The name SAR is an
acronym derived from the first letters of its three constituent clades;[a] it has been alternatively spelled "RAS".[6][8] The term "Harosa" (at the subkingdom level) has also been used, with Stramenopiles replaced by its synonym Heterokonta in this variant of the acronym.[9]
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.[2] 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.[10] This clade has been found by later phylogenomic studies to be robustly characterized compared to other supergroups.[7]
This groups excludes haptophytes and cryptomonads, hypothesized to have acquired plastids in separate endosymbiosis events,[11] leading Okamoto et al. (2009) to propose the clade
Hacrobia to accommodate them.[12]
It has been estimated that SAR encompasses up to half of all eukaryotic diversity.[2]
Owing to the clade's discovery through
phylogenomics, there are no known synapomorphies uniting its various members.[3] 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.[15]
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.[7]
Internal phylogeny
A 2021 analysis places Alveolata and Stramenopiles in
Halvaria, as sister to Rhizaria.[11]
^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.