Agriculture Reference
In-Depth Information
et al., 1992; Delaney et al., 1995a,b; Mickelson-
Young et al., 1995; Gill et al., 1996a,b). More
recently, some 16,000 EST loci have been
mapped across the seven wheat homoeologous
groups (Qi et al., 2004).
Comparisons between the genetic maps and
deletion maps have shown that genes are located
mainly toward the ends of chromosomes. In fact,
the decrease in marker density from the centro-
meric to the telomeric regions that can be seen in
the genetic maps is inversely correlated with the
gene density gradient on the physical maps. The
distal chromosome regions tend to be gene-rich
and have high recombination rates. These are also
the regions that harbor most of the agronomic
traits (Qi et al., 2004). Centromeric regions, on
the other hand are gene-poor and largely devoid
of recombination leading to marker clustering on
the genetic map. These trends are clear even
when only a small number of deletion lines is used
per chromosome arm and bin sizes are relatively
large.
A few studies have tried to quantify the size of
the gene-rich regions in wheat. On the short arm
of the group 1 chromosomes, it was estimated
that 70% of the genes were contained within
two gene-rich regions that occupied only 14%
of the arm (Sandhu et al., 2001). Global projec-
tions for the entire wheat genome were that
94% of the genes were located within gene-rich
regions comprising 29% of the wheat genome
(Erayman et al., 2004). Knowing the precise
organizational patterns of genes is very impor-
tant when considering strategies for sequencing
the wheat genome (see “Toward sequencing the
wheat genome”).
information, the early map-based comparative
analyses could be refi ned and complemented
with detailed studies of colinearity at the DNA
sequence level. The comparative information
enhanced the use of related germplasm as a source
of novel alleles in breeding, helped to elucidate
mechanisms of genome evolution, and allowed
exploitation of knowledge and resources across
species. The latter was particularly important for
large-genome species such as wheat. The 17,000-
Mb wheat genome had long been considered
intractable for map-based cloning, and the poten-
tial to use small-genome models such as rice
(
Oryza sativa
L.) as intermediates to conduct
chromosome walking opened the door to gene
isolation in wheat. Although technical advances
made chromosome walking in wheat a reality at
the start of the 21st century, knowledge about
comparative relationships remained important.
It allowed whole-genome sequence data from
rice and other grass species to be exploited for
targeted marker development in wheat. In the
current era of new sequencing technologies, com-
parative information of model species may aid
with the assembly of shotgun sequence of large-
genome relatives. In subsequent sections, we will
review the knowledge about wheat—grass com-
parative relationships.
Comparative mapping
Triticeae tribe
The Triticeae tribe contains wheat and its rela-
tives, with
Triticum
and
Aegilops
species being the
most closely related to wheat at an estimated
divergence age of less than 5 million years ago
(MYA) (Huang et al., 2002b; Devos et al., 2005a),
followed by rye (
Secale cereale
) at approximately
7 MYA (Huang et al., 2002a), and barley (
Hordeum
vulgare
) at approximately 10-13 MYA (Wolfe
et al., 1989; Gaut 2002) (Fig. 15.1). Wheat rela-
tives such as rye and various
Aegilops
species are
often used as donors of novel genes or alleles for
wheat improvement (McIntosh et al., 2003). The
fi rst comparative genetic maps constructed
between species within the Triticeae tribe had as
a goal to identify the extent of synteny between
COMPARATIVE GENETICS
Cross-species genetic mapping was initiated in
the early 1990s. The use of common sets of DNA
probes, mostly RFLP markers, in the construc-
tion of genetic maps of related species provided
insight into genome relationships. Following the
development of genomic resources such as large
EST collections, bacterial artifi cial chromosome
(BAC) libraries, and whole-genome sequence
