Alveolate

The alveolates (meaning "pitted like a honeycomb") are a group of protists, considered a major clade and superphylum within Eukarya. They are currently grouped with the stramenopiles and Rhizaria among the protists with tubulocristate mitochondria into the SAR supergroup. The most notable shared characteristic is the presence of cortical (near the surface) alveoli (sacs). These are flattened vesicles (sacs) arranged as a layer just under the membrane and supporting it, typically contributing to a flexible pellicle (thin skin). In armored dinoflagellates they may contain stiff plates. Alveolates have mitochondria with tubular cristae (invaginations), and cells often have pore-like intrusions through the cell surface. The group contains free-living and parasitic organisms, predatory flagellates, and photosynthetic organisms. Almost all sequenced mitochondrial genomes of ciliates and apicomplexa are linear. The mitochondria almost all carry mtDNA of their own but with greatly reduced genome sizes. Exceptions are Cryptosporidium which are left with only a mitosome; ciliates; and Janouškovec et al 2013 demonstrated that Acavomonas diverged early and thus have retained some gene-encoding mtDNA. The mitochondrial genome of Babesia microti is circular. This species is also now known not to belong to either of the genera Babesia or Theileria and a new genus will have to be created for it. The relationship of apicomplexa, dinoflagellates and ciliates had been suggested during the 1980s, and this was confirmed in the early 1990s by comparisons of ribosomal RNA sequences, most notably by Gajadhar et al. Cavalier-Smith introduced the formal name Alveolata in 1991, although at the time he considered the grouping to be a paraphyletic assemblage. Many biologists prefer the use of the colloquial name 'alveolate'. Alveolata include around nine major and minor groups. They are diverse in form, and are known to be related by various ultrastructural and genetic similarities: Ciliates – very common protozoa with many short cilia arranged in rows, and two nuclei Acavomonidia Colponemidia Dinoflagellates s.