Biology Reference
In-Depth Information
The same project activities in Sudan were
much more advance, and recently resulted
in the release of four
Striga
-resistant vari-
eties in the genetic backgrounds of popu-
lar, but
Striga
-susceptible, improved sorghum
varieties “Tabat,” “Wad Ahmed,” and “AG8”
(personal communication with Dr. Abdalla
Mohamed, Sudan). The backcross/QTL vali-
dation project advanced to the second back-
cross generation (BC2) in several locally adapted
farmer-preferred open-pollinated varieties. The
resulting early-generation backcross progenies,
although
Striga
resistant, were not agronomi-
cally elite enough to be submitted to national
trials and considered for release. The national
programs in Sudan, Eritrea, and Kenya, led
by Dr. Abdalla Mohamed and with ICRISAT
providing backstopping, then obtained funding
through the regional agricultural science net-
work (ASARECA Competitive Grant System for
2006) to fine-map the
Striga
resistance QTLs and
complete the task of recovering recurrent parent
eliteness for materials in the genetic backgrounds
of farmer-preferred improved sorghum varieties
from Sudan. Three more backcrosses were exe-
cuted along with foreground selection, with QTL
flanking SSRs at each stage and with back-
ground selection with DArT markers in BC4F1
progenies.
Standard variety trials were conducted in
Striga
sick plots over three rainy seasons (2009
to 2011) at the Gezira Research Station (GRS),
Damazine, Sinnar, and Gedaref in Sudan. Results
from these trials revealed that backcross-derived
lines T1BC3S4, AG6BC3S4, AG2BC3S4, and
W2BC3S4 were
Striga
resistant and agronomi-
cally superior, giving 180% to 298% increases
in grain yield over their recurrent parents in
the infested sick plots. These four experimen-
tal varieties in Sudan were approved by the
National Crop Variety Release Committee, as
“ASARECA.T1” (T1BC3S4), “ASARECA.W2
Striga” (W2BC3S4), “ASARECA.AG3” (AG2
BC3S4), and “ASARECA.AG4” (AG6BC3S4).
There were several hurdles, which in turn had
led to significant new knowledge for handling the
issues related to MABC of complex traits such
as
Striga
resistance. Most of the
Striga
resis-
tance QTLs targeted for this introgression work
were characterized by large confidence interval
between flanking markers, scant availability of
flanking SSRs (Table 6.1), and lack of polymor-
phism between donor and recurrent parents. This
had many implications for attempting introgres-
sion of
Striga
resistance QTLs in several target
genetic backgrounds. The lack of enough SSRs
at initial stage of project (till BC2-generation)
spread across QTL region meant high proba-
bility of losing the QTL even after flanking
marker confirmation because of the possibility
of recombination occurring in the putative QTL
regions, linkage drag with unfavourable traits,
and ultimately lower recurrent parent recov-
ery. This was evident from the first phase of
the project were we had lower recurrent par-
ent recovery in BC2-progenies. Before we can
make any further progress toward this end, we
were also stuck with unavailability of tightly
linked SSRs with target QTLs and hence identi-
fication of confirmed QTL heterozygote(s). Also
the underlying mechanisms/traits for each tar-
get QTL were not fully understood, which was
linked to lack of proper phenotyping assays for
those mechanism/traits. All of the phenotyp-
ing was done with field-level screening. These
issues were addressed by advancing large BC-
progenies until BC4 with foreground selection
to reduce the probability of losing the QTL
due to crossover between the large QTL inter-
val. We simultaneously assayed the RIL pop-
ulation based on cross-(N13
E 36-1), used
for QTL identification, with additional SSRs
and DArT markers used for MABC. This led
to identification of additional markers for tar-
get QTL interval. These additional SSRs were
subsequently used for foreground selection of
BC4S4-population of
×
150 progenies, followed
by background selection with DArT markers.
We identified 31 BC4S4-progenies, which were
screened across several years and locations,
resulting in identification and release of the four
varieties.
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