (C-14 reductase and α-AAR respectively). The genus name is derived from the habitat in which it was first found: coral reef lagoons. The single species name is derived from the limax-shaped ( slug -shaped) spores that are produced by
667-492: The newest holozoan member, is still unresolved. Three different phylogenetic positions of Tunicaraptor have been obtained from analyses: as the sister group to Filasterea , as sister to Filozoa , or as the most basal group of all Holozoa. Environmental DNA surveys of oceans have revealed new diverse lineages of Holozoa. Most of them nest within known groups, mainly Ichthyosporea and Choanoflagellata . However, one environmental clade does not nest within any known group and
696-712: The periphery, a movement that could be explained by differential cell-cell adhesion . These occurrences are consistent with extant unicellular holozoans, which are known to form multicellular stages in complex life cycles. Proposed Ediacaran fossil " embryos " of early metazoans , discovered in the Doushantuo Formation , have been reinterpreted as non-animal protists within Holozoa. According to some authors, although they present possible embryonic cleavage , they lack metazoan synapomorphies such as tissue differentiation and nearby juveniles or adults. Instead, its development
725-468: The relationship of the two most basal groups, Ichthyosporea and Pluriformea . They may be sister to each other, forming the putative clade Teretosporea . Alternatively, Ichthyosporea may be the earliest-branching of the two, while Pluriformea is sister to the Filozoa clade comprising filastereans, choanoflagellates and animals. This second outcome is more strongly supported after the discovery of Syssomonas . The position of Tunicaraptor ,
754-484: The wider animal ancestry that it contains. Holozoa has since been supported as a robust clade by every posterior analysis, even after the discovery of more taxa nested within it (namely Filasterea since 2008, and the pluriformean species Corallochytrium and Syssomonas since 2014 and 2017 respectively). As of 2019, the clade is accepted by the International Society of Protistologists, which revises
783-423: The α-AAR gene, put Corallochytrium as a sister group to fungi, however trees using other genes, such as C-14 reductase, have been inconclusive in their placement in relation to animals or fungi (Sumanthi et al. 2006). Corallochytrium is a small (around 5–20μm in diameter), round, non-photosynthetic choanoflagellate (Torruella et al. 2015, Cavalier-Smith & Allsopp, 1996). It possesses no cilia (despite being
812-500: Was considered on the basis of morphology and ultrastructure . Early phylogenetic analyses gave contradicting results, because the amount of available DNA sequences was insufficient to yield unambiguous results. The taxonomic uncertainty was such that, for example, some Ichthyosporea were traditionally treated as trichomycete fungi. Holozoa was first recognized as a clade in 2002 through a phylogenomic analysis by Franz Bernd Lang, Charles J. O'Kelly and other collaborators, as part of
841-473: Was determined not to be a thraustochytrid, but rather related to choanoflagellates . Cavalier-Smith assigned a new order and class for Corallochytrium under the phylum Choanozoa : Corallochytrida and Corallochytrea respectively (Cavelier-Smith 1995). Recently, Torruella et al. (2015) revealed that Corallochytrium has a sister group: Ichthyosporea and clusters within Teretosporea, an early branching lineage of unicellular organisms that are thought to be one of
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