The plastids (chloroplasts) of photosynthetic eukaryotes are the product of an ancient symbiosis between a heterotrophic eukaryote and a free-living Cyanobacterium. It is widely believed that this process, known as primary endosymbiosis, occurred only once and that all plastids descend from a single common ancestor. However, plastids have also moved laterally amongst unrelated eukaryotic cells by secondary endosymbiosis, a process that has occured multiple times and has given rise to a staggering array of photosynthetic organisms [diagram, see Archibald & Keeling 2002, Trends Genet. 18, 577- for review].
The Cryptomonads and Chlorarachniophytes are two microalgal lineages that are of particular interest with respect to secondary endosymbiosis. Unlike all other secondary plastid-containing algae, these organisms have retained the nucleus of the eukaryotic endosymbiont in a highly derived form called a nucleomorph. The nucleomorph genomes of Chorarachniophytes and Cryptomonads are very fast evolving and are the smallest eukaryotic genomes known, having transferred most of their genetic material to the nuclear genome of their respective host cells. Within the two groups, nucleomorph genome size varies considerably from lineage to lineage.
Continued evolutionary surprises among dinoflagellates -- Morden and Sherwood 99 (18): 11558 -- Proceedings of the National Academy of Sciences:"It is well established that chloroplasts in green and red algae are derived from a primary endosymbiotic event between a cyanobacterium and a eukaryotic organism 1 billion years ago (Fig. 1; refs. 1 and 2). Although these two groups account for many of the world's photosynthetic species, most other major taxonomic groups of photosynthetic organisms (stramenopiles including diatoms, phaeophytes, chrysophytes, and haptophytes) have plastids derived from a photosynthetic eukaryote implying a secondary endosymbiosis (1, 2). Still other groups, such as the dinoflagellates, have more complicated associations believed to be derived from tertiary endosymbioses involving the engulfment of a secondary endosymbiont. Each endosymbiotic event has characteristic structural changes associated with it, the most notable of which is the addition of two membranes surrounding the plastid (the inner representing the cell membrane of the engulfed organism and the outer representing the phagocytosis vacuole membrane) (2). Dinoflagellates, although believed to be tertiary endosymbionts, have only 3 membranes surrounding their plastids (1, 2), suggesting that the acquisition of too many membranes may be functionally unstable and can cause some to be lost. "
Clifford W. Morden, and Alison R. Sherwood Continued evolutionary surprises among dinoflagellates PNAS September 3, 2002 vol. 99 no. 18 11558-11560