Four genera of moles within the subfamily Scalopinae, namely, Scalopus
(eastern mole), Condylura
(star-nosed mole), Parascalops
(hairy-tailed mole), and Scapanus
(western North American mole), and one genus in the subfamily Talpinae, namely, Neurotrichus
(American shrew mole), are found in the United States. Previously, we reported evidence for host switching during the evolution of a hantavirus hosted by the American shrew mole (23
). We now report a previously unrecognized, distinctly “rodent-like” hantavirus in the eastern mole, the most widely distributed mole species in North America (57
At least 16 subspecies of Scalopus aquaticus
are currently recognized (17
), but the phylogeographic variation in this species has not been assessed. Due to inadequate sequence coverage for the eastern mole, it was impossible to establish the subspecies of the RKPV-infected eastern moles in this study. However, it is likely that they are of the subspecies texanus
. To what extent other subspecies or geographic variants of Scalopus aquaticus
harbor genetic variants of RKPV or entirely different hantaviruses requires further investigation. However, the detection of RKPV in eastern moles only from Rockport, TX, and the failure to detect RKPV in eastern moles from Florida, Kansas, South Carolina, and Tennessee raise interesting possibilities. RKPV in the eastern mole may simply represent spillover, with the eastern mole serving as a secondary host to an as-yet-unidentified present-day rodent reservoir host. Based on the basal position of RKPV in the phylogenetic trees, however, it is more likely that RKPV represents a bona fide
mole-borne hantavirus resulting from cross-species transmission in the past, with subsequent host-specific divergence. The focal finding of RKPV in Texas provides the basis for detailed investigations on the transmission of present-day hantaviruses in phylogenetically diverse but distinct small-mammal communities.
Male eastern moles are generally solitary, although they may share burrows or tunnels with other moles in areas where their home ranges overlap (16
). However, their generally low population density and a subterranean existence, compared to the high population density and above-ground existence of sympatric rodent species, presumably offer limited opportunities for direct contact. Nevertheless, cross-species transmission of hantaviruses might occur through infectious secretions and excretions. Our previous studies of ASAV (3
) and OXBV (23
), two shrew mole-borne hantaviruses, indicate probable host switching with soricine shrews. Similarly, the polyphyletic relationship of RKPV and rodent-borne hantaviruses is suggestive of a host-switching event deep in the evolutionary history of these clades. Three of the four hantaviruses described from the family Talpidae (ASAV, OXBV, NVAV, and RKPV) have discordant coevolutionary relationships. The role of this unique host group in the evolution of hantaviruses, as a source or sink for host switching requires further investigation.
Consistently with recent molecular phylogenetic studies (3
), our findings confirm that moles serve as hosts of hantaviruses. RKPV in the eastern mole is a genetically distinct hantavirus species by virtue of amino acid sequence differences of 20.8% and 36.9% for the NP and Gn/Gc glycoprotein, respectively, which satisfies the criteria set forth by the International Committee for Taxonomy of Viruses (11
). New criteria, based on an exhaustive analysis of hantavirus genomes, have been reported for the demarcation of hantavirus into species (amino acid distance of >10% for S or >12% for M) and into groups (amino acid distance of >24% for S or >32% for M) (32
). Based on the these guidelines, RKPV and cricetid-rodent-borne hantaviruses belong to the same group, with S segment amino acid distances of 22.9% for SNV, 20.8% for ANDV, 27.8% for PUUV, 25.2% for PHV, and 22.7% for TULV. This grouping conformed to the results of our phylogenetic analysis.
Apart from the fact that RKPV represents the first example of a hantavirus harbored by a New World mole in the subfamily Scalopinae, the phylogenetic analyses further expand conventional thinking about the complex evolutionary history of hantaviruses. The emerging conceptual framework indicates multiple independent host-switching events through deep evolutionary time, or across deep divergences, followed by local host-specific adaptation and establishment of parallel enzootic cycles. Moreover, the collective data suggest that soricomorph-borne hantaviruses are somewhat more catholic in their host range than present-day rodent-borne hantaviruses, suggesting that ancestral shrews or moles may have served as the early hosts of primordial hantaviruses.
The published literature consists of only a few articles estimating the age of hantaviruses. For example, based on the rates of nucleotide substitutions per site per year for the SNV M and S segments, Black and colleagues concluded that SNV evolved within the past 37 to 106 years (5
Using a mean rate of 4.245 × 10−4
substitutions per site per year, calculated for hantaviruses by Ramsden and coworkers (40
), we estimated that RKPV, ANDV, and SNV shared a common ancestor 900 years before present (±233 years; 95% highest posterior density [HPD]) based on the S segment maximum clade credibility tree. On the other hand, by using a mean rate of 3.62 × 10−6
substitutions per site per year, derived from the work of Hughes and Friedman (20
) and Sironen and coworkers (45
), RKPV, ANDV, and SNV were shown to have last shared a common ancestor 106,449 years ago (±26,786 years; 95% HPD). Such age estimates, however, are biologically implausible, because they fail to explain how hantaviruses can be found in myriad species within two phylogenetically disparate orders of small mammals that have evolved in widely separated geographic regions across five continents over millions of years.
Although Ramsden and coworkers demonstrated that the divergence dates of hantaviruses were more recent than those of their hosts (40
), divergence dates between viruses and hosts sometimes fail to coincide, mainly because RNA viruses evolve so rapidly that the signal is lost (overwhelmed by noise due to error-prone RdRP) long before time scales over which the host diverged are reached. Holmes (18
) has argued that evolution in RNA viruses becomes incalculable with respect to rates and timing due to saturation of changes (homoplasy) after 50,000 years (e.g., for divergences of >50,000 years before present).
Because the sequence database of hantaviruses from shrews, moles, and other soricomorphs remains incomplete, it is premature to definitively conclude that recent host-switching events coupled with subsequent divergence are singularly responsible for the similarities between the phylogenies of hantaviruses and their mammalian reservoir hosts. The issue is not whether the evolution of hantaviruses is a direct consequence of either host switching or cophylogeny. Rather, both mechanisms apparently influenced the evolution of hantaviruses. That is, when viewed within the context of molecular phylogeny and zoogeography, the close association between distinct hantavirus clades and specific subfamilies of rodents, shrews, and moles is likely the result of alternating and periodic codivergence through deep evolutionary time. By more fully exploring the vast genetic diversity and phylogenetic divergence of present-day hantavirus species (including as-yet-unidentified soricid- and talpid-borne hantaviruses), the temporal and spatial scales for these events in this fascinating host/pathogen system will become more clear.