the Potexviridae. Evolution The evolution of the picornaviruses based on an analysis of their RNA polymerases and helicases appears to date to the divergence of eukaryotes. Their putative ancestors include the bacterial group II retroelements, the family of HtrA proteases and DNA bacteriophages. Partitiviruses are related to and may have evolved from a totivirus ancestor. Hypoviruses and barnaviruses appear to share an ancestry with the potyvirus and sobemovirus lineages respectively. Double-stranded RNA viruses This analysis also suggests that the dsRNA viruses are not closely related to each other but instead belong to four additional classes—Birnaviridae, Cystoviridae, Partitiviridae, and Reoviridae—and one additional order (Totiviridae) of one of the classes of positive ssRNA viruses in the same subphylum as the positive-strand RNA viruses. One study has suggested that there are two large clades: One includes the families Caliciviridae, Flaviviridae, and Picornaviridae and a second that includes the families Alphatetraviridae, Birnaviridae, Cystoviridae, Nodaviridae, and Permutotretraviridae. Negative strand RNA viruses These viruses have multiple types of genome ranging from a single RNA molecule up to eight segments. Despite their diversity it appears that they may have originated in arthropods and to have diversified from there. Satellite viruses A number of satellite viruses—viruses that require the assistance of another virus to complete their life cycle—are also known. Their taxonomy has yet to be settled. The following four genera have been proposed for positive sense single stranded RNA satellite viruses that infect plants—Albetovirus, Aumaivirus, Papanivirus and Virtovirus. A family—Sarthroviridae which includes the genus Macronovirus—has been proposed for the positive sense single stranded RNA satellite viruses that infect arthropods. Group III – dsRNA viruses There are twelve families and a number of unassigned genera and species recognised in this group. Family Amalgaviridae Family Birnaviridae Family Chrysoviridae Family Cystoviridae Family Endornaviridae Family Hypoviridae Family Megabirnaviridae Family Partitiviridae Family Picobirnaviridae Family Reoviridae – includes Rotavirus Family Totiviridae Family Quadriviridae Genus Botybirnavirus Unassigned species Botrytis porri RNA virus 1 Circulifer tenellus virus 1 Colletotrichum camelliae filamentous virus 1 Cucurbit yellows associated virus Sclerotinia sclerotiorum debilitation-associated virus Spissistilus festinus virus 1 Group IV – positive-sense ssRNA viruses There are three orders and 34 families recognised in this group. In addition, there are a number of unclassified species and genera. Order Nidovirales Family Arteriviridae Family Coronaviridae – includes Human coronavirus (common cold viruses HCoV-229E, HCoV-HKU1, HCoV-NL63, and HCoV-OC43), MERS-CoV, SARS-CoV-1 and SARS-CoV-2 Family Mesoniviridae Family Roniviridae Order Picornavirales Family Dicistroviridae Family Iflaviridae