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(DAdV-2); and ORF51, instead of ORF0, is found in GoAdV-A genome of mastadenoviruses and aviadenoviruses, two struc-
(GoAdV-4) only (Table 10.2) (Kaján et al., 2012). tural genes map at the left end of the atadenoviral genome: p32K
The left end region contains gene clusters that are thought and LH3. Non-structural genes within this region include LH1
to have originated through gene duplication during evolution and LH2 (Harrach et al., 2011). The right end of the genome
(Washietl and Eisenhaber, 2003; Corredor et al., 2006; Kaján et consists of clusters of leftward-oriented genes that are related
al., 2012). These clusters are mastadenovirus E4ORF6-like genes to each other: E4 34 K (E4.2 and E4.3) and RH homologues
(ORFs 24 and 14) (Gilson et al., 2016) and parvovirus NS-1 (RH1, 2, 4, 6 and 5). ORF content downstream of RH homo-
homologues (ORFs 2, 13 and 12) (Washietl and Eisenhaber, logues can differ among members of the genus Atadenovirus. For
2003). Previous studies placed ORFs 2, 24, 14, 13 and 12 within example, DAdV-1 contains seven uncharacterized ORFs related
the superfamily III helicases related to parvovirus NS-1 (Washi- to each other and the VA RNA region (Harrach et al., 2011).
etl and Eisenhaber, 2003). ORF content within these clusters Sequence analysis between DAdV-1 strains that poorly replicate
seems to be species-specific. For example, E4ORF-6-like genes of and replicate well in chicken embryo liver cells shows amino
FAdV-C include ORFs 24, 14A, 14, 14B and 14C. This cluster in acid differences in IVa2, DNA polymerase, endopeptidase and
FAdV-B, PiAdV-A and TAdV-B-D consists of ORFs 14A and 14 DNA-binding protein. Such differences have been suggested as
only. ORFs 14A and 14 of TAdV-5 (TAdV-D) appear as a fusion potential determinants of virus tropism and/or virulence (Kang
ORF (ORF14A + 14) (Corredor et al., 2006; Marek et al., 2014b). et al., 2017).

Right end Siadenovirus genome
The right end varies in gene content and size among aviadenovi- The genomes of siadenoviruses are the shortest among all
ruses. Variations of nucleotide sequences of ORFs and intergenic adenoviruses sequenced to date (26 kb) with G + C contents
regions can be also found in virus strains within the same sero- of 34.9 to 38.5%, and ITRs of 29–39 bp. The ORFs encoding
type. In general, the right end genome consists of leftward- and a putative sialidase and highly hydrophobic protein at the left
rightward-oriented ORFs and some intergenic regions containing end of the genome are unique to the genus Siadenovirus. The
tandem repeats with variable numbers of repetition units (Ojkic region located between pVIII and fibre gene, named E3, has no
and Nagy, 2000; Corredor et al., 2008; Griffin and Nagy, 2011). sequence homology to the mastadenovirus E3 region. ORFs 7
The number of repetition units as well as deletions in some ORFs and 8 are unique ORFs located downstream of the fibre gene.
(e.g. FAdV-4 ORFs 19 and 27) are thought to be associated with DNA sequence analysis of 12 TAdV3 isolates suggests ORF1,
virulence (Liu et al., 2016; Pan et al., 2017a). E3 region and fibre gene (knob domain) as virulence factors
Leftward-oriented ORFs 22, 20A, 20 and 19 are present in (Beach et al., 2009a).
all aviadenoviruses sequenced to date (Table 10.2) (Corredor et
al., 2008; Marek et al., 2014a,b, 2016). As mentioned previously,
ORF22 in concert with ORF8 (Gam-1) promotes cell cycle Phylogeny and evolutionary relationships
progression (Lehrmann and Cotten, 1999). Rightward ORF8, Each genus consists of species groups that include one or more
known for its essential role in FAdV-1 replication (Glotzer et al., serotypes based on phylogenetic distance (> 5–15% based on
2000), is present in most aviadenoviruses except in DAdV-2 and DNA polymerase amino acid sequence), genome composi-
GoAdV-4 (Kaján et al., 2012; Marek et al., 2014a). The absence tion, cross-neutralization, RFLP profiles, haemagglutination,
of Gam-1 in DAdV-2 and GoAdV-4 suggest alternative ORFs and host range, etc. (Harrach et al., 2011). Members of the genus
mechanisms that promote cell cycle progression and regulate Aviadenovirus are grouped into two major clusters based on
apoptosis. nucleotide sequence of whole viral genomes and analysis of
ORF19 encodes a putative lipase with identities to homo- the hexon L1 gene region: the first cluster includes DAdV-B
logues of avian pathogenic herpesviruses including duck enteritis and GoAdV-A, which infect anseriform birds, while the second
virus (DEV), Marek’s disease virus (MDV), or Meleagrid her- cluster consists of FAdVs, TAdVs and PiAdV-A, which infect
pesvirus 1 (MeHV-1). Lipase and transmembrane domains are galliformes (turkeys and chickens) and pigeons (Marek et al.,
present in all ORF19 homologues in FAdV species (Corredor 2010a, 2016). The second cluster consists of six subclusters
et al., 2008). Most aviadenovirus genomes contain one ORF19 that group FAdV-A and TAdV-D; FAdV-C; PiAdV-A and TAdV-
homologue, whilst 2 ORF19 homologues are present in the B; FAdV-B and TAdV-C; and FAdV-D and FAdV-E (Marek et
genomes of DAdV-2, GoAdV-4 and FAdV-4 (Table 10.2): ORFs al., 2016). Nucleotide sequence from the analysed viral strains
19 and 19B (both with leftward orientations) in DAdV-2 and representing species groups in such subclusters reveal similar
GoAdV-4 and ORFs 19 and 19A (leftward and rightward orienta- genome organization between FAdV-D and FAdV-E; FAdV-A
tions, respectively) in FAdV-4 (Corredor et al., 2008; Marek et al., and TAdV-D; and FAdV-B and TAdV-C (Fig. 10.7) (Marek
2014a,b, 2016). et al., 2014b). Interspecies and intraspecies variations in the
nucleotide sequence within these groups have been observed.
Atadenovirus genome For example, members of species FAdV-D and FAdV-E have
The high A + T content and unique genes at the right and left identical genome organization with nucleotide sequence
ends of the genome are characteristic of atadenoviruses. Interest- identities between 71.2 and 75.4%. Intraspecies variations in
ingly, while no structural genes are found at the left end of the nucleotide sequence identity range between 89.4 and 97.1%