Agriculture Reference
In-Depth Information
(8)
We should already start thinking beyond
the genomic sequence. Functional tool
development has been initiated and
includes the generation of populations by
TILLING (Targeted Induced Local
Lesions IN Genomes), virus-induced gene
silencing (VIGS), and improved transfor-
mation protocols. Knowledge of all genes
and their function will allow highly effi -
cient and targeted improvement of wheat.
MAPPING
clones as markers. In hexaploid wheat, genes are
generally triplicated. While polyploidy compli-
cates genetic mapping to a degree, it also provides
advantages. The buffering capacity of the hexa-
ploid background has allowed the development of
series of nullisomic-tetrasomic (NT) lines. In
each NT line, a chromosome pair has been
replaced with a homoeologous pair (Sears 1954).
These lines have been extremely useful for deter-
mining the chromosomal location of markers.
Hybridization of RFLP markers to DNA extracted
from the NT lines has revealed that nearly all
cDNAs and about 50% of
Pst
I genomic clones
detect copies on all three genomes in hexaploid
wheat (Devos et al., 1992). When these markers
are used for mapping, however, polymorphisms
are rarely obtained between the parents of even
wide mapping populations for all three copies.
The main problem in the construction of detailed
wheat genetic maps, therefore, was not the hexa-
ploid nature of the crop, but the low level of
variation.
To enhance polymorphism levels, and thus the
number of markers that can be incorporated in a
genetic map, segregating populations were often
generated from wide crosses involving synthetic
wheat (Gale et al., 1995; Nelson et al., 1995c).
Alternatively, mapping was done in a diploid
wheat progenitor or relative such as
Aegilops
tauschii
and
Triticum monococcum
, which showed
higher levels of variation (Dubcovsky et al., 1996;
Boyko et al., 1999). Loci detected with the same
probe in different genomes mapped to homoeolo-
gous locations. This showed that marker orders
were highly conserved among the A, B, and D
genomes of wheat.
To determine the location of centromeres on
the genetic maps, markers were hybridized to a
set of ditelosomic lines. Ditelosomic lines are
disomic for the absence of a chromosome arm,
and they exist for the majority of the chromo-
Genetic mapping in wheat started in earnest in
the mid-1980s following the development of
restriction fragment length polymorphisms
(RFLPs) as a marker system (Botstein et al.,
1980). Since then, detailed maps have been devel-
oped for diploid, tetraploid, and hexaploid wheat
(Chao et al., 1989; Gill et al., 1991b; Liu and
Tsunewaki 1991; Devos et al., 1992; Hart et al.,
1993; Xie et al., 1993; Gale et al., 1995; Nelson
et al., 1995a,b,c; Jia et al., 1996; Marino et al.,
1996; Blanco et al., 1998; Röder et al., 1998;
Messmer et al., 1999; Peng et al., 2000; Zhang et
al., 2004b). In addition to genetic maps, chromo-
some bin maps have been generated using sets of
overlapping deletion lines (Kota et al., 1993;
Delaney et al., 1995a,b; Mickelson-Young et al.,
1995; Gill et al., 1996a; Conley et al., 2004;
Hossain et al., 2004; Linkiewicz et al., 2004;
Miftahudin et al., 2004; Munkvold et al., 2004;
Peng et al., 2004; Randhawa et al., 2004).
Deletion mapping does not require intervarietal
polymorphism and thus provides a mechanism
for generating high-density maps in crops such as
wheat that have a narrow genetic base. The physi-
cal maps provided a fi rst insight into the distribu-
tion patterns of genes in the large wheat genome
and are a valuable tool for studying genome orga-
nization. Genetic maps have important agronomic
applications. They form the framework for the
mapping of traits using quantitative trait analyses
as further elaborated in Chapter 14, for gene-
tagging and, more recently, for the isolation of
genes underlying these traits (see subsequent
section on map-based cloning).
Genetic mapping
The fi rst RFLP mapping in wheat was accom-
plished using cDNAs and low copy
Pst
I genomic