Biology Reference
In-Depth Information
genotyping-by-sequencing data to provide full-
genome scans across
QTL Identification
100,000 SNP (single-
nucleotide polymorphism) loci in each member
of a portion of a global reference collection of
sorghum germplasm that has been phenotyped
with a lysimetric system to explore the allelic
variation available for key components of the
stay-green phenotype. Similar phenotype and
genoptype data for available sets of stay-green
QTL introgression lines in several genetic back-
grounds are also being used to better characterize
the genomic regions associated with each of six
stay-green QTLs having favorable alleles from
donor B35 [
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The late 1990s and early 2000s have seen a
plethora of studies aimed at identifying stay-
green QTLs. Several studies used as the stay-
green donor parent B35 (
BTx642), a BC1
derivative of IS12555, a durra sorghum from
Ethiopia (Rosenow et al. 1983). Six QTLs
for pre-flowering stress tolerance were mapped
(Tuinstra et al. 1996), with B35 as a source of
stay-green, in a cross with pre-flowering drought
tolerant line Tx7078. The stress was imposed by
withholding irrigation for several weeks during
the vegetative period, until flowering of about
80% of the lines. Drought tolerance was assessed
either by the absolute yield under stress, or by the
ratio of yield, seed number, or plant height under
stress to the same parameters measured under
fully-irrigated conditions. In a later study, using
the same mapping population, three trials were
conducted. Although two of these trials did not
have any drought effect and the water stress was
applied by withholding irrigation at flowering,
a 40% grain yield reduction was achieved under
imposed drought treatment. Stay-green was eval-
uated by scoring each plot for this trait, using a 1-
to-5 scale, weekly from flowering until maturity.
Several QTLs for stay-green were mapped, with
two of these, on linkage group F (SBI-10) and I
(SBI-10), also co-mapped with yield under either
only drought or under both drought and fully-
irrigated conditions (Tuinstra et al. 1997). Crasta
and colleagues (1999) also used B35 as a stay-
green donor parent and Tx430 as a senescent par-
ent to produce a recombinant inbred line (RIL)
population of 96 individuals to map seven differ-
ent QTLs (StgA, StgD, StgG, as major QTLs, and
StgB, StgI.1, StgI.2 and StgJ, as minor QTLs).
The scoring of stay-green was done by visual
assessment, with the experiment carried out in
the field, with plants exposed to post-flowering
stress. Tao and colleagues (2000) identified five
QTLs for stay-green in trials that were conducted
in five locations over three years. They used
QL41 as a donor source, and QL39 as a drought-
sensitive elite parent. QL41 is a derivative from a
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BTx642].
Last but not least, the phenotypic evaluation
of traits involved in drought adaptation is limited
by the number of years and sites in which these
experimentations can be carried out. This is
so because many of the productive processes
eventually generating yield are influenced sub-
stantially by environmental cues (e.g., Reymond
et al. 2003). Given the large variability of agro-
climatic (including weather and soil) conditions,
and the difficulty of acquiring reliable yield
estimates across environments, it is risky to
rely on only experimental data to assess the
value of a given trait, such as stay-green and
its components, for yield improvement under
water-limited conditions. The literature is rid-
dled with reports of genotype-by-environment
interactions, in one recent study, in the yield per-
formance of a reference collection of groundnut
(Hamidou et al. 2012): other studies simply
address the environment-specific identification
of stay-green QTLs across locations and years
(Tuinstra et al. 1997; Crasta et al. 1999; Subhudi
et al. 2000; Tao et al. 2000; Xu et al. 2000;
Kebede et al. 2001; Haussmann et al. 2002;
Sanchez et al. 2002). Accordingly, here we
review how the use of crop-simulation modeling
can contribute to an ex-ante assessment of the
likely impact of traits shown to be associated
with stay-green, in term of the probability of
success, the possible range of yield increase,
and geographic determination of where gains
can be made.
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