Oxymonadea phylum Metamonada

The oxymonads also lack both the Golgi and mitochondrion. These species are larger (mostly ~50 ^m), strictly symbiotic anaerobes in the digestive tract of wood-eating insects (mostly known from termites and cockroaches). Typical genera include Monocercomoides, Oxymonas, Pyrsonympha and Saccinobacculus (Fig. 1.3). They lack a cytostome and ingest fragmented wood chips and microdetritus from the host gut, by phagocytosis from the posterior region of the cell. Pinocytosis occurs over the whole cell surface. There are two pairs of anterior basal bodies connected by a thick fibre. The cilia are trailing posteriorly and spiral-wrap around the cell body. From one basal body, a sheet of tightly cross-linked microtubules (the pelta) overlays the nucleus and supports the anterior region. A ribbon of cross-linked microtubules and other supporting cytoskeletal elements combine to form the axostyle. It is a characteristic organelle which runs the length of the cell and participates in locomotion. It is a strong organelle that bends the cell and contributes to its wriggling sinusoidal locomotion. There is no micro-tubule network supporting the cell membrane, which is therefore flexible. Newly divided cells can be seen swimming in the gut, but eventually attach at the pelta to the host digestive tract lining. Mitosis is closed, with an intranuclear spindle. Sexual stages with meiosis occur. The surface of the Oxymonadea is covered by other symbiotic bacteria, which also participate in the digestion process inside the gut.

Fig. 1.3. A termite gut oxymonad, Pyrsonympha, with four cilia (C) spiralling around the body, an internal axostyle (Ax) that extends the length of the cell and an anterior nucleus (N). Scale bar 10 ^m.

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