Holozoa

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Holozoans
Temporal range: Early Tonian - Present
PluriformeaDermocystidaFilastereaIchthyophonidaChoanoflagellataChoanoflagellataPoriferaCtenophoraCnidariaDeuterostomiaSpiraliaEcdysozoa
Holozoan diversity. Top half displays representatives of holozoan protists. Bottom half displays representatives of metazoans.
Scientific classification Edit this classification
Domain: Eukaryota
Clade: Amorphea
Clade: Obazoa
(unranked): Opisthokonta
(unranked): Holozoa
Lang et al., 2002[1]
Clades[4]

Incertae sedis

Synonyms
  • Choanofila Cavalier-Smith, 2009[5] (plus animals)

Holozoa (from

paraphyletic
.

The holozoan protists play a crucial role in understanding the evolutionary steps leading to the emergence of

fossils
of possible metazoans have been reinterpreted as holozoan protists.

Characteristics

Composition

Holozoa is a

filastereans, ichthyosporeans, and the distinct genera Corallochytrium, Syssomonas, and Tunicaraptor.[6][2]

  • sponges.[9] The mysterious Proterospongia is an example of a colonial choanoflagellate that was thought to be related to the origin of sponges.[10] The affinities of the other single-celled holozoans only began to be recognized in the 1990s.[11]
  • Syssomonas multiformis. These organisms have varied shapes, including cellular aggregations, amoebae, flagellates, and amoeboflagellates.[6]
  • Tunicaraptor unikontum is the newest discovered clade, whose position within Holozoa has yet to be resolved. It is a flagellate with a specialized "mouth" structure absent in other holozoans.[2]

Genetics

The first

fungi. Animal genomes are usually larger (e.g. human genome, 2900 Mbp; fruit fly, 180 Mbp), with some exceptions.[15]

Evolution

Phylogeny

Holozoa, along with a clade that contains

phylogenomic analyses, the cladogram of Holozoa is shown below:[17][18][6][2]

Opisthokonta
Holomycota
1110 Mya
Holozoa
1070 Mya
1250 Mya

Uncertainty remains around the relationship of the two most basal groups, Ichthyosporea and Pluriformea.[4] They may be sister to each other, forming the putative clade Teretosporea.[19] 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.[2][6]

The position of Tunicaraptor, 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.[2][20]

Choanoflagellata. However, one environmental clade does not nest within any known group and is a potential new holozoan lineage. It has been tentatively named MASHOL (for 'marine small Holozoa').[21]

Unicellular ancestry of animals

Unicellular holozoans do not go through animal embryonic development, but they display developmental processes using similar molecules. An actomyosin network controls the cellularization of both an ichthyosporean coenocyte (A) and a fruit fly blastoderm (B). Similarly, actomyosin contraction allows both the shaping of choanoflagellate colonies (C) and the gastrulation of animal embryos.

The quest to elucidate the

genetic pathways between animals and their closest living unicellular relatives. The genetic content of these single-celled holozoans has revealed 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).[7][6]
For example:

Additionally, many biological processes seen in animals are already present in their unicellular relatives, such as sexual reproduction and gametogenesis in the choanoflagellate Salpingoeca rosetta and several types of multicellular differentiation.[7]

Fossil record

A fossilized sample of Bicellum brasieri, a billion-year-old potential holozoan.

A

cellular migration to 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.[3]

Proposed

synapomorphies such as tissue differentiation and nearby juveniles or adults. Instead, its development 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)[23] was the method of dispersal and propagation.[24]

Taxonomy

History

Prior to 2002, a relationship between

trichomycete fungi.[1]

Holozoa was first recognized as a clade in 2002 through a

Ancient Greek ὅλος (holos) 'whole', and ζῷον (zoion) 'animal'), meaning 'whole animal', referencing the wider animal ancestry that it contains.[1]

Holozoa has since been supported as a robust clade by every posterior analysis,[20] even after the discovery of more taxa nested within it (namely Filasterea since 2008,[13] and the pluriformean species Corallochytrium and Syssomonas since 2014[25] and 2017[6] respectively). As of 2019, the clade is accepted by the International Society of Protistologists, which revises the classification of eukaryotes.[4]

Classification

Further information: Taxonomy of Protista

In classifications that use traditional

monophyletic groupings over traditional ranks, which are increasingly perceived as redundant and superfluous. Because Holozoa is a clade, its use is preferred over the paraphyletic taxon Choanofila.[4]

Notes

  1. ^
    Choanoflagellata. However, these terms have not been formally described or adopted, and were rejected in favor of a renamed Choanozoa to fit the clade Metazoa+Choanoflagellata.[4]

References

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  25. ^ Torruella G, de Mendoza A, Grau-Bové X, Donachie S, Pérez-Cordón G, Sitjà-Bobadilla A, Paley R, Manohar CS, Nichols K, Eme L, del Campo J (2014). "Phylotranscriptomics reveals ancient and convergent features in Corallochytrium and Ministeria (Holozoa, Opisthokonta)". Phylogeny and evolutionary perspective of Opisthokonta protists (PDF) (PhD thesis). Vol. 75. Universitat de Barcelona. pp. 1–9.
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