Agriculture Reference
In-Depth Information
A number of research projects in cereals have used association genet-
ics to analyze disease resistance. One successful GWA study was for
powderymildew resistance inMediterranean barley as part of a breeding
program with high-density DArT markers (Comadran et al. 2009). DArT
and SSR markers were used to search for disease resistance QTL against
various diseases (three types of rust and powdery mildew) in historical
bread wheat populations from CIMMYT across many locations and over
years (Crossa et al. 2007). Yu et al. (2011) used the same marker types and
GWA to evaluate 276 spring wheat breeding lines for stem rust race Ug99
resistance in Kenya over three years with 15 associated loci identi
ed.
This work is of great importance due to the aggressive nature of this rust
strain. AM can also be useful when evaluating multiple strains of the
disease against a set of germplasm. Gurung et al. (2011) evaluated 567
spring wheat landraces for 5 races of
Pyrenophora tritici-repentis
, which
causes tan spot for associations with 832 DArT markers. Similarly, AM
approaches with DArTs were used to
find QTL for resistance to
Phaeos-
phaeria nodorum
in the USDA National Small Grains Collection of
spring wheat landraces (Adhikari et al. 2011).
The convergence of high-throughput genotyping platforms with the
development of appropriate statistical methods has meant that the
impasse in successful implementation of AM has now largely been
overcome with GWA scans in plants (Cockram et al. 2010). Proliferation
of association studies in the cereals is re
ected by the number of
researchers delving into this
field of association genetics in the past
10 years. Also, this increase in association studies is proof of the interest
and credibility that AM has acquired especially through CG and GWA
studies in maize and barley that continue to be the leaders among the
crop communities undertaking association genetics. Marker numbers
are likely to increase even further from the initial studies in cereals as
assays for SNP polymorphisms improve.
Already, the affymetrix and in
nium chips supersede the number of
markers provided by GoldenGate technology. Genotyping by sequencing
will be the next large test of AM protocols with very large number of
polymorphisms that may increase the power of AM. The advantage in
genome technologies of certain cereals such as barley is evident in their
progress in AM studies compared with the less-well funded tropical
cereals such as millet and sorghum where only a few initial attempts at
AM have been made (Brown et al. 2008; Bhosale et al. 2012). The
advantage of full-genome sequence is evident as for sorghum, where a
recent study used genotyping by sequencing (GBS) to uncover
265,000
SNPs in 971worldwide accessions and associations with plant height
components and in
∼
orescence branch length (Morris et al. 2012).
