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In-Depth Information
G93-9223—were each developed and released
from each of the three PIs above with resis-
tance to Mi, Ma, and Mj, respectively (Luzzi
et al. 1996a, 1996b, 1997). Harris et al. (2003)
found additional PIs for resistance to Mi and
Ma nematodes, which showed different patterns
for egg production and galling, indicating con-
trasting mechanisms for resistance. Yates et al.
(2010) found that PI 594403, PI 594427C, and
PI 594651B contain useful resistance genes from
those previously characterized in PI 200538.
These PIs likely contain unique resistance genes
that, when combined with PI 200538-derived Ma
resistance, could improve the level of Ma resis-
tance in soybean cultivars.
Molecular markers associated with PI 96354,
PI 200538, and PI 230977 have been investi-
gated (Tamulonis et al. 1997a, 1997b, 1997c).
One QTL from PI 96354 was associated with
the single gene
Rm1
previously found in Forrest
(Luzzi et al. 1994b) for southern RKN resistance.
SNP markers for two southern RKN genes that
can be used in MAS have been identified (Ha
et al. 2007).
ratory, unpublished data). Those PIs have been
employed for further molecular characterization
and for the development of new genetic mate-
rials, aiming to identify novel QTL or genes
conveying resistance to these nematode pests.
Genetic markers associated with the QTL or
genes can be efficiently utilized for marker-
assisted backcrossing or genomic selection (GS),
which
facilitate
soybean
molecular
breeding
programs.
New Genomics Approaches and
Biotechnology
Genomics-Based Crop Improvement
In the past, QTL mapping was an important
approach to identify loci of agronomic interest.
This approach is typically conducted by ana-
lyzing the co-segregation of traits with mark-
ers in bi-parental populations (Hamblin et al.
2011). However, major disadvantages of tra-
ditional QTL mapping include the long time
needed for population establishment, the lim-
ited inference made from alleles in just two
parental lines, and the small number of recombi-
nant events (Morrell et al. 2012).
In recent years, next-generation sequencing
(NGS) technology has had a transforming effect
on population-level studies linking genetic varia-
tion to gene function (Harrison 2012). NGS con-
fers the ability to sequence the whole genome
of many related organisms and makes feasi-
ble the development of high-throughput, dense
genotyping (Metzker 2010). This progress has
led to a shift from traditional QTL mapping
to genome-wide association studies (GWAS).
GWAS can identify more novel functional vari-
ation that may be deployed in cultivar improve-
ment through MAS (Hamblin et al. 2011).
Furthermore, GWAS enables a much greater
genetic resolution than QTL mapping due to
a long history of recombination events cap-
tured in most association panels. Combined with
dense, genome-wide marker coverage, GWAS
can potentially map causative loci to individual
Resistance to Multiple Nematode
Species
Several species of nematodes can occur in the
same field, causing damage and complicating
control using host resistance. It has been men-
tioned that there are soybean cultivars that have
resistance to both SCN and RKN. There are also
cultivars that have shown resistance or tolerance
to three or more nematode species. Examples are
Forrest, Bedford, and Jake soybeans (Hartwig
and Epps 1973a; Shannon et al. 2007) with resis-
tance to SCN, RKN, and RN. It is evident that
many of the 118 sources of SCN resistance also
carry resistance to RKN, RN, or both (Shannon
et al. 2007).
As mentioned earlier in this chapter,
recent evaluations of more than 600 soybean
germplasm accessions (MG III-V) showed that
many new exotic soybean PIs were highly resis-
tant to multi-nematode species (Nguyen Labo-
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