Lacanobia oleracea nucleopolyhedrovirus (LaolNPV): A new European species of alphabaculovirus with a narrow host range

Genomics of alphabaculovirus isolates infecting Mamestra species from North America and Eurasia

Whole genome sequencing and multiplex PCR analysis were used to characterize previously isolated baculovirus isolates from Mamestra populations in Eurasia. Although these viruses have been previously described as Mamestra brassicae nucleopolyhedrovirus (MbNPV/MabrNPV), we demonstrate here that these isolates represent strains of the baculovirus species Alphabaculovirus maconfiguratae (MacoNPV-A) and Alphabaculovirus altermaconfiguratae (MacoNPV-B). The MabrNPV-Bu and -Uk isolates had 96% nucleotide (nt) identity to the type isolate MacoNPV-A 90/2 at the whole genome level and in addition contained a lef-7 homologue which is found in MacoNPV-A but not MacoNPV-B. MabrNPV-Si, -De and -Nl had 96.6, 96.6 and 98.5% nt identity to the type isolate MacoNPV-B 96/2 at the whole genome level, respectively and contained a helicase-2 homologue. Gene content, synteny and K-2-P lef-8, lef-9 and polh analysis also confirmed the presence of both MacoNPV-A and MacoNPV-B isolates in Eurasia. Thus, both these alphabaculovirus species have wide Holarctic distributions in Mamestra host species. MacoNPV-A and MacoNPV-B have wide host ranges and in addition we showed that MacoNPV-B isolates trended to higher infectivity for T. ni larvae.

Evolutionary analysis and biological characterization of a novel alphabaculovirus isolated from Mythimna separata

Baculoviruses are insect-specific pathogens. Novel baculovirus isolates provide new options for the biological control of pests. Therefore, research into the biological characteristics of newly isolated baculoviruses, including accurate classification and nomenclature, is important. In this study, a baculovirus was isolated from Mythimna separata and its complete genome sequence was determined by next-generation sequencing. The double-stranded DNA genome was 153 882 bp in length, encoding 163 open reading frames. The virus was identified as a variant of Mamestra brassicae multiple nucleopolyhedrovirus (MbMNPV) and designated Mamestra brassicae multiple nucleopolyhedrovirus CHN1 (MbMNPV-CHN1) according to ultrastructural analysis, genome comparison and phylogenetic analysis. Phylogenetic inference placed MbMNPV-CHN1 in a clade containing isolates of MacoNPV-A, MacoNPV-B and MbMNPV, which we have designated the Mb-McNPV group. The genomes of isolates in the Mb-McNPV group exhibited a high degree of collinearity with relatively minor differences in the content of annotated open reading frames. The development of codon usage bias in the Mb-McNPV group was affected mainly by natural selection. MbMNPV-CHN1 shows high infectivity against seven species of Lepidoptera. The yield of MbMNPV-CHN1 in the fourth- and fifth-instar M. separata larvae was 6.25×109-1.23×1010 OBs/cadaver. Our data provide insights into the classification, host range and virulence differences among baculoviruses of the Mb-McNPV group, as well as a promising potential new baculoviral insecticide.

Baculovirus Kimura two-parameter species demarcation criterion is confirmed by the distances of 38 core gene nucleotide sequences

Kimura two-parameter nucleotide distance comparisons based on polyhedrin/granulin (polh/gran), late expression factor 8 (lef-8) and late expression factor 9 (lef-9) are a widely applied method for species demarcation for lepidopteran-specific baculoviruses. Baculoviruses are considered to belong to the same species when a pairwise distance threshold of 0.015 is not exceeded and are considered as possibly belonging to the same species with a distance of up to 0.050. In the present work this method was revised and extended for 172 entirely sequenced lepidopteran, hymenopteran and dipteran baculovirus genomes by applying the nucleotide sequences of all 38 known baculovirus core genes for pairwise distance calculations. On the basis of this large dataset, the previously established standard thresholds for baculovirus species demarcation were adjusted for pairwise nucleotide distances estimated from the alignments of all 38 core genes. With the newly applied thresholds for the 38 core-gene dataset, a more sophisticated Kimura two-parameter method was established, avoiding the possible influence of the chimerical polh gene of the Autographa californica multiple nucleopolyhedrovirus. Based on the new dataset, the present classification of baculovirus species was confirmed. Thereby the Kimura two-parameter method for baculovirus demarcation was extended to include the information from all 38 Baculoviridae core genes, which represent the established standard information for baculovirus phylogeny to date.