Bidensovirus

Bidensovirus is a genus of single stranded DNA viruses that infect invertebrates. The species in this genus were originally classified in the family "Parvoviridae" (subfamily "Densovirinae") but were moved to a new genus because of significant differences in 
the genomes.
Taxonomy.
There is one species in this genus currently recognised: "Bombyx mori bidensovirus".
Host.
As the name suggests this virus infects "Bombyx mori", the silkworm.
Virology.
The virions are icosahedral, non enveloped and ~25 nanometers in diameter. They contain two structural proteins.
The genome is bipartite, unique among ssDNA viruses, with two linear segments of ~6 and 6.5 kilobases (kb). These segments and the complementary strands are that are packaged separately giving rise to 4 different types of full particles.
Both segments have an ambisense organization, coding for a structural protein in one sense and the non-structural proteins on the complementary strand.
The open reading frame 4 (VD1-ORF4) is 3318 nucleotides (bases) in length and encodes a predicted () 1105 amino acid protein which has a conserved DNA polymerase motif. It appears to encode at least 2 other proteins including one of ~53 kDa that forms part of the virion.
Evolution.
Comprehensive analysis of bidnavirus genes has shown that these viruses have evolved from a parvovirus ancestor from which they inherit a jelly-roll capsid protein and a superfamily 3 helicase. It has been further suggested that the key event that led to the separation of the bidnaviruses from parvoviruses was the acquisition of the PolB gene. A likely scenario has been proposed under which the ancestral parvovirus genome was integrated into a large virus-derived DNA transposon of the Polinton/Maverick family (polintoviruses) resulting in the acquisition of the polintovirus PolB gene along with terminal inverted repeats. Bidnavirus genes for a minor structural protein (putative receptor-binding protein) and a potential novel antiviral defense modulator were derived from dsRNA viruses ("Reoviridae") and dsDNA viruses ("Baculoviridae"), respectively.