Veterinary parasitologists: the time has come to talk about the use of the expressions "Protozoan" and "Protista"

Abstract

The classification of eukaryotic organisms has evolved significantly over the past years. For a long time, the five-kingdom model proposed in 1969, which included the kingdoms Monera, Protista, Fungi, Plantae, and Animalia, dominated biological classification. However, recent advances in molecular biology, particularly phylogenomic studies, have challenged this classification as it does not accurately represent the evolutionary patterns of a vast diversity of organisms, especially those formerly known as protozoa. Currently, Protista is no longer considered a valid taxon, as the organisms previously classified in this group are highly divergent and not monophyletic. Modern approaches now classify eukaryotes into several supergroups, with "protozoa" now dispersed among different groups. For example, parasites once grouped as "protozoa," such as Babesia (Apicomplexa), Trypanosoma (Euglenozoa), and Entamoeba (Evosea), are now placed into distant branches of the tree of life and within different supergroups. Although this supergroup classification may change in the coming years, it provides a more accurate representation of evolutionary relationships among eukaryotes. However, this issue has not been adequately discussed by the veterinary parasitology community. This article advocates revisiting these terms in light of modern classification systems to ensure a more accurate and biologically realistic terminology that reflects current knowledge.

Figure 1. Drawing of a tree showing some of the major currently accepted super-groups of living organisms. This schematic representation was freely drawn by the authors, mostly based on the scheme and topology of the tree of life from Burki et al. (2020). Here, some controversial/orphan groups, not classically associated with parasitism of animals, were intentionally omitted. The schematic tree highlights super-groups in which most living eukaryotes are included. To facilitate the understanding of our major audience (veterinary parasitologists and veterinary students), we included in dashed boxes the current taxonomic position of the most important genera (and the Phylum they belong) of parasites of animals formerly classified as “protozoa” or “Protist”. Amorphea and Diaphoretickes are no-rank classifications proposed by Adl et al. (2019) which denote the evolutionary relationship among the super-groups. All distances are merely illustrative and do not indicate any genetic relationship.