Functional characterization of Microplitis demolitor bracovirus genes that encode nucleocapsid components

Abstract

Bracoviruses (BVs) are endogenized nudiviruses in parasitoid wasps of the microgastroid complex (order Hymenoptera: Family Braconidae). BVs produce replication-defective virions that adult female wasps use to transfer DNAs encoding virulence genes to parasitized hosts. Some BV genes are shared with nudiviruses and baculoviruses with studies of the latter providing insights on function, whereas other genes are only known from nudiviruses or other BVs which provide no functional insights. A proteomic analysis of Microplitis demolitor bracovirus (MdBV) virions recently identified 16 genes encoding nucleocapsid components. In this study, we further characterized most of these genes. Some nucleocapsid genes exhibited early or intermediate expression profiles, while others exhibited late expression profiles. RNA interference (RNAi) assays together with transmission electron microscopy indicated vp39 , HzNVorf9-like2 , HzNVorf93-like , HzNVorf106-like , HzNVorf118-like , and 27b are required to produce capsids with a normal barrel-shaped morphology. RNAi knockdown of vlf-1a , vlf-1b-1 , vlf-1b-2 , int-1, and p6.9-1 did not alter the formation of barrel-shaped capsids but each reduced processing of amplified proviral segments and DNA packaging as evidenced by the formation of electron translucent capsids. All of the genes required for normal capsid assembly were also required for proviral segment processing and DNA packaging. Collectively, our results deorphanize several BV genes with previously unknown roles in virion morphogenesis. IMPORTANCE Understanding how bracoviruses (BVs) function in wasps is of broad interest in the study of virus evolution. This study characterizes most of the Microplitis demolitor bracovirus (MdBV) genes whose products are nucleocapsid components. Results indicate several genes unknown outside of nudiviruses and BVs are essential for normal capsid assembly. Results also indicate most MdBV tyrosine recombinase family members and the DNA binding protein p6.9-1 are required for DNA processing and packaging into nucleocapsids.

Identification of a viral gene essential for the genome replication of a domesticated endogenous virus in ichneumonid parasitoid wasps

“…didymator pupal stages 1 and 3 further shows that all HdIV genome components are concurrently amplified in calyx cells while our qPCR data indicate amplification begins between pupal stage 1 and stage 2 and reaches its maximum in stage 4. This temporal pattern is overall similar to what has previously been shown to occur for BVs [ 27 , 35 ]. In the braconid wasp Chelonus inanitus , DNA content of calyx cells also increases through endoreduplication [ 36 ], which we have not yet examined in H .…”

“…Similarly, the initiation time of amplification during pupal development and IV virion production remains unknown. The specific role of IV core genes in virion production is also poorly documented when compared to BVs [ 26 , 27 ]. The limited sequence homology of IVSPER genes with genes in other viruses provides minimal insights into potential functions.…”

Identification of a viral gene essential for the genome replication of a domesticated endogenous virus in ichneumonid parasitoid wasps

“…Functional studies, guided in part by these similarities, have provided insights into several key processes underlying BV virion production. Identification of BV core genes that regulate the expression of other BV core genes encoding structural proteins [25], are involved in BV virion formation [25, 26, 35], or are required for processing proviral segments into circular DNA molecules packaged into capsids [25, 26] have been documented. In contrast, identifying the components of IV genomes and functions of IV genes regulating replication is more challenging because the hypothesized NCLDV ancestor is unknown.…”

“…However, all of these concatemers are similarly processed into circular DNAs by recombination at a precise site within DRs, which is a tetramer conserved in all BV segments [47, 48]. Nudiviral genes encoding tyrosine recombinases are further known to mediate this homologous recombination event [25, 26]. These types of molecules could also be present in IV genomes and need to be discovered.…”

“…Similarly, the initiation time of amplification during pupal development and IV virion production remains unknown. The specific role of IV core genes in virion production is also poorly documented when compared to BVs [25, 26]. The limited sequence homology of IVSPER genes with genes in other viruses provides minimal insights into potential functions.…”