Abstract
Preaxostyla comprises Oxymonadida, containing 14 genera of gut endosymbionts plus two genera of free-living bacterivorous flagellates from low oxygen sediments (Trimastix and Paratrimastix). The group was recognized on the basis of 18S rRNA phylogenies, and ultrastructural investigations have revealed a synapomorphy in the organization of the "I" fiber that supports microtubular root R2.
Trimastix and Paratrimastix are typical excavates with three anterior/lateral flagella and the recurrent flagellum passing through a conspicuous ventral feeding groove. The cellular structure of oxymonads is more derived, and a particularly striking diversity of large cellular forms is observed in genera inhabiting guts of lower termites and wood-eating cockroaches.
Here the large oxymonad species and their bacterial ecto- and endosymbionts are probably involved in the cellulose digestion, similarly to the large species of parabasalids. All Preaxostyla live in low oxygen environments, and this has affected their metabolism and organelle complement.
Glycolysis is apparently the main source of cellular ATP and mitochondria are either reduced to hydrogenosome-like compartments (in Trimastix and Paratrimastix) or lost completely (in oxymonads). Peroxisomes are absent in the whole group.
Stacked Golgi bodies are unknown in oxymonads; however, genes encoding proteins functional in Golgi are present, indicating the existence of a cryptic Golgi. Phylogenomic analyses have shown that Preaxostyla represent one of the three main lineages of Metamonada (within Excavata).
Because oxymonads are the only known eukaryotes that have completely lost the mitochondrial organelle, they may serve as models for studies of amitochondriality and mitochondrial evolution.