Title: Parasite evolution through the lens of a unique microbial alliance
Short summary:
Biological interactions are a key component in the maintenance of our ecosystems and
nature in general, where all species interact with at least one other species. However,
the effect of these interactions, or symbioses, is not always beneficial to all parties. Take
viruses for example. Whether or not they are considered organisms, viruses are infectious
agents of virtually all life forms on our planet and are well known for their direct impact
on plant, animal and human populations (e.g. the relatively recent outbreaks of SARS-
CoV-2, Mpox and Influenza H5N1). However, there are other viruses that may cause more
complex and indirect effects on populations through the infection of other organisms (e.g.
bacteria or protozoa) that are, in turn, pathogenic to others (e.g. humans). In these
cases, the presence of viruses may complicate the pathogen’s disease dynamics in the
host, leading to more severe disease outcomes, treatment failure or symptomatic relapse.
These types of viruses were able to remain under the radar for a long time, but have been
increasingly detected over the past twenty-five years.
One of the best-established examples of such a virus-parasite interaction is the one
between Leishmania parasites and their double-stranded RNA viruses, Leishmaniavirus
(LRV). Here, LRV and Leishmania appear to have a mutualistic relationship where the
virus facilitates parasite survival through interaction with the host’s immune response –
leading to increased disease progression, exacerbation, and treatment failure – in exchange
for the exploitation of Leishmania-encoded proteins required for viral replication and sur-
vival. Because of these potentially adverse pathological effects, it will be important to
account for the presence of these viruses in the context of the control and elimination
of diseases like leishmaniasis. It is therefore important to obtain a better fundamental
understanding of how such virus-parasite symbioses work, not only from a biological and
immunological perspective, but also from an evolutionary point of view.
The work presented in this thesis was performed to gain a better understanding of the
eco-epidemiology and evolutionary history of Leishmania parasites through the lens of
their unique microbial alliance with LRV. This revealed two novel findings: (i) the com-
pelling evidence of a strong co-evolutionary history between the virus and parasite across
the different taxonomic levels of both Leishmania and LRV (chapters 3 and 5); and (ii)
the multifaceted importance of Leishmania hybridization in the eco-epidemiology of leish-
maniasis and its apparent association with maintaining the genetic diversity of LRV as
well as its distribution (chapters 2, 4 and 5). With this, our work strongly highlights
the need for large-scale and continuous surveillance of pathogens, especially those caus-
ing neglected tropical diseases, and raises the importance of research on symbioses in
general.
Link to the text
You can find the full text and supplementary material here.