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alleles for recessively inherited thick mesocarp.
Similarly
Stg1
and
Stg2
had overlapping confi-
dence intervals on SBI-03, and exhibited linkage
with one of two epistatically interacting genes
controlling pericarp pigmentation (
Y
/
y
on SBI-
01 and
R
/
r
on SBI-03) and a gene controlling
the absence/presence of awns (
A
/
a
). The Indian
post-rainy season (
rabi
) sorghum has a very spe-
cific grain quality requirement of thin meso-
carp with creamy/lustrous bold grain. Utilizing
the intermediate BC
2
-progeny for further back-
crossing helped to generate segregation for desir-
able phenotypes at the mesocarp and pericarp
loci, recombined with B35 alleles at stay-green
QTL-flanking SSR markers. This permitted iso-
lation of white-grained, thin mesocarp progenies
such as 'K369white' in later backcross gener-
ations. Such “clean” single-QTL introgression
lines can now be used individually as donor par-
ents of specific stay-green QTLs shown to have
the largest favourable phenotypic effects across
several genetic backgrounds.
Similarly, for stay-green QTL introgression
inS35
now available in several genetic backgrounds
and can be used as parents directly in breeding
programs to feed the current and future product
pipeline.
We also tried utilizing another well char-
acterized stay-green source, namely Ethiopian
landrace germplasm accession E 36-1 (Hauss-
mann et al. 2002). The major problem for uti-
lizing this source was lack of marker (SSR)
polymorphism, as E 36-1 belongs to same set
of of zera-zera landraces that have contributed
extensively in development of the agronomically
elite caudatum and guinea-caudatum derivatives
across global sorghum breeding programs over
the past fifty years. Further, the major stay-
green QTL from E 36-1 maping to SBI-10
had a very large confidence interval and is
linked with unfavourable alleles at neighboring
shoot fly resistance component QTLs associated
with seedling leaf blade trichome density and
seedling glossines score. We are currently devel-
oping a fine-mapping population to break this
unfavorable linkage by crossing an intermedi-
ate derivative of E 36-1 with E 36-1 alleles at
the stay-green QTL on SBI-10 chromosome and
a QTL introgression line with shoot fly resis-
tance QTLs, utilizing newer marker techniques
such as DArT, and single-nucleotide polymor-
phisms (SNPs) identified with a genotyping-
by-sequencing (GbS) pipeline exploiting one of
the new generation sequencing (NGS)-platforms
(Elshire et al. 2011).
Fine-mapping populations for the best of stay-
green QTLs validated in R 16 and S 35 back-
grounds are being developed. These populations
will be screened for several physiological param-
eters. This, along with newer marker systems
such as DArTs and GbS-SNPs, will help to
achieve deeper genome coverage for tracking
important recombinants across large genomic
region(s) present between the QTL-flanking SSR
markers. These recombinants will help to iden-
tify near-isogenic lines for each of the stay-green
QTLs, with no or minimal negative linkage drag,
for direct utilization by breeding programs and
as well for use in pyramiding multiple QTLs
ICSV 111-background, backcrossing
was continued up to BC
4
and BC
5
with complete
foreground selection and incomplete background
selection during each backcross generation. This
has finally produced sets of BC
n
F
4
/BC
n
+
1
F
3
progenies. These progenies have been evalu-
ated across locations and years for their stay-
green expression in field, pot, and lysimeter stud-
ies under fully-irrigated non-stress conditions
and rain-fed or supplementally-irrigated termi-
nal drought-stress conditions during the cool, dry
post-rainy (
rabi
) season in peninsular India. Sev-
eral progenies still segregated for plant height
and flowering time, and so were advanced by
selfing for several additional generations follow-
ing pedigree selection. Even though it took four
to five seasons more to isolate clean versions
of stay-green QTL introgression lines in this
genetic background, the fact that it was achieved
with limited SSR availability in the early days
of marker-assisted backcrossing must be appre-
ciated. These recently developed and validated
“clean” stay-green QTL introgression lines are
=
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