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SAR Temporal range: 736–0 Ma Pha. Proterozoic Archean Had.
Scientific classification
Domain:	Eukaryota
Clade:	Diaphoretickes
Clade:	TSAR
Clade:	SAR Burki et al., 2007
Subtaxa
Halvaria Stramenopiles (heterokonts) Alveolata Rhizaria
Synonyms
Harosa Cavalier-Smith, 2010

SAR or Harosa is a highly diverse clade of eukaryotes, often considered a supergroup, that includes stramenopiles (heterokonts), alveolates, and rhizarians. It is a node-based taxon, including all descendants of the three groups' last common ancestor, and comprises most of the now-rejected Chromalveolata. Their sister group has been found to be telonemids, with which they make up the TSAR clade.

Etymology

The name SAR is an acronym derived from the first letters of its three constituent clades; it has been alternatively spelled "RAS". The term "Harosa" (at the subkingdom level) has also been used, with Stramenopiles replaced by its synonym Heterokonta in this variant of the acronym.

History of discovery

Before the discovery of the SAR supergroup, stramenopiles and alveolates were classified in the supergroup Chromalveolata alongside haptophytes and cryptomonads, being believed to have acquired plastids through secondary endosymbiosis of red algae through a common ancestor. Meanwhile, Rhizaria was traditionally considered to be a separate supergroup. More recent phylogenetic studies confirmed that stramenopiles and alveolates diverged with rhizarians as part of the SAR lineage. This clade has been found by later phylogenomic studies to be robustly characterized compared to other supergroups.

This groups excludes haptophytes and cryptomonads, hypothesized to have acquired plastids in separate endosymbiosis events, leading Okamoto et al. (2009) to propose the clade Hacrobia to accommodate them.

Diversity

The SAR supergroup encompasses a variety of morphologies and ecological niches, from microscopic zoo- and phytoplankton to massive kelp forests. The group includes both photosynthetic and non-photosynthetic forms. Photosynthesis arose independently across various stramenopiles and alveolates lineages through secondary or higher-order endosymbiosis events, acquiring plastids of red algal origin, while chlorarachniophyte rhizarians captured plastids from green algae, retaining vestigial nucleomorphs.

It has been estimated that SAR encompasses up to half of all eukaryotic diversity.

Owing to the clade's discovery through phylogenomics, there are no known synapomorphies uniting its various members. This was already the case for its subclade Rhizaria, established earlier through similar means. On the other hand, Stramenopiles is well-defined morphologically, characterized by an anterior flagellum with tripartite bristles (mastigonemes), while Alveolata is united by the presence of cortical alveoli.

Nonetheless, studies of telonemids, believed to be the sister group to SAR, have revealed characteristics such as tripartite hair and peripheral vacuoles, potentially homologous to similar structures in stramenopiles and alveolates. This brings into light the possibility of these structures being ancestrally shared by the clade, with cortical alveoli originating from peripheral vacuoles under this hypothesis.

Internal phylogeny

A 2021 analysis places Alveolata and Stramenopiles in Halvaria, as sister to Rhizaria.

See also

• Amoebozoa
• Archaeplastida
• Excavata
• Opisthokonta

Notes

1. Some recent studies do not recover the TSAR clade and find telonemids to branch within or sister to Haptista, albeit with moderate support. In such case TSAR would be a polyphyletic group.
2. As a formal taxon, "Sar" has only its first letter capitalized, while the earlier abbreviation, SAR, retains all uppercase letters. Both names denote the same group of organisms, unless further taxonomic revisions deem otherwise.

References

1. Laura Wegener Parfrey, Daniel J G Lahr, Andrew H Knoll, Laura A Katz (16 August 2011). "Estimating the timing of early eukaryotic diversification with multigene molecular clocks" (PDF). Proceedings of the National Academy of Sciences of the United States of America. 108 (33): 13624–9. Bibcode:2011PNAS..10813624P. doi:10.1073/PNAS.1110633108. ISSN 0027-8424. PMC 3158185. PMID 21810989. Wikidata Q24614721.
2. ^ Burki, Fabien; Roger, Andrew J.; Brown, Matthew W.; Simpson, Alastair G.B. (January 2020). "The New Tree of Eukaryotes". Trends in Ecology & Evolution. 35 (1): 43–55. Bibcode:2020TEcoE..35...43B. doi:10.1016/j.tree.2019.08.008. PMID 31606140.
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7. ^ Strassert JF, Jamy M, Mylnikov AP, Tikhonenkov DV, Burki F (April 2019). Shapiro B (ed.). "New Phylogenomic Analysis of the Enigmatic Phylum Telonemia Further Resolves the Eukaryote Tree of Life". Molecular Biology and Evolution. 36 (4): 757–765. doi:10.1093/molbev/msz012. PMC 6844682. PMID 30668767.
8. Yazaki, Euki; Yabuki, Akinori; Imaizumi, Ayaka; Kume, Keitaro; Hashimoto, Tetsuo; Inagaki, Yuji (2022). "The closest lineage of Archaeplastida is revealed by phylogenomics analyses that include Microheliella maris". Open Biol. 12 (4): 210376. doi:10.1098/rsob.210376. PMC 9006020.
9. Torruella, Guifré; Galindo, Luis Javier; Moreira, David; López-García, Purificación (27 August 2024). "Phylogenomics of neglected flagellated protists supports a revised eukaryotic tree of life". bioRxiv.org. doi:10.1101/2024.05.15.594285. Retrieved 12 November 2024.
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11. Cavalier-Smith T (June 2010). "Kingdoms Protozoa and Chromista and the eozoan root of the eukaryotic tree". Biology Letters. 6 (3): 342–5. doi:10.1098/rsbl.2009.0948. PMC 2880060. PMID 20031978.
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15. McFadden, G. I. (2001). "Primary and secondary endosymbiosis and the origin of plastids". Journal of Phycology. 37 (6): 951–959. Bibcode:2001JPcgy..37..951M. doi:10.1046/j.1529-8817.2001.01126.x. S2CID 51945442.
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• Protist taxa