Monodnaviria

Viruses in Monodnaviria appear to have come into existence independently multiple times from circular bacterial and archaeal plasmids that encode the HUH endonuclease.

Members of Monodnaviria are also known to frequently become integrated into the DNA of their hosts, and they experience a relatively high rate of genetic mutations and recombinations.

The 3'-end of the nicked strand remains as a free hydroxyl (-OH) end that acts as a signal for the host DNA polymerase to replicate the genome.

[8][9] Apart from the aforementioned replication methods, ssDNA viruses in Monodnaviria share a number of other common characteristics.

The sole exception are viruses in the family Anelloviridae, proposed members of Monodnaviria, which have a negative-sense genome.

In any case, ssDNA viruses have their genomes converted to a dsDNA form prior to transcription, which creates the messenger RNA (mRNA) needed to produce viral proteins from translation by ribosomes.

[1][3][4][10] Lastly, ssDNA viruses have a relatively high rate of genetic recombinations and substitution mutations.

Genetic recombination, or mixture, of ssDNA genomes can occur between closely related viruses when a gene is replicated and transcribed at the same time, which may cause the host cell's DNA polymerases to switch DNA templates (negative strands) during the process, which causes recombination.

[3] The high substitution rate seen in ssDNA viruses is unusual since replication is performed primarily by the host cell's DNA polymerase, which contains proofreading mechanisms to prevent mutations.

[3] Comparison of genomes and phylogenetic analyses of the HUH endonucleases, superfamily 3 helicases (S3H), and capsid proteins of viruses in Monodnaviria have shown that they have multiple, chimeric origins.

HUH endonucleases of CRESS-DNA viruses are most similar to those found in small, RCR bacterial and archaeal plasmids, extra-chromosomal DNA molecules inside bacteria and archaea, and appear to have evolved from them at least three times.

CRESS-DNA viruses therefore represent a notable instance of convergent evolution, whereby organisms that are not directly related evolve the same or similar traits.

Plant viruses in the families Geminiviridae and Nanoviridae infect economically important crops and cause significant damage to agricultural productivity.

Bacilladnaviruses, which primarily infect diatoms, are thought to have a significant role in controlling algal blooms.

Endogenization can occur through several means such as the integrase or transpose enzymes or by exploiting the host cell's recombination machinery.

The poem describes a yellowing or vein clearing disease of Eupatorium plants that was likely caused by a geminivirus.

Centuries later, a circovirus infection that caused balding in birds was observed in Australia in 1888, which marked the first reference to ssDNA viruses in modern times.

[1][3][12] In recent years, analyses of viral DNA in various contexts such as fecal matter and marine sediments have shown that ssDNA viruses are widespread throughout nature, and the increased knowledge of their diversity has helped to greater understand their evolutionary history.

A diagram of HUH endonuclease binding to single-stranded DNA.
HUH endonuclease binding to single-stranded DNA.
A plant infected with Abutilon mosaic virus.
A plant infected with Abutilon mosaic virus .