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In-Depth Information
in the growing area of infected field is breeding
for resistance.
BaMMV/BaYMV, genotypic differences con-
cerning the reaction to the different members of
the barley yellow mosaic virus complex were
observed, but it turned out that resistance to
BaMMV and BaYMV is quite frequent within
the primary gene pool of barley (e.g., Kawada
et al. 1991; Ordon and Friedt 1993; Habekuss
et al. 2008). Genetic analysis has shown that
different recessive resistance genes are present
in barley (Goetz and Friedt 1993; Ordon and
Friedt 1993). In addition, two dominant resis-
tance genes have been identified in the secondary
gene pool, namely in
Hordeum bulbosum
(Ruge
et al. 2003; Ruge-Wehling et al. 2006), and very
recently the first dominant resistance gene has
been identified also in cultivated barley (Kai
et al. 2012).
In contrast to BaMMV/BaYMV, no complete
resistance against BYDV/CYDV is known in
barley, but different genes conferring tolerance
have been identified. The first was
ryd1
, detected
in the spring barley cultivar “Rojo” (Suneson
1955). This gene was not used in barley breeding
due to its low efficiency. Furthermore,
Ryd2
and
Ryd3
, with similar effects against BYDV-PAV
and BYDV-MAV, were identified in Ethiopian
landraces (Schaller et al. 1964; Niks et al. 2004).
The effect of the semi-dominant
Ryd2
gene
located on chromosome 3HL (Collins et al. 1996)
depends on the genetic background, environmen-
tal conditions, and virus isolate (Schaller et al.
1964; Schaller 1984). Besides this, different alle-
les may be present at this locus (Catherall and
Hayes 1970; Chalhoub et al. 1995). Recently, a
gene called
Ryd4
Hb
, conferring complete resis-
tance to BYDV-PAV, has been transferred from
Hordeum bulbosum
to cultivated barley (Scholz
et al. 2009). Furthermore, additional QTL for tol-
erance against BYD have been identified in bar-
ley (Kraakman et al. 2006; Scheurer et al. 2001;
Toojinda et al. 2000). Results of these screen-
ing programs and genetic analyses revealed that
there is a broad genetic variation available con-
cerning resistance to BaMMV/BaYMV and suf-
ficient variation for tolerance to BYDV/CYDV,
which can be employed in barley breeding.
BarleyYellowDwarfVirus/Cereal
YellowDwarfVirus
Barley yellow dwarf is caused by a group of
related viruses (see below) inducing leaf dis-
coloration and dwarfing, resulting in high yield
losses (e.g., Lister and Ranieri 1995). It was
first detected in 1951 in California (Oswald
and Houston 1951). The agents of BYD are
single-stranded RNA viruses with a genome
size of 5.3-5.7 kb and isometric particles of
about 25 nm in diameter, which are transmit-
ted in a persistent manner by aphids. In a first
step, according to the different transmission effi-
ciency of different aphid species, five different
strains of BYDV were distinguished (Rochow
1969; Rochow and Muller 1971). Today, this
group of viruses is subdivided into three species
belonging to the genus
Luteovirus
, namely
Bar-
ley yellow dwarf virus
(BYDV)-PAV, BYDV-
PAS, and BYDV-MAV, two species known as
Cereal yellow dwarf virus
(CYDV)-RPS and
CYDV-RPV belonging to the genus
Polerovirus
,
and another three species, namely BYDV-GPV,
BYDV-RMV, and BYDV-SGV, also classified as
members of the
Luteoviridae
family but not yet
being assigned to any genus (
http://ictvonline
2009). Du
e to
global warming and longer periods of warm tem-
peratures in autumn and winter, it is expected
that these aphid-transmitted viruses will become
even more important in the future in some parts
of the world, for example, in northern Europe
(Habekuss et al. 2009).
=
Breeding for Virus Resistance -
Some Case History
Sources and Genetics of Resistance
The starting point for each breeding program for
virus resistance in barley has been the exten-
sive screening for resistance. With respect to
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