Abstract
Trypanosomatids are protozoan parasites of the class Kinetoplastida predominately restricted to invertebrate hosts (i.e. possess a monoxenous life-cycle). However, several genera are pathogenic to humans, animals and plants, and have an invertebrate vector that facilitates their transmission (i.e. possess a dixenous life-cycle). Phytomonas is one dixenous genus that includes several plant pathogens transmitted by phytophagous insects. Trypanosoma and Leishmania are dixenous genera that infect vertebrates, including humans, and are transmitted by hematophagous invertebrates. Traditionally, monoxenous trypanosomatids such as Leptomonas were distinguished from morphologically similar dixenous species based on their restriction to an invertebrate host. Nonetheless, this criterion is somewhat flawed as exemplified by Leptomonas seymouri which reportedly infects vertebrates opportunistically. Similarly, Novymonas and Zelonia are presumably monoxenous genera yet sit comfortably in the dixenous clade occupied by Leishmania. The isolation of Leishmania macropodum from a biting midge (Forcipomyia spp.) rather than a phlebotomine sand fly calls into question the exclusivity of the Leishmania-sand fly relationship, and its suitability for defining the Leishmania genus. It is now accepted that classic genus-defining characteristics based on parasite morphology and host range are insufficient to form the sole basis of trypanosomatid taxonomy as this has led to several instances of paraphyly. While improvements have been made, resolution of evolutionary relationships within the Trypanosomatidae is confounded by our incomplete knowledge of its true diversity. The known trypanosomatids probably represent a fraction of those that exist and isolation of new species will help resolve relationships in this group with greater accuracy. This review incites a dialogue on how our understanding of the relationships between certain trypanosomatids has shifted, and discusses new knowledge that informs the present taxonomy of these important parasites.
Highlights
The Trypanosomatidae are a diverse family of protozoan parasites that are predominately monoxenous
With the exception of the gambiense form of Human African Trypanosomiasis (HAT) and infections caused by Leishmania tropica and Leishmania donovani, for which humans are considered the main reservoir, trypanosomatid-associated diseases are predominately zoonotic, with animal reservoirs playing a key role in maintaining endemicity [8, 12]
Important developments have been made in the field of trypanosomatid taxonomy in recent years, facilitated by the identification of novel trypanosomatid species in addition to advances in molecular biology and phylogenetics
Summary
The Trypanosomatidae are a diverse family of protozoan parasites that are predominately monoxenous. While the phrase “lower trypanosomatid” remains in general use, the concept is imperfect as dixenous parasitism has evolved independently multiple times in trypanosomatids, meaning that some dixenous genera (e.g. Trypanosoma) are basal to certain monoxenous species (e.g. Crithidia) [27, 81]. Whilst they are considered non-pathogenic to vertebrates, reports of infections with monoxenous species resulting in human illness date back to the 1980s [34, 88] (Table 2).
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