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physiological mechanisms underlying each of
these stay-green QTLs and their interactions - at
least little that has been published towards trait
expression.
a result, most of the stay-green QTLs targeted
for this introgression work were characterized
by large confidence intervals between flanking
markers and scant availability of flanking SSR
polymorphisms between the donor and recurrent
parents. These limitations on readily available
flanking marker polymorphism had many impli-
cations for this attempt to introgress stay-green
QTLs into several target genetic backgrounds.
The lack of enough polymorphic SSRs spread
across each QTL region resulted in a high prob-
ability of losing the QTL even after flanking
marker confirmation because of the possibility
of recombination occurring within one or more
of the putative QTL target regions, linkage drag
with unfavourable traits, and ultimately a lower
level of recurrent parent genome recovery. Sim-
ilarly lack of polymorphic SSRs between B35
(donor) and recurrent parents (especially Indian
durras
) meant that no progress in stay-green
QTL introgression work was possible in many
target backgrounds until more markers were
available. Accordingly, the MABC project was
focused on two genetic backgrounds, R 16 and
S35
QTL Introgression -Current
Progress atICRISAT
The initial stay-green QTL mapping studies used
RFLPs (restriction fragment length polymor-
phisms) and AFLPs (amplified fragment length
polymorphisms; Tuinstra et al. 1997), but later
studies extensively used SSRs (simple sequence
repeats; Subudhi et al. 2000, Haussmann et al.
2002, Harris et al. 2007) and DArTs (Sabadin
et al. 2012). Most of the stay-green QTL intro-
gression and QTL validation studies reported to
date (Tuinstra et al. 1996, Harris et al. 2007) have
used RFLPs and/or SSRs as flanking markers for
foreground selection - along with RAPD (ran-
dom amplified polymorphic DNA) and/or AFLP
markers.
Researchers at ICRISAT-Patancheru selected
six candidate QTLs for the stay-green trait from
donor B35, including
Stg1
,
Stg2
,
Stg3
, and
Stg4
reported by Subudhi and colleagues (2000),
Sanchez and colleagues (2002), and Harris and
colleagues (2007), as well as additional QTLs
on SBI-01 (
StgA
) and SBI-02 (
StgB
), and initi-
ated marker-assisted backcross (MABC) trans-
fer of these into a number of genetically diverse,
tropically-adapted elite sorghum varieties hav-
ing a range of drought tolerance (Hash et al.
2003). Recurrent parents included highly senes-
cent post-rainy season-adapted durra variety R
16, short 2-dwarf tan white-grained caudatum
variety ISIAP Dorado, and tall 2-dwarf tan,
white-grained, sweet-stemmed caudatum sister-
line varieties S 35 and ICSV 111. The posi-
tions of flanking SSRs for these six target QTLs
were putatively inferred from stay-green QTL
mapping studies published in the late 1990s
and early 2000s (Table 8.1). The unavailabil-
ity of alternate SSRs and lack of polymorphism
was a major constraint for this activity in the
early 2000s, when this work was initiated. As
ICSV 111.
For the R 16-background, BC
2
-progenies
were developed with foreground selection each
generation to confirm the presence of alleles
from the donor parent at the SSR loci flanking
each of the six putative stay-green QTLs, com-
bined with limited background selection, in order
to hasten the recovery of recurrent parent alleles
in genomic regions distant from one or more of
the target QTLs. These progenies were evalu-
ated for stay-green expression (Kassahun et al.
2010), and one of the backcross progenies, RSG
04005, was confirmed as carrying three QTLs
(
Stg3
,
Stg4
and
StgB
). This entry was used as
the stay-green donor in another round of back-
crosses to R 16 to derive single-QTL introgres-
sion lines. Two of the stay-green QTLs,
StgB
and
Stg3
, are mapped on sorghum chromosome SBI-
02. These two QTLs are linked to the morpho-
logical marker gene
Z
/
z
controlling the impor-
tant grain quality trait of mesocarp thickness,
with the stay-green alleles from B35 linked with
=
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