Virus Resistance in Barley - Translational Genomics for Crop Breeding: Biotic Stress - page 66

In general, virus resistance is identified in
genetic resources that are rather un-adapted to
productive growing systems (e.g., Ordon and
Friedt 1994). Therefore, to combine virus resis-
tance with superior agronomic performance,
time-consuming backcrossing procedures are
needed. This holds especially true if reces-
sive resistance genes like in the case of
BaMMV/BaYMV must be incorporated, as a
selfing generation is needed after each back-
cross to identify homozygous recessive geno-
types on the phenotypic level. In contrast to
this, in marker-based procedures, the recessive
resistance encoding allele can be directly fol-
lowed by a co-dominant marker or a dominant
one showing an additional fragment linked to
the resistance-encoding allele (Ordon et al. 2003,
2009), thus saving one generation per backcross-
ing cycle. Furthermore, backcrossing schemes
can be additionally enhanced if in parallel the
genomic portion of the recurrent parent is deter-
mined, for example, by efficient high-throughput
Single Nucleotide Polymorphism (SNP) geno-
typing (Close et al. 2009), as in many species
(Uptmoor et al. 2006) a strong deviation from
the theoretically expected portion of 75% in BC 1
was observed.
Furthermore, molecular markers facilitate
efficient pyramiding of resistance genes, espe-
cially in combination with doubled haploids
(Werner et al. 2005, 2007). With respect to
BaMMV/BaYMV, pyramiding may become of
special importance in the future, as most of the
resistance genes (Table 5.1) have been over-
come already by new pathotypes of BaYMV or
BaMMV (Habekuss et al. 2008). This approach,
which may be conducted following strate-
gies involving one or two DH-line production
steps (Werner et al. 2005, 2007), facilitates
the extended use of respective resistance genes
in barley breeding: for example, the combina-
tion of rym5 being effective in Europe against
BaMMV, BaYMV, and BaYMV-2, with rym9
being effective against BaMMV, BaMMV-SIL
and BaMMV-Teik, will result in complete resis-
tance to all strains known in Europe until now.

Molecular Markers for Virus Resistance

With respect to the insect-transmitted BYDV/
CYDV, viruliferous aphids have to be available
for efficient breeding for resistance/tolerance on
the phenotypic level, while effective selection
procedures against BaMMV/BaYMV require
uniformly infested fields or mechanical inocu-
lation procedures in the greenhouse followed by
DAS-ELISA (for details, cf. Palloix and Ordon
2011). Furthermore, it has to be taken into
account that rearing of viruliferous aphids and
artificial infection are difficult to integrate into
applied barley-breeding schemes, and that with
respect to BaMMV/BaYMV, symptom develop-
ment is strongly influenced by the climatic con-
ditions during winter and spring time, leading
to the situation in which a reliable selection for
virus resistance on the phenotypic level cannot
be conducted every year.
Therefore, soon after the first molecular
marker techniques, such as Restriction Frag-
ment Length Polymorphisms (RFLPs), became
available, followed by PCR-based techniques
like Random Amplified Polymorphic DNA
(RAPDs), Simple Sequence Repeats (SSRs),
and Amplified Fragment Length Polymorphisms
(AFLPs), attempts were made to develop molec-
ular markers for virus resistance genes in barley,
facilitating efficient marker-based selection pro-
cedures. An overview on markers available for
major virus resistance genes in barley is given in
Table 5.1.
Based on these markers, different marker-
based selection procedures have been devel-
oped. Together with doubled haploid techniques,
which are routinely used in barley breeding
today, respective markers facilitate a reliable and
fast selection for virus resistance; for example,
doubled haploid populations may be screened
directly in vitro and only those plantlets car-
rying the resistance encoding allele have to be
transferred to the greenhouse (for overview on
molecular breeding for virus resistance, cf. Friedt
and Ordon 2007; Ordon et al. 2009; Palloix and
Ordon 2011).