Genetic Variation and Biological Activity of Two Closely Related Alphabaculoviruses during Serial Passage in Permissive and Semi-Permissive Heterologous Hosts

Phylogenetic analyses suggest that Mamestra brassicae multiple nucleopolyhedrovirus (MbMNPV) and Helicoverpa armigera multiple nucleopolyhedrovirus (HearMNPV) may be strains of the same virus species. Most of the studies comparing their biological activities have been performed in their homologous hosts. A comparison of host range and stability in alternative hosts was performed. The host range of these viruses was compared using high concentrations of inoculum to inoculate second instars of six species of Lepidoptera. One semi-permissive host (Spodoptera littoralis) and one permissive host (S. exigua) were then selected and used to perform six serial passages involving a concentration corresponding to the ~25% lethal concentration for both viruses. Restriction endonuclease analysis showed fragment length polymorphisms in every host-virus system studied. In S. littoralis, serial passage of MbMNPV resulted in decreased pathogenicity and an increase in speed-of-kill, whereas no significant changes were detected for HearMNPV with respect to the initial inoculum. In contrast, both viruses showed a similar trend in S. exigua. These results highlight the low genetic diversity and a high phenotypic stability of HearMNPV with respect to the original inoculum after six successive passages in both insect hosts. This study concludes that host-baculovirus interactions during serial passage are complex and the process of adaptation to a novel semi-permissive host is far from predictable.

Predictability of helminth parasite host range using information on geography, host traits and parasite community structure

SUMMARYHost–parasite associations are complex interactions dependent on aspects of hosts (e.g. traits, phylogeny or coevolutionary history), parasites (e.g. traits and parasite interactions) and geography (e.g. latitude). Predicting the permissive host set or the subset of the host community that a parasite can infect is a central goal of parasite ecology. Here we develop models that accurately predict the permissive host set of 562 helminth parasites in five different parasite taxonomic groups. We developed predictive models using host traits, host taxonomy, geographic covariates, and parasite community composition, finding that models trained on parasite community variables were more accurate than any other covariate group, even though parasite community covariates only captured a quarter of the variance in parasite community composition. This suggests that it is possible to predict the permissive host set for a given parasite, and that parasite community structure is an important predictor, potentially because parasite communities are interacting non-random assemblages.

Fluid Spatial Dynamics of West Nile Virus in the United States: Rapid Spread in a Permissive Host Environment

ABSTRACTThe introduction of West Nile virus (WNV) into North America in 1999 is a classic example of viral emergence in a new environment, with its subsequent dispersion across the continent having a major impact on local bird populations. Despite the importance of this epizootic, the pattern, dynamics, and determinants of WNV spread in its natural hosts remain uncertain. In particular, it is unclear whether the virus encountered major barriers to transmission, or spread in an unconstrained manner, and if specific viral lineages were favored over others indicative of intrinsic differences in fitness. To address these key questions in WNV evolution and ecology, we sequenced the complete genomes of approximately 300 avian isolates sampled across the United States between 2001 and 2012. Phylogenetic analysis revealed a relatively star-like tree structure, indicative of explosive viral spread in the United States, although with some replacement of viral genotypes through time. These data are striking in that viral sequences exhibit relatively limited clustering according to geographic region, particularly for those viruses sampled from birds, and no strong phylogenetic association with well-sampled avian species. The genome sequence data analyzed here also contain relatively little evidence for adaptive evolution, particularly of structural proteins, suggesting that most viral lineages are of similar fitness and that WNV is well adapted to the ecology of mosquito vectors and diverse avian hosts in the United States. In sum, the molecular evolution of WNV in North America depicts a largely unfettered expansion within a permissive host and geographic population with little evidence of major adaptive barriers.IMPORTANCEHow viruses spread in new host and geographic environments is central to understanding the emergence and evolution of novel infectious diseases and for predicting their likely impact. The emergence of the vector-borne West Nile virus (WNV) in North America in 1999 represents a classic example of this process. Using approximately 300 new viral genomes sampled from wild birds, we show that WNV experienced an explosive spread with little geographical or host constraints within birds and relatively low levels of adaptive evolution. From its introduction into the state of New York, WNV spread across the United States, reaching California and Florida within 4 years, a migration that is clearly reflected in our genomic sequence data, and with a general absence of distinct geographical clusters of bird viruses. However, some geographically distinct viral lineages were found to circulate in mosquitoes, likely reflecting their limited long-distance movement compared to avian species.