What’s in a Name? The Curious Case of Trypanosome Species and Ecotypes

Throughout a large part of his magnum opus On the Origin of Species, Charles Darwin ponders the question of how to discern a species from a mere variety. Approximately 80 years ago, Ernst Mayr developed the concept of a biological species which is defined as a group of all individuals that can interbreed and produce fertile offspring while being isolated from other species by reproductive barriers. In reality, the reproductive barriers are not perfect, as exemplified by multiple interspecific hybrids. However, when we examine a cladogram (the ‘pedigree’ of selected taxa), the concept of a biological species predicts that all individuals belonging to the same species will group together on the cladogram – they will be related more to each other than to members of other species. However, if individuals of the putative species freely intermingle with other species, then the species in question is no longer a species and its status needs to be revised. Alternatively, we may discover that a species comprises two or more separate lineages which may then be recognised as cryptic species. However, it is the former situation that poses a larger issue.

This issue is known from human lice: the body louse (Pediculus humanus), a vector of pathogens causing diseases such as epidemic typhus or trench fever, is not a ‘proper species’, but rather an assemblage of a few independently evolved lineages of the head louse (Pediculus capitis). A similar problem in African trypanosomes (Trypanosoma, subgenus Trypanozoon) has now been tackled by parasitologists, including Jan Votýpka from the Faculty of Science.

Trypanosoma in a blood smear from a patient afflicted with sleeping sickness. Public domain.

The best-known African trypanosomes are the causative agents of sleeping sickness (human African trypanosomiasis, HAT) – Trypanosoma brucei f. gambiense and Trypanosoma brucei f. rhodesiense vectored by tsetse flies (Glossina). The former causes chronic Western HAT, the latter acute Eastern HAT. Furthermore, tsetse flies transmit Trypanosoma brucei f. brucei. This trypanosome is lysed by a trypanolytic factor in human blood, thus causing no pathology, although it infects cattle, horses, and certain other mammals where it is responsible for a disease called nagana.

Domesticated, as well as wild, mammals also host Trypanosoma brucei f. evansi, a causative agent of surra. The transmission of T. b. f. evansi is not limited to tsetse flies, as it can be mechanically transmitted with blood in the proboscis of other hematophagous insects (e.g., stable flies and tabanids). A broader host spectrum allows this trypanosome to spread beyond the distribution range of tsetse flies; for example, to South America where it may be vectored not only by insects, but also by vampire bats (Desmodus rotundus). Mechanical transmission also occurs in Trypanosoma brucei f. equiperdum which went so far as to lose a vector altogether. In addition, it is transmitted during coitus in horses where it causes a disease known as dourine.

Readers may notice that the scientific names of the various trypanosomes do not correspond to the currently used species (T. evansi or T. gambiense) or subspecies (T. brucei gambiense). The nomenclature used above instead refers to ecotypes (T. brucei f. gambiense, where f. is an abbreviation of ‘form’), as proposed by the authors of the original paper. Ecotypes are groups of individuals of a single species who are adapted to specific conditions and exhibit traits that set them apart from other conspecifics. The change from (sub)species to the concept of an ecotype may seem small and insignificant, but it serves as a paradigm shift – the current view of trypanosome (sub)species indicates the existence of separate and effectively standalone taxonomic units, a misleading view with potentially detrimental implications.

A cladogram (‘pedigree’) of Trypanosoma brucei ecotypes; each ecotype is represented by a different colour. Note the positions of ecotypes on the cladogram – a condition incompatible with the concept of the (sub)species. The image is redrawn from the original paper.

The existence of African trypanosome (sub)species is pragmatically based on different hosts and vectors, resultant pathologies, and the presence or absence of certain genes. The issue that arises is that all of these traits evolved independently and repeatedly in multiple lineages. Moreover, the authors stress the fact that it is possible to create the ‘(sub)species’ in laboratorio, and that these lab-created lineages are genetically similar or even identical to their wild counterparts. Further, the changes leading to the creation of, for example, T. b. f. evansi or T. b. f. equiperdum, are fairly simple – these two ecotypes may arise from T. b. f. brucei by the mere disruption of a single protein. They may also evolve in natural conditions as a reaction to the treatment of animals with trypanocides. Finally, the distinction between ‘(sub)species’ is, in certain instances, blurry at best; for example, there have been reports of diseases with a combination of symptoms of surra/dourine or acute/chronic HAT.

Thus, considering the various polyphyletic lineages of African trypanosomes to be the same (sub)species is about as sensible as considering all red-haired people on earth to be a (sub)species separate from the rest of humanity. The shift to the concept of ecotype maintains the pragmatic view used by human or veterinary medicine while rectifying the issue of the concept of a (sub)species presenting a false hypothesis regarding the relationship between individual lineages.

Kateřina Bezányiová

Lukeš, J., Kachale, A., Votýpka, J., Butenko, A., & Field, M. C. (2022). African trypanosome strategies for conquering new hosts and territories: the end of monophyly? Trends in Parasitology.