**2. Two types of virus progeny serve at different steps of host invasion**

Alphabaculoviruses and betabaculoviruses produce virions of two phenotypic classes: occlusion-derived virus (ODV) and budded-virus (BV). Nucleocapsids of both types of virus particles are assembled in the nucleus of the infected cell. During the late phase of infection, when proteins of the nucleocapsid are expressed, BVs are produced as the newly formed nucleocapsids exit the cell, acquiring their envelopes from the cell membrane during the budding process. BVs disseminate the infection within the host by entering other cells via a mechanism of receptor-mediated endocytosis. ODVs arise at very late times, when nucleocapsids are enveloped in membrane units derived from the nuclear envelope to finally be embedded in the OBs. OBs persist in the environment after liquefaction of the insect cadaver and are responsible for the horizontal transmission of the virus between hosts. Upon ingestion by a susceptible insect larva, OBs are dissolved in the alkaline environment of the midgut and ODVs are released. The ODVs move through the peritrophic membrane and nucleocapsids are delivered into midgut epithelial cells through a mechanism of membrane fusion mediated by specific viral proteins known as *per os* infectivity factors (PIFs). This primary infection in the midgut is followed by a secondary infection, consisting in the dissemination of BVs to other tissues. In contrast to this infection cycle, which is typical of lepidopteran-specific baculoviruses, infections of gammabaculovi‐ ruses and deltabaculoviruses are restricted to the midgut of their hosts.

## **3. Nuclear events associated with infection**

Baculovirus infection causes cells to enlarge and stop dividing; the nucleus swells and forms the virogenic stroma (VS), which is the nuclear compartment where the viral DNA is replicated and the nucleocapsids of virus progeny are assembled. The host chromatin adopts a marginal distribution at 24 hours post infection (hpi), when the VS becomes evident. At this time, it was shown that histone H4 fused to fluorescent protein markers colocalizes with the chromatin in the periphery but not with the VS [5]. The new distribu‐ tion of the chromatin is determined by the replication of the virus and may have effects in changes operated in the expression of host genes and the progression of the cell cycle. At late times, ODVs become occluded into OBs in the periphery of the VS.

Apparently, the nucleocapsids of NPVs enter the nucleus through the nuclear pores, whereas the genome of GVs is probably injected [6]. Virions of baculoviruses are devoid of histones, in turn the DNA in the nucleocapsid is packed in association with viral protein P6.9, a basic DNA-binding protein. This small polypeptide is rich in arginine, serine and threonine residues, a feature similar to proteins called protamines present in the nuclei of spermatids in many animals and plants. The positive arginine residues in protamines neutralize the negative charges in the DNA backbone while serine and threonine medi‐ ate interaction between protamine molecules, resulting in a high condensation of genom‐ ic DNA. Once it is uncoated into the nucleus, the DNA dissociates from P6.9 through the phosphorylation of the protein. During infection the viral DNA appears to be organized in the form of nucleosomal-like structures in association with P6.9, as suggested by experiments of micrococcal nuclease digestion of isolated nuclei [7].
