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to 12 depending on the genus, 19–32 kilobasepairs (kbp) in total. (Jansen van Vuren et al., 2016). When comparing the amino acid
The family is divided into two subfamilies: Subfamily Spinareo- sequence of more divergent outer capsid proteins, > 55% identity
virinae [with virions having 12 ‘spikes’ (turrets) protruding from indicates one species and < 35% indicates distinct species (Jansen
the core through the outer capsid, hence ‘turreted’ viruses] and van Vuren et al., 2016). Nucleic acid sequence identity of homol-
Subfamily Sedoreovirinae (with virions having a smooth unspiked ogous segments of > 75% indicates same species and < 60% a new
appearance and three capsid shells, hence ‘non-turreted’ viruses) species (Jansen van Vuren et al., 2016). The genus consists of five
(Attoui et al., 2012). In addition to this structural classification, formally recognized species: Avian orthoreovirus (ARV), Baboon
amino acid identities of > 30% in the RNA-dependent RNA orthoreovirus (BRV), Mammalian orthoreovirus (MRV), Nelson
polymerase (RdRp) sequence and the presence and sequence of Bay orthoreovirus (NRV) and Reptilian orthoreovirus (RRV)
conserved terminal nucleotide motifs (Antczak et al., 1982) have (Attoui et al., 2012). At least three new Orthoreovirus species
been used as criteria for the classification of genera within this have been proposed: Broome orthoreovirus (BroV), isolated from
family (Auguste et al., 2015). an Australian fruit bat (Thalmann et al., 2010), Mahlapitsi virus
The family Reoviridae is the largest and the best studied of (MAHLV), isolated from bat flies associated with Egyptian fruit
all dsRNA virus families (Mertens 2004). Notwithstanding the bats (Rousettus aegyptiacus) collected in South Africa (Jansen van
‘orphan’ connotation, the ability to cause disease is variable in the Vuren et al., 2016), and the highly divergent novel orthoreoviruses
different genera in this family, and in some cases, it depends on isolated from a Steller sea lion in Canada (Palacios et al., 2011), a
the virus strain and host species (Kibenge and Godoy, 2016). psittacine bird in Germany (de Kloet, 2008), and a brown-eared
To date, the 15 genera in family Reoviridae include seven bulbul in Japan (Ogasawara et al., 2015) that have been placed in a
genera containing animal viruses (Orthoreovirus, Aquareovirus, new recently proposed species Wild bird orthoreovirus (Ogasawara
Coltivirus, Orbivirus, Rotavirus, Seadornavirus, Cardoreovirus), et al., 2015). Additional highly divergent novel orthoreovirus iso-
three genera containing plant viruses (transmitted by arthropods: lates from corvids, Tvärminne avian virus (TVAV) (Huhtamo et
Oryzavirus, Fijivirus, Phytoreovirus), three genera containing al., 2007; Dandár et al., 2014) and American crow orthoreovirus
insect viruses (Cypovirus, Dinovernavirus, Idnoreovirus), one genus (Kalupahana, 2017), may also represent a separate new species
of fungus virus (Mycoreovirus), and one genus of marine protist within the genus Orthoreovirus.
(Mimoreovirus). Recently discovered highly divergent novel fish The Orthoreovirus genus is divided into non-fusogenic (MRV)
reoviruses piscine orthoreovirus (PRV) from Atlantic salmon and fusogenic orthoreoviruses (for example ARV, BRV, NRV,
(Palacios et al., 2011; Kibenge et al., 2013), coho salmon (Godoy RRV, BroV) (Duncan, 1999; Thalmann et al., 2010) by posses-
et al., 2016; Takano et al., 2016; Bohle et al., 2018), and rainbow sion of a fusion-associated small transmembrane (FAST) protein
trout (Olsen et al., 2015; Godoy et al., 2016; Dhamotharan et al., (also referred to as p10, p13, p14, p15, p16 or p18 in the different
2018), and largemouth bass reovirus from wild Largemouth bass reoviruses) (Shmulevitz and Duncan, 2000; Jansen van Vuren et
(Sibley et al., 2016) with 10 genome segments and with close al., 2016).
resemblance to genus Orthoreovirus are proposed to belong to a
new genus (Kibenge et al., 2013; Takano et al., 2016). Crustacean Strain variation in avian reoviruses
reoviruses like Eriocheir sinensis reovirus (EsRV816) with 10 seg- The ARVs can be classified into at least 5 to 11 serotypes (Wood
ments (Zhang and Bonami, 2012), Macrobrachium nipponense et al., 1980; Duncan et al., 1999), with considerable cross-
reovirus with 12 segments (Zhang et al., 2015), and Scylla serrata neutralization among heterogenous types (Robertson and
reovirus with 13 segments (Chen et al., 2011b) have not yet been Wilcox, 1986). Muscovy duck reovirus strains occur in two sero-
assigned to a genus. The RdRp gene has been used to elucidate types (Chen et al., 2011b) with no cross-reactivity to the chicken
the evolutionary relationships among the viruses in the genera of serotype S1133 (Heffels-Redmann et al., 1992). Liu et al. (2003),
the family Reoviridae because the polymerase protein is the most and Ayalew et al. (2017) using phylogenetic analysis of the σC
conserved of the viral protein products (Attoui et al., 2000, 2002). (S1) gene identified six genotypes among ARVs (I to VI). Addi-
Phylogenetic relationships within the family based on the RdRp tional divergent novel orthoreoviruses from wild birds belong to
gene are illustrated in Fig. 6.1. at least two new recently proposed species [Wild bird orthoreovi-
The genus Orthoreovirus belongs to the subfamily Spin- rus (Ogasawara et al., 2015) and a species of novel orthoreovirus
areovirinae. Orthoreovirus virions are non-enveloped with a isolates from corvids (Huhtamo et al., 2007; Dandár et al., 2014;
double-layered icosahedral capsid of ≈ 75 nm diameter, and a Kalupahana, 2017)]. Thus, while avian reoviruses originating
genome consisting of ten segments of linear dsRNA in three size from different avian species, geographical regions and even lesion
classes based on their electrophoretic mobility on a polyacryla- type or pathogenicity can be shown to be genetically distinct, few
mide gel: large (L1–L3), medium (M1–M3) and small (S1–S4) of these viruses are well characterized from either an epizootio-
(Spandidos and Graham, 1976; Gouvea and Schnitzer, 1982; logical or clinical disease perspective (Sellers, 2017).
Dermody et al., 2013). Ogasawara et al. (2015) and Jansen van
Vuren et al. (2016) reviewed the criteria used for modern spe- Morphology
cies demarcation for orthoreoviruses (Attoui et al., 2012). For The genus Orthoreovirus belongs to the subfamily Spinareoviri-
conserved core proteins, an amino acid identity > 85% for homol- nae [with virions having 12 ‘spikes’ (turrets) protruding from
ogous proteins indicates that two viruses belong to the same the ‘core’ through the ‘outercapsid’, hence ‘turreted’ viruses].
species, while identity < 65% indicates a possible new species Orthoreovirus virions are non-enveloped with a nearly spherical