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Many colorless flagellates such as the non-photosynthetic euglenoids have retained plastids with a reduced genome, because they are still essential to the organisms. Loss of photosynthetic capacity has reduced the number of genes in the plastids, but other functions such as production of rna products apparently preserved within them and not transferred to the nucleus. The same is the case for colorless holoparasitic higher plants such as Lathraea clandestine (purple toothwort), Orobanche lutea (yellow broomrape) and Neottia nidus-avis (birdsnest orchid), as well as apicomplexan parasites such as Plasmodium and Toxoplasma (Barbrook, Howe and Purton 2006). Both colorless and photosynthetically active euglenoids are auxotrophic, i.e. dependent on dissolved organic carbon including vitamins B1 and B12. Several species are phagotrophic, capable of ingesting particulate carbon including bacteria and small photosynthetic algae, some of which the host retains as organelles rather than digesting them. Thus they have the capacity to obtain plastids and become secondarily photosynthetic (Graham and Wilcox 2000.
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Barbrook, A.C., C.J. Howe and S. Purton 2006. Why are plastid genomes retained in non-photosynthetic organisms? Trends in Plant Science 11(2):101-108. online Graham, L. and L.W. Wilcox 2000. Algae. Prentice Hall (699 pp).
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