Agriculture Reference
In-Depth Information
somes (Sears and Sears 1979). The centromeres
typically colocalized with large clusters of
markers. Although marker orders were generally
highly conserved among the A, B, and D genomes,
the maps confi rmed that the wheat genome had
undergone several chromosomal translocations.
The best characterized rearrangement involved
chromosome arms 4AL, 5AL, and 7BS (Liu et
al., 1992; Devos et al., 1995; Nelson et al., 1995a).
The reciprocal translocation between chromo-
some arms 4AL and 5AL took place in the A-
genome diploid progenitor of wheat. This was
followed by a pericentric inversion in chromo-
some 4A which resulted in the long arm becom-
ing the physically shorter arm. A second reciprocal
translocation took place between the 5AL region
on 4AL and 7BS. In addition to the 4AL/5AL/
7BS translocation, a 2BS/6BS translocation has
been mapped (Devos et al., 1993b; Qi et al.,
2006), and pericentric inversions have been iden-
tifi ed in 'Chinese Spring' chromosomes 2B, 3B,
4A, 4B, 5A, and 6B (Qi et al., 2006). No rear-
rangements have been observed in the D genome,
and the D genome is therefore considered to
have retained the most ancestral chromosome
organization.
As new polymerase chain reaction (PCR)-based
marker systems were developed, these markers
were used to populate the wheat genetic maps
(Röder et al., 1998; Stephenson et al., 1998;
Somers et al., 2004). Microsatellite markers tend
to detect relatively high polymorphism levels and
thus are a great tool for marker-assisted selection
(Prasad et al., 1999; Qiu et al., 2006; Hiebert et
al., 2007; Houshmand et al., 2007; Kumar et al.,
2007; Sharma et al., 2007; Tsilo et al., 2007, 2008).
However, microsatellites also tend to be chromo-
some-specifi c and hence provide little informa-
tion on the relationship between the three wheat
genomes. This is particularly true for microsatel-
lites isolated from genomic sequences, but it is
also often the case for microsatellites derived from
5′ or 3′ untranslated regions of expressed sequence
tags (ESTs). Other marker systems that are cur-
rently in use in wheat are single nucleotide poly-
morphisms (Somers et al., 2003; http://probes.
pw.usda.gov:8080/snpworld/disclaimer.jsp)
and Diversity Array Technology (DArT) markers
(Akbari et al., 2006). The latter technology is
hybridization-based and allows the genotyping in
parallel of several thousand loci (Jaccoud et al.,
2001).
Typical features of the wheat genetic maps
include the high conservation of colinearity
between the homoeologous genomes and the clus-
tering of markers in the centromeric regions. The
latter is due to a reduction in recombination
around the centromere. While mapping the cen-
tromeres on the genetic maps was easy due to the
existence of ditelosomic lines, capping the genetic
maps with telomeres proved to be a greater chal-
lenge. The ends of seven chromosome arms have
been defi ned by mapping telomere-associated
sequences beyond the most distal marker on a
linkage group (Mao et al., 1997). The maps that
have been generated over the years can be viewed
on GrainGenes (http://wheat.pw.usda.gov/
GG2/index.shtml). In addition, a composite
wheat map resulting from the integration of 12
maps and 2,700 loci has been compiled (http://
rye.pw.usda.gov/cmap/).
Deletion mapping
A number of alien chromosomes, including chro-
mosomes 2C of
Ae. cylindrica
, 3C of
Ae. triuncia-
lis
, and 2S
l
and 4S
l
of
Ae. sharonensis
, carry
gametocidal genes (
Gc
) (McIntosh et al., 1998).
It has been proposed that the primary function
of a
Gc
factor is to induce chromosome breaks in
gametes that lack the
Gc
factor (Endo 1990). The
broken chromosome ends form telomeres and
can thus be stably transmitted to the progeny.
This characteristic of
Gc
genes has been exploited
to generate a large collection of terminal dele-
tions of varying size (Endo and Gill 1996). By
using overlapping sets of deletions, a chromo-
some arm is
de facto
divided into small chromo-
some regions or bins. For mapping, each deletion
line is scored for the presence or absence of a
marker and this process allocates the marker to a
particular bin. The disadvantage of deletion
mapping is that markers within a bin cannot be
ordered. The advantage is that no polymorphism
is required for mapping. The fi rst deletion maps
were produced with RFLP markers (Werner
