The malaria parasite's ancestors may once have lived on sunlight
Alveolates are a huge group of single-celled organisms that includes pond ciliates, glowing marine dinoflagellates and parasites such as Toxoplasma. What unites them is a layer of tiny sacs under the cell surface. Stranger still, their shared ancestor probably carried a plastid inherited from a red alga, so even today's parasites descend from photosynthesisers.
The name comes from those cortical alveoli: flattened sacs lined up beneath the membrane that stiffen it into a flexible skin, and in armoured dinoflagellates can hold rigid plates. Alveolates also share mitochondria with tube-shaped folds. Together with stramenopiles and Rhizaria they form the SAR supergroup, and a 2015 classification ranked them as a superphylum. Membership spans free-living predators, photosynthetic plankton and parasites.
The main branches include the ciliates, common protozoa covered in rows of short hairlike cilia and carrying two nuclei; the dinoflagellates, mostly marine and often photosynthetic; and the Apicomplexa, parasites that have lost most locomotion except in their sex cells. Apicomplexans and dinoflagellates seem closer to each other than either is to ciliates. Both carry a cone of microtubules at the cell's tip; parasites use it to break into host cells, while some colourless dinoflagellates use a similar structure to feed on prey.
Researchers suspected these groups were related in the 1980s, and ribosomal RNA comparisons confirmed it in the early 1990s. Thomas Cavalier-Smith coined the formal name Alveolata in 1991. Sampling of ocean plankton in 2001 turned up two previously unknown lineages, one with no cultivated relatives. Their mitochondrial genomes are odd too: mostly linear and shrunken, while Toxoplasma's is shattered into 21 blocks that recombine, and Cryptosporidium keeps only a remnant organelle.
A Bayesian estimate puts the group's origin at around 850 million years ago. How they got plastids was long argued; the current evidence suggests alveolates, dinoflagellates and heterokont algae share a plastid acquired from a red alga, making it likely that the common ancestor of alveolates and heterokonts photosynthesised. Hunting algae appears to have been a major engine of their evolution, supplying new partners for endosymbiosis, which is why the informal label Myzozoa, 'siphoning the contents from prey', is used for much of the group.
Source: Alveolate
