The Euglenozoa is a monophyletic group consisting of single-celled flagellates with very different modes of nutrition, including predation, osmotrophy, parasitism, and photoautotrophy. Predatory euglenozoans are phylogenetically widespread within the group and show a wide diversity of feeding apparatus structure, feeding strategies and prey preferences (Leander et al. 2001; Leander 2004). For instance, some predatory species prefer small prey such as bacteria (e.g., Bodo and Entosiphon); other species, such as Peranema and diplonemids frequently consume larger prey, such as other eukaryotic cells, by either engulfing them whole ( ‘true’ phagotrophy) or by piercing the prey cell and consuming the contents (myzocytosis). Most predatory euglenids are adapted to move and feed on surfaces and they are important components of the microbial biota in many surface sediments. Osmotrophic euglenozoans are heterotrophs that lack a feeding apparatus and absorb nutrients directly from their environments (e.g., Distigma and Rhabdomonas). Photoautotrophy is restricted to a specific subclade of euglenid euglenozoans and originated via secondary endosymbiosis between a eukaryovorous euglenid and a green algal cells (Gibbs 1978; Leander 2004, Leander et al. 2007) (e.g. see the third cell, starting from the left, in the title image). Parasitic (and commensalistic) euglenozoans appear to have evolved independently several times within the group (Simpson et al. 2002, 2006). One subgroup, the trypanosomatids (e.g. see right-hand cell in the title image), include the organisms that cause important human illnesses such as "sleeping sickness", leishmaniases, and Chagas’ disease. Euglenozoans, whether they are parasitic or photoautotrophic, share several derived cytoskeletal features associated with the flagellar apparatus and the feeding apparatus.
- Closed mitosis with an intranuclear spindle
- Paddle-shaped, discoidal mitochondrial cristae
- (Ancestrally with) two flagella: an anteriorly directed dorsal flagellum and a posteriorly directed ventral flagellum
- (Ancestrally with) a flagellar apparatus consisting of three microtubular roots: dorsal root, intermediate root and ventral root.
- Microtubule-reinforced ventral or anterior feeding apparatus (MtR pocket)
- Heteromorphic paraxonemal rods - relatively thick flagella (synapomorphy)
- Tubular extrusomes (synapomorphy)
Description of Euglenozoa
Evolution and Systematics
Discussion of Phylogenetic Relationships
Most euglenozoans are members of two major subgroups: kinetoplastids and euglenids. The best morphological synapomorphy for kinetoplastids is a highly structured mitochondrial inclusion of DNA called a "kinetoplast"; the best morphological synapomorphy for euglenids is a distinctive cortical system of overlapping proteinaceous strips that run longitudinally over the cell surface; this system is called the "euglenid pellicle" (Leander et al. 2007) (e.g. see the third cell, starting from the left, in the title image). The third, much smaller group, diplonemids, are free-living phagotrophs and facultative parasites with sack-shaped cells and short, thin flagella (Kivic and Walne 1984; Kent et al. 1987; Marande et al. 2005; Roy et al. 2007) (e.g. see left-hand cell in the title image). Diplonemid mitochondria contain a few very large flattened cristae, whereas almost all other Euglenozoa have smaller and more numerous discoidal cristae. Most recent molecular phylogenetic analyses support a sister relationship between kinetoplastids and diplonemids (Maslov et al. 1999; Simpson and Roger, 2004; Breglia et al. 2007). Several other euglenozoan lineages, such as Postgaardi and Calkinsia (which both inhabit low-oxygen marine environments, form symbioses with epibiotic bacteria) lack the diagnostic features of the three main subgroups and, therefore, have unclear phylogenetic positions within the Euglenozoa (Simpson et al. 1997; Yubuki et al. In review).