Date Published: January 19, 2017
Publisher: Public Library of Science
Author(s): Robert L. Harrison, Daniel L. Rowley, Joseph Mowery, Gary R. Bauchan, David A. Theilmann, George F. Rohrmann, Martin A. Erlandson, Xiu-Qing Li.
The betabaculovirus originally called Pseudaletia (Mythimna) sp. granulovirus #8 (MyspGV#8) was examined by electron microscopy, host barcoding PCR, and determination of the nucleotide sequence of its genome. Scanning and transmission electron microscopy revealed that the occlusion bodies of MyspGV#8 possessed the characteristic size range and morphology of betabaculovirus granules. Barcoding PCR using cytochrome oxidase I primers with DNA from the MyspGV#8 collection sample confirmed that it had been isolated from the true armyworm, Mythimna unipuncta (Lepidoptera: Noctuidae) and therefore was renamed MyunGV#8. The MyunGV#8 genome was found to be 144,673 bp in size with a nucleotide distribution of 49.9% G+C, which was significantly smaller and more GC-rich than the genome of Pseudaletia unipuncta granulovirus H (PsunGV-H), another M. unipuncta betabaculovirus. A phylogeny based on concatenated baculovirus core gene amino acid sequence alignments placed MyunGV#8 in clade a of genus Betabaculovirus. Kimura-2-parameter nucleotide distances suggested that MyunGV#8 represents a virus species different and distinct from other species of Betabaculovirus. Among the 153 ORFs annotated in the MyunGV#8 genome, four ORFs appeared to have been obtained from or donated to the alphabaculovirus lineage represented by Leucania separata nucleopolyhedrovirus AH1 (LeseNPV-AH1) during co-infection of Mythimna sp. larvae. A set of 33 ORFs was identified that appears only in other clade a betabaculovirus isolates. This clade a-specific set includes an ORF that encodes a polypeptide sequence containing a CIDE_N domain, which is found in caspase-activated DNAse/DNA fragmentation factor (CAD/DFF) proteins. CAD/DFF proteins are involved in digesting DNA during apoptosis.
Viruses of family Baculoviridae possess a double-stranded circular DNA genome that is packaged into enveloped, rod-shaped capsids . These viruses, which have been isolated exclusively from insects, have been shown to produce two virion forms, budded virus (BV) and occlusion-derived virus (ODV) . BVs, which are produced first, acquire an envelope as nucleocapsids bud through the host cell plasma membrane. At later times during replication, nucleocapsids of genera Alphabaculovirus, Gammabaculovirus, and Deltabaculovirus acquire an envelope within the host cell nucleus to form the occlusion-derived virus (ODV). ODVs of genus Betabaculovirus obtain their envelope in a nucleo-cytoplasmic milieu formed after breakdown of the host nuclear envelope. ODVs are subsequently assembled into occlusion bodies consisting largely of single, highly expressed viral protein, known as polyhedrin (alpha-, delta- and gammabaculoviruses) or granulin (betabaculoviruses).
The MyunGV#8 genome sequence reported here highlights how the analysis of baculovirus genomes continues to reveal details about the relationships, history and divergence of these viruses. The MyunGV#8 genome disclosed a pattern pointing to an exchange of genetic material between viruses of lineages represented by MyunGV#8 and LeseNPV-AH1, or their recent ancestors. The LeseNPV-AH1 virus was originally isolated from the Oriental armyworm, Mythimna (Leucania) separata. M. separata and M. unipuncta are closely related, although populations of these species do not share the same geographic range. Larvae of M. unipuncta and M. separata, or a common ancestor or related species, may have served as hosts to viruses of both lineages, with co-infection of larvae occurring to an extent that four ORFs were exchanged between isolates of these lineages. A similar exchange of genes between a distantly related alphabaculovirus and a betabaculovirus has been observed with Mamestra configurata nucleopolyhedrovirus-B (MacoNPV-B) and a XecnGV-related betabaculovirus .