Euglenozoa

The Euglenozoa comprise two major groups of flagellate protozoa, the kinetoplastids and euglenids, together with the diplonemids and possibly a few other forms. They include various common free-living forms as well as a few important parasites, some of which are parasitic in humans. Most are small, around 15-40 Ám in size, although many euglenids get up to 500 Ám long. They were first grouped together by Cavalier-Smith in 1981.

Most Euglenozoa have two flagella, usually one leading and one trailing, which insert parallel to each other in an apical or subapical pocket. In most forms there is an associated cytostome (mouth) supported by one of three microtubule groups that arise from the flagellar bases. These are characteristic of the group; the other two support the dorsal and ventral surfaces of the cell, and in the euglenids support proteinaceous strips that form a pellicle. A number of other ultrastructural peculiarities also distinguish the group, most notably the presence of a paraxial rod in each flagellum, which respectively have tubular and latticed structures.

Most Euglenozoa feed by ingesting smaller organisms, typically bacteria, or by absorption. A number of euglenids, however, possess chloroplasts and so produce energy through photosynthesis. These have generally lost the cytostome and often have other adaptations to an autotrophic life, such as light-sensitive eyespots. The chloroplasts are contained in three membranes and are pigmented similarly to the plants, suggesting they were retained from some captured green alga.

All Euglenozoa have mitochondria with discoid cristae, which in the kinetoplastids characteristically have a DNA containing granule or kinetoplast associated with the flagellar bases. No examples of sexual reproduction in the group have been found. Reproduction is exclusively through cell division, characteristically with closed mitosis involving an internal spindle. The Heterolobosea and other excavates, and the peculiar flagellate Stephanopogon, have been considered as possible relatives but at the moment the position of the Euglenozoa remains uncertain.


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