Genomic support for speciation and specificity of baculoviruses

Keywords: baculovirus, insects, speciation, genomics, phylogeny, host specificity The Baculoviridae comprise a large family of double-stranded DNA viruses infecting arthropods. In this thesis two baculoviruses, Leucoma salicis nucleopolyhedrovirus (LesaNPV) and Agrotis segetum (Agse) NPV, were characterized in detail. Both viruses are potential biocontrol agents of the insects from which they were isolated. A close genetic relationship between LesaNPV and Orgyia pseudotsugata multiple NPV (OpMNPV) was found. O. pseudotsugata is known from North America and contains two baculoviruses, OpSNPV and OpMNPV. L. salicis is a European insect species that was accidentally introduced in the beginning of the 20th century into North America. Results from the current study suggest that LesaNPV was imported along with L. salicis into North America, where it infected O. pseudotsugata and adapted to this new host in coexistence with OpSNPV. As such, this case provides a snapshot of baculovirus evolution through speciation. The genome sequence of AgseNPV showed a striking co-linearity with Spodoptera exigua (Se) MNPV, although these viruses vary in biological properties such as host specificity. AgseNPV can infect S. exigua orally, but SeMNPV is not infectious for A. segetum larvae. SeMNPV causes a systemic infection in A.segetum only when the midgut barrier is bypassed through injection of the virus into the hemocoel. SeMNPV was able to enter A. segetum midgut epithelial cells and to transcribe its early genes, but was unable to replicate and produce progeny virus in these cells. The AgseNPV / SeMNPV case provides an excellent model to study baculovirus specificity by analyzing the changes in the genome sequence that lead to the differences in host range. The collected data support the view that molecular characterization is essential for proper virus classification and for assessing the phylogenetic relationships with other viruses.