Abstract
The Parabasalia are a clade of single-celled, anaerobic flagellates that are mainly obligate symbionts or parasites of insects and vertebrates. The group includes the common and widespread human sexually transmitted species Trichomonas vaginalis.
Many species are found exclusively in the guts of termites and the wood-feeding roach Cryptocercus, where they contribute to wood digestion as part of a complex microbial community that sustains the insects. These insect symbionts often harbor an extensive and diverse assortment of ecto- and endo- symbionts.
The Parabasalia are characterized by a parabasal body (Golgi complex supported by a parabasal fiber), which is associated with the flagellar apparatus. Their mitochondria have evolved into hydrogenosomes, double-membrane- bounded organelles that derive energy from the breakdown of pyruvate to acetate, CO2, and H2.
They vary in size from the minute Tricercomitus, which is only a few microns long, to the half-a-millimeter-long Mastotermes gut symbiont Mixotricha paradoxa. Historically, the Parabasalia have been treated as two groups: the smaller, simpler "trichomonads" which bear up to six flagella and the typically much larger, multiflagellate "hypermastigotes." Ultrastructural and molecular evidence have shown that together these groups form a monophyletic Parabasalia, and though neither "trichomonads" nor "hypermastigotes" are monophyletic, they continue to be useful as descriptive terms.