The alveolates are a major line of protists. There are three main groups, which are very divergent in form, but are now known to be close relatives based on various ultrastructural and genetic similarities:

The most notable shared characteristic is the presence of cortical alveoli, flattened vesicles packed into a continuous layer supporting the membrane, typically forming a flexible pellicle. In dinoflagellates they often form armor plates. Alveolates have mitochondria with tubular cristae, and their flagella or cilia have a distinct structure.

The Apicomplexa and dinoflagellates are closer to each other than they are to the ciliates. Both have plastids, which likely have a common origin. Most also share a bundle or cone of microtubules at the top of the cell. In apicomplexans this forms part of a complex used to enter host cells, while in some colorless dinoflagellates it forms a peduncle used to ingest prey. Various other genera are closely related to these two groups, mostly flagellates with a similar apical structure. They include free-living members in Oxyrrhis, Acrocoelus, Colpodella, and perhaps Colponema, and parasites in Perkinsus, Parvilucifera, Cryptophagus, and the ellobiopsids .

Relationships between the major groups were suggested during the 1980s, and confirmed by a genetic study by Gajadhar et al. in 1991, after which the name Alveolata was introduced by Cavalier-Smith. Some studies suggested the haplosporids , mostly parasites of marine invertebrates, might belong here but they lack alveoli and are now placed among the Cercozoa.

References

• Gajadhar, A. A. et al. (1991). Ribosomal RNA sequences of Sarcocystis muris, Theilera annulata, and Cryptothecodinium cohnii reveal evolutionary relationships among apicomplexans, dinoflagellates, and ciliates. Molecular and Biochemical Parasitology 45: 147-153.
• Cavalier-Smith, T. (1993). Kingdom Protozoa and its 18 Phyla. Microbiological Reviews 57:953-994. PubMed