Caracterização de um baculovírus isolado da lagarta-preta-da-soja Spodoptera cosmioides

Abstract

The Spodoptera genus, Guenée (1852) contains important polyphagous pests of agriculture. In Brazil, the occurrence of S. cosmioides used to be sporadic, although in recent years it has emerged as a primary pest in monocultures such as soybean, corn, and cotton. Here a new baculovirus isolated from S. cosmioides was characterized. The virus was found in extracts of dead black armyworm with signals of viral infection caused by baculovirus and named Spodoptera cosmioides nucleopolyhedrovirus isolated CNPSo-72 (SpcoNPV-CNPSo-72). SpcoNPV-CNPSo-72 occlusion bodies (OB) had a polyhedral shape with several occluded virions and multiple nucleocapsids per envelope. The new virus was lethal to healthy third-instar S. cosmioides larvae with an LC50 of 2.9 x 103 OB/ml, but not to larvae of S. frugiperda at the tested concentrations. The genome of the new virus was sequenced using Roche 454 technology, and the sequencing produce 14,499 reads that were assembled into a single circular contig of 147,763 bp with a G+C content of 44.8%. We annotated 161 ORFs and found only 20 genes as unique in baculovirus and five homologous regions. About 86% of the genome was composed of protein coding sequences, while 13% represent intergenic spaces and homolog regions. We found the 38 currently defined baculoviral core genes in the genome of SpcoNPV-CNPSo-72 and also a set of 26 genes shared between infectious baculoviruses in lepidopterans (members of genera Alphabaculovirus and Betabaculovirus). Interestingly, we found homologs of cathepsin and chitinase, which were much likely to the observed host tegument liquefaction when healthy larvae were exposed to virus OBs, as a post mortem phenotype similar to other viruses that infect members of Spodoptera. The virus also presented a tRNA-coding gene that corresponded to the amino acid arginine (Arg). We carried out a phylogenetic analysis of the new species based on the core genes and found that the new virus is closely related to members of Alphabaculovirus, especially those infectious to Spodoptera. We also performed a gene content comparison SpcoNPV-CNPso-72 and those most closely related and found 241 different genes, with only 119 genes shared among all analyzed species. Moreover, we applied the evolutive model of kimura-2-parameter adjusted to conserved regions of lef-8, lef-9, and polyhedrin and the number of substitutions per site suggested that SpcoNPV could constitute a new species within gens Alphabaculovirus. Baculoviruses are pathogens that lethally infect insect larvae and their study allows for a better understanding of the evolution of large DNA viruses, providing bases for the improvement of viruses as a tool in biological control like the discovery of genes associated with both specificity and host range.