Trichomonads

Abstract

Trichomonads are a diverse collection of flagellate protozoa that are grouped in the phylum Parabasalia identified by the presence of an intracellular fibrous band composed of microtubules termed the axostyle. They encompass free‐living, symbiotic and parasitic representatives, with representatives of the latter group being the most studied, for example the human sexually transmitted parasite Trichomonas vaginalis , the cattle sexually transmitted parasite Tritrichomonas foetus , and the Colombian and galliform bird pathogen Trichomonas gallinae . With the exception of Monocercomonoides sp. they all contain a divergent mitochondrion‐like organelle, termed the hydrogenosome which is considered to have evolved due to the secondary adaptation to low oxygen environments. Key Concepts Trichomonads are in the clade Metamonada, a diverse group of anaerobic or microaerophilic protists that possess modified mitochondria lacking cytochromes, a functional TCA cycle and a genome. The Parabasalia are a phylum within the Metamoada which include all cells with two or more striated parabasal fibres connecting the Golgi apparatus to the ciliary apparatus (parabasal apparatus). The parabasalia are further divided into eight orders including the Trichomonadida, Tritrichomonadida and the Hypotrichomonadida; which are distinguished by the number of flagella, type of karyomastigont, axoneme, undulating membrane and costa. Trichomonads have adapted to an anaerobic metabolism and therefore lack an oxidative metabolism and classic mitochondria, instead of possessing hydrogenosomes. Hydrogenosomes have retained many features of classic mitochondria, including chaperonins; heat shock proteins; outer (TOM 40) and inner membrane (TIM17/22/23) components required for protein import and metabolite exchange; mitochondrial paralogs that are responsible for Fe–S protein assembly; trichomonad nuclear valyl tRNA synthetase gene has a high degree of sequence homology with the mitochondrial valyl tRNA synthetases.