Biological activity and genome composition of a Tunisian isolate of Spodoptera littoralis nucleopolyhedrovirus (SpliNPV-Tun2)

Background: The baculovirus Spodoptera littoralis nucleopolyhedrovirus (SpliNPV) is an entomopathogenic virus
utilized as a biological control agent of the Egyptian cotton leaf worm, Spodoptera littoralis. Several studies have
focused on the identification of different SpliNPV isolates from a biological and molecular point of view, but few of
them conducted in-depth analyses of the genomic composition of these isolates.
Results: Identification of a novel isolate of SpliNPV, termed Tun2, which was purified from infected S. littoralis larvae
from Tunisia was reported. This isolate was propagated in vivo and its median lethal concentration (
LC50) was determined
to be 1.5 × 104
occlusion bodies (OBs)/ml for third instar S. littoralis larvae at 7 days of post-infection. OB
production in late fourth instar larvae was estimated to be at least 2.7 × 109
OBs/g larval weight. The completely
sequenced genome of SpliNPV-Tun2 was 137,099 bp in length and contained 132 open reading frames (ORF).
It showed a 98.2% nucleotide identity to the Egyptian isolate SpliMNPV-AN1956, with some striking differences;
between both genomes, insertion and deletion mutations were noticed in 9 baculovirus core genes, and also in the
highly conserved polyhedrin gene. The homologs of ORF 106 and ORF 107 of SpliNPV-AN1956 appeared to be fused
to a single ORF 106 in SpliNPV-Tun2, similar to the homologous ORF 110 in SpltNPV-G2.
Conclusion: SpliNPV-Tun2 is proposed as a new variant of SpliNPV and a potential candidate for further evaluation as
a biocontrol agent for S. littoralis and probably other Spodoptera species.