Mycovirus

Many double-stranded RNA elements that have been described in fungi do not fit this description, and in these cases they are referred to as virus-like particles or VLPs.

However, this system is only being used in Europe routinely because of the relatively small number of vegetative compatibility groups (VCGs) on the continent.

[13] The lack of genomic data often hampers a conclusive assignment to already established groups of viruses or makes it impossible to erect new families and genera.

The latter is true for many unencapsidated dsRNA viruses, which are assumed to be viral, but missing sequence data has prevented their classification so far.

[14][15] There are also viruses that simply use fungi as vectors and are distinct from mycoviruses because they cannot reproduce in the fungal cytoplasm.

[13][24] It has also been suggested that it is very likely that plant viruses containing a movement protein evolved from mycoviruses by introducing an extracellular phase into their life cycle rather than eliminating it.

Furthermore, the recent discovery of an ssDNA mycovirus has tempted some researchers[10] to suggest that RNA and DNA viruses might have common evolutionary mechanisms.

This "plant virus hypothesis" may not explain how mycoviruses developed originally, but it could help to understand how they evolved further.

[citation needed] A significant difference between the genomes of mycoviruses to other viruses is the absence of genes for ‘cell-to-cell movement’ proteins.

It is therefore assumed that mycoviruses only move intercellularly during cell division (e.g. sporogenesis) or via hyphal fusion.

[14][26] Mycoviruses might simply not need an external route of infection as they have many means of transmission and spread due to their fungal host's life style: However, there are potential barriers to mycovirus spread due to vegetative incompatibility and variable transmission to sexual spores.

However, it is not known how fungi overcome the genetic barrier; whether there is some form of recognition process during physical contact or some other means of exchange, such as vectors.

Although it is not known yet whether viral transport is an active or passive process, it is generally assumed that fungal viruses move forward by plasma streaming.

However, some researchers have found them located next to septum walls,[2][33] which could imply that they ‘got stuck’ and were not able to move actively forward themselves.

Others have suggested that the transmission of viral mitochondrial dsRNA may play an important role in the movement of mitoviruses found in Botrytis cinerea.

[24][45] A three-part system involving a mycovirus of an endophytic fungus (Curvularia protuberata) of the grass Dichanthelium lanuginosum has been described, which provides a thermal tolerance to the plant, enabling it to inhabit adverse environmental niches.

[47] In medically important fungi, an uncharacterized A78 virus of A. fumigatus causes mild hypervirulent effect on pathogenicity when tested on Galleria mellonella (Greater wax moth).

[21] These could imply mycoviruses may play important roles in the pathogensis of human pathogenic fungi.