The alveolates ("with cavities") are a major line of protists.
There are four phyla, 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 may be more closely related to each other than to the ciliates. Both have plastids, and most 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. These include free-living members in Oxyrrhis and Colponema, and parasites in Perkinsus, Parvilucifera, Rastrimonas and the ellobiopsids. In 2001, direct amplification of the rRNA gene in marine picoplankton samples revealed the presence of two novel alveolate linages, called group I and II. Group I has no cultivated relatives, while group II is related to the dinoflagellate parasite Amoebophrya, which was classified until now in the Syndiniales dinoflagellate order.
Relationships between some of these the major groups were suggested during the 1980s, and a specific relationship between all three was confirmed in the early 1990s by genetic studies, most notably by Gajadhar et al. Cavalier-Smith, introduced the formal name Alveolata in 1991, although at the time he actually considered the grouping to be a paraphyletic assemblage, rather than amonophyletic group.
Some studies suggested the haplosporids, mostly parasites of marine invertebrates, might belong here but they lack alveoli and are now placed among the Cercozoa.
The development of plastids among the alveolates is uncertain. Cavalier-Smith proposed the alveolates developed from a chloroplast-containing ancestor, which also gave rise to the Chromista (thechromalveolate hypothesis). However, as plastids only appear in relatively derived (as opposed to ancestral) groups, others argue the alveolates originally lacked them and possibly the dinoflagellates and Apicomplexa acquired them separately.
It seems likely that the common ancestor of this group was a myzocytotic predator with two heterodynamic flagella, micropores, trichocysts, rhoptries, micronemes, a polar ring and a coiled open sidedconoid. This ancestor also probably possesed a plastid but it is presently not clear whether it was photosynthetic. Furthermore it is not clear whether extant perkinsids or colpodellids have retained this organelle.
Given that the alveolates, the dinoflagellates and the heterokont algae acquired their plastids from a red algae it seems likely that their ancestor was photosynthetic.