Biology Reference
In-Depth Information
resistance. It is clear that a much greater knowl-
edge of
S
.
hermonthica
and
S
.
asiatica
diver-
sity/race structure in relation to host species and
cultivar specificity is urgently needed to inform
breeding programs in different regions of Africa.
There is growing evidence from field studies
and molecular analysis that there is host speci-
ficity and adaptation in
S. asiatica
and
S. her-
monthica
populations. Different
Striga
popula-
tions show specific genetic adaptation to host
and host genotypes displaying variable virulence
(Riopel and Timko 1995). In addition to varia-
tion in the genetic diversity between populations
of a species of
Striga
, there is likely to be a
difference between the species in within pop-
ulation diversity.
Striga hermonthica
is a self-
incompatible outbreeder, whereas
S. asiatica
is
autogamous, self-pollinating prior to floral open-
ing, and, as such, it is highly inbred. The differ-
ence in reproductive biology is likely to have
a significant impact on the within-population
diversity of these species,
S. hermonthica
hav-
ing a higher within-population genetic diversity
than
S.asiatica
(Safa et al. 2006; Scholes et al.
2007). What implication this has for plasticity of
populations regarding host specificity needs to
be determined. Evaluation of the genetic diver-
sity of
Striga
populations and determination of
the influence of parasite genotype on virulence
in differing hosts would better enable
Striga
researchers to ensure that potential control prod-
ucts are fully evaluated. This knowledge is key
to the generation of more durable technologies
and will enable better targeting of dissemina-
tion of control technologies to specific localities.
A recent study by Estep et al. (2011) investi-
gated genetic diversity of
S
.
hermonthica
popu-
lations collected from four different regions in
Mali using SSR markers. The
Striga
populations
were characterized by broadly distributed allelic
diversity with little genetic differentiation and
large amount of gene flow. It was also observed
that population structure did not correlate with
local environment or host species (sorghum ver-
sus pearl millet). These understandings can help
plant breeders identify race/population specific
resistance genes/genotypes, which can further
be used to identify individual resistance genes
in crop (host) germplasm and pyramid multi-
ple resistant genes into a targeted crop plant.
In order to fully characterize the existence of
“races” and the factors driving their formation,
further collections of
S
.
hermonthica
populations
and their hosts are needed. The recent devel-
opment of a high-throughput microarray-based
Diversity Arrays Technology (DArT) (Wenzl
et al. 2004) for several crop species, includ-
ing sorghum, and Next Generation Sequencing
(NGS) based Genotyping-by-Sequencing (GbS)
tools could substantially accelerate knowledge
of
Striga
diversity. The development of a
Striga
-
specific assays would allow screening of thou-
sands of molecular markers in parallel and facil-
itate comparison of
Striga
populations from
micro (within a field) to macro (between coun-
tries) scales.
QTL Analysis and Marker-Assisted
Selection for Improving
Striga
Resistance
The availability of sequence information from
EST and genome-sequencing projects has led
to the development of dense molecular genetic
maps for many cereals including rice, sorghum,
and maize (Varshney et al. 2004; Mace et al.
2009). Most resistance to
Striga
appears to be
polygenic. The use of mapping populations, QTL
analysis, and advanced backcross QTL analysis
(AB-QTL) (for transferring important traits from
wild relatives into a crop variety (Tanksley et al.
1996) combined with marker-assisted selection
(MAS) is a promising approach that is beginning
to yield results for the development of resistant
cultivars. Several QTL and AB-QTL studies have
been performed under laboratory conditions to
identify the genetic basis of resistance in culti-
vated and wild relatives of sorghum (Haussmann
et al. 2004; Grenier et al. 2007). An advanced
backcross mapping population derived from
a cross between PQ434 (low-HIF-producing
wild sorghum) and Shanqui Red (cultivated,
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