There are no earlier data on hantaviruses in Buryatia. The taiga region south of Lake Baikal has acted as a corridor for the palearctic fauna and Buryatian rodents were therefore of special interest in the study of hantavirus evolution in Eurasia. Myodes rufocanus is found throughout the palearctic taiga, and Microtus fortis is at the northwest corner of its distribution range in Buryatia. Samples of both species were found positive in our immunoblotting assay; this suggested a possible involvement of HOKV and KHAV or VLAV. Earlier, HOKV genome sequences were recovered from M. rufocanus captured in Japan, Far-East Russia and China [10, 18]. M. fortis was known as the natural host for KHAV . Our earlier analysis of KHAV and TOPV suggested that a host-switch had occurred in the evolution of these hantaviruses .
There was also a report describing the recovery of partial hantaviral S segment sequence of from M. fortis captured near Vladivostok . In our experiments, RT-PCR followed by sequencing confirmed the presence of HOKV and VLAV in M. rufocanus and M. fortis, respectively. These data support the notion on M. fortis as a natural host for VLAV. Furthermore, our findings of VLAV sequences in M. oeconomus suggest that this rodent species could carry VLAV as well. Notably, both vole species belong to the same subgenus Alexandromys in the genus Microtus, i.e. genetically they are closely related to each other . Of course, spillover of VLAV from its real host (whatever it is) to other sympatric rodent species, cannot be excluded and therefore further investigation of this issue is needed.
Phylogenetic analysis of newly recovered Buryatian hantavirus sequences was complicated by limited datasets available for HOKV and especially VLAV genetic variants. This, in our opinion, was the very reason for the lower than desired resolution (seen as <70% bootstrap support values for a number of branching points). Our previous experience tells that, at least in some cases, an addition of one-two "critical" sequences to the dataset could remarkably improve the phylogenetic resolution [23, 24]. Some improvement could also be achieved by the recovery of longer M segment sequences directly from rodent tissue samples. So far, this presented a real problem for our Buryatian collection. Isolation of HOKV and VLAV in cell culture would, undoubtedly, speed the progress in this direction. Despite these drawbacks, the general phylogeny of HOKV genetic variants looked logical and supportive to the hypothesis of hantavirus-host co-evolution (Fig. 1 and Fig. 2).
Our finding of VLAV sequences in M. oeconomus was a bit surprising and thus added a new twist to the already quite intriguing relationships between TOPV, KHAV, and VLAV. KHAV and VLAV, both carried by Microtus voles, do not cluster on the phylogenetic trees with other hantaviruses carried by Microtus (TULV, PHV, ISLAV, and BLLV) but instead appeared monophyletic with TOPV and the KHAV-VLAV-TOPV trio forms a sister taxon to the Myodes-carried hantaviruses: PUUV, HOKV, and MUJV (Fig. 1). This clustering is in apparent disagreement with strict hantavirus-host co-evolution and would suggest host-switching event(s). One could imagine three possible scenarios. The first scenario includes a host-switch of an ancient hantavirus from Myodes to Lemmus yielding an ancestor for TOPV, KHAV, and VLAV, followed by two independent, and separated in time, host-switching events of a Lemmus-associated virus to Microtus yielding KHAV and VLAV. The second scenario involves a "jump" of an ancient hantavirus from Myodes to Microtus followed by diversification of hosts for KHAV and VLAV and another "jump" of pre-KHAV from Microtus to Lemmus yielding TOPV. The third scenario implies that Microtus-associated hantaviruses are the most ancestral ones among the group carried by Arvicolinae rodents. According to this scenario, in a single host-switching event an ancient Microtus-carried virus gave origin to the ancestor of all Myodes-carried viruses and later pre-KHAV virus "jumped" from Microtus to Lemmus, producing TOPV.
Future studies, incuding analysis of larger sets of TOPV, KHAV and VLAV variants, preferably originated from a wider geographical area and showing substantial genetic diversity, would be needed to evaluate these hypotheses.