Evolutionary relationships between arenaviruses and their rodent hosts
16 June 2015
UAntwerp, Campus Middelheim, Room A.143 - Middelheimlaan 1 - 2020 Antwerp
Organization / co-organization:
Department of Biology
Herwig Leirs & Joëlle Goüy de Bellocq
PhD defence Sophie Gryseels - Faculty of Science, Department of Biology
Arenaviruses are RNA viruses that have predominantly been detected in rodents. Some of them are occasionally transmitted to humans in whom they may cause severe diseases. In Africa, 24 arenaviruses (of which five newly described in this thesis) are known to occur in restricted geographic areas, and this may have profound consequences: human haemorrhagic fever cases caused by the arenavirus Lassa (LASV) only occur in West Africa, because its reservoir host, the multimammate mouse Mastomys natalensis, only carries this virus in this region, while carrying divergent, non-pathogenic arenaviruses in the rest of its pan-African range.
For all arenaviruses, including LASV, their persistence relies solely on the availability of the reservoir rodent host(s). Therefore, the key to understanding their restricted distribution ranges as well as their potential to switch between host taxa lies in the evolutionary relationship they have with their natural hosts. In this thesis we study the arenavirus-host relationship at different phylogenetic levels as reflections of distinct evolutionary timeframes.
In chapters 2 to 5 intraspecific host phylogenetic levels are explored, using Tanzanian Mastomys natalensis and its arenaviruses as a model system. Firstly, an analysis of fine-scale population genetics of M. natalensis in the Morogoro Region shows that this generalist rodent has recently diverged into distinct genetic units, likely due to the rise of an urban site amidst the long-standing rural environment. Because this genetic divergence did not entail contact-limiting allopatry, the genetic structure of Morogoro virus (MORV) -the arenavirus that is carried by these rodent populations- followed an independent spatial distribution.
Across a larger spatial scale in our study area, M. natalensis was structured into two divergent phylogenetic lineages (B-IV and B-V). In the B-IV lineage we discovered a novel arenavirus species, Gairo virus (GAIV). By sampling M. natalensis in detail across the contact zone of lineages B-IV and B-V, we showed that a gene-flow-reducing permeable species barrier spatially separates the lineages, likely maintained by counter-selection on lineage hybrids. The same host hybrid zone, and not a contact-limiting environmental barrier, formed the borders of both GAIV’s and MORV’s distributions, indicating that these intraspecific taxa and not the entire M. natalensis species represent the reservoirs for these arenaviruses.
Next we studied the evolutionary history of currently known African arenaviruses, which each have so far been detected solely in a single rodent species. By comparing the phylogenies of the arenaviruses with those of their rodent hosts, it was clear that in the past several host switching events have occurred. However, these shifts were strongly constrained between members of the same rodent tribe (a taxonomic order between genus and family) while overlap of distribution ranges could not significantly explain these shifts.