New arguments in the discussion about the nature of picobirnaviruses

Picobirnaviruses (PBVs), members of the Picobirnaviridae family, are found in a wide range of hosts, including eukaryotes (both higher and lower), fungi, and bacteria. However, scientists are unsure about their “true master” or primary host. While often found in animals, including cases of gastroenteritis, they are also detected in environmental samples and have shown genetic links to bacterial and fungal viruses. The lack of a reliable cell culture or animal model for PBV propagation further complicates determining their host specificity. Due to the discovery of prokaryotic regions (motifs) in segments of the PBV genome, it was suggested that their hosts are prokaryotic. However, even this discovery did not pin one specific host to PBVs; since then PBV-like genomes not characteristic of the studied PBV strains, with a mitochondrial genetic code characteristic of lower eukaryotes (molds and invertebrates), were discovered. And recently, a new version of the origin of PBVs from vertebrate viruses and fungi has appeared, denying their phage nature. To understand the nature of genetically diverse PBV strains detected in different organisms, researchers were guided by information about the presence of motifs specific to the viral family in the genome, the genetic code used, and the method of distribution. Recent research suggests that PBVs, previously thought to have a vertebrate origin, may have also evolved from fungal sources denying their phage nature. Some PBV-like sequences have been found to utilize the fungal mitochondrial genetic code, indicating a possible fungal origin or a close relationship with fungal viruses like mitoviruses. This discovery challenges the previously held view of PBVs as exclusively vertebrate viruses and suggests a more complex evolutionary history. The information available today inspires confidence in the imminent conclusion of the ongoing discussion about the possible PBV hosts. In particular, a hypothesis has recently emerged demonstrating a possible mechanism for the replacement of the genetic code in RNA viruses, which makes it possible to explain the origin of PBV forms with the mitochondrial genetic code capable of reproduction in cells of lower eukaryotes using the example of phages. However, an evolutionarily deterministic model demonstrating the path of PBV formation with the genetic code of mold and invertebrate cells has not yet been presented. According to the authors of this review, this evolutionary path is due to the endosymbiotic relationships between the putative PBV hosts, contributing to the horizontal virus spread. The purpose of this review article is to attempt to describe a possible path of formation from the ancestral PBV form and its derived evolutionary forms, some of which inherited a genome with a prokaryotic motif and a standard genetic code, while others acquired a non-standard form of the genome with the code of lower eukaryotes. This review article focuses on the leading role of horizontal transmission in the formation of non-standard intermediate PBV forms.

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