Agriculture Reference
In-Depth Information
Chapter 15
Genome Organization and
Comparative Genomics
Katrien M. Devos, Jaroslav Doleˇel, and Catherine Feuillet
SUMMARY
challenges of gene isolation in wheat are
demonstrated in nine success stories.
(5) A global physical map of the polyploid
wheat genome is needed to facilitate genetic
and genomic analyses in wheat. The com-
plexity of this task can be reduced by con-
structing physical maps in diploid wheat
species. An alternative approach is to isolate
individual chromosomes or chromosome
arms using fl ow-sorting and build physical
maps for single chromosomes. The physi-
cal map of hexaploid wheat chromosome
3B has been described as a case study.
(6) The component of the wheat genome that
is of most interest is the gene space. Current
projections for the number of genes present
in the hexaploid wheat genome vary from
108,000 to 300,000. Genes are not ran-
domly distributed in the wheat genome,
but are organized in gene islands that
increase in number and size from the cen-
tromere to the telomere. The distal chro-
mosome regions have higher evolutionary
rates which, in turn, lead to lower levels of
intergenomic colinearity.
(7) The next big target is to obtain the
sequence of the wheat genome. Questions
that have been raised include whether the
entire wheat genome or only the gene
space should be sequenced, and whether
Sanger sequencing technology and/or
new generation sequencing technologies
should be used. Information content and
cost will have to be balanced in the fi nal
decision.
(1) Detailed genetic maps have been generated
for wheat using restriction fragment length
polymorphism (RFLP), simple sequence
repeat (SSR), single nucleotide polymor-
phism (SNP), and diversity array technol-
ogy (DArT) markers. In addition, markers
have been located to chromosome bins using
overlapping sets of deletion lines. All maps
are available on GrainGenes.
(2) Common sets of markers can be used
to establish the relationship between the
wheat genomes and those of other species
within the Triticeae tribe, the Pooideae
subfamily, and even the
Poaceae
family.
Knowledge about the relationship between
different genomes has allowed exploita-
tion of information and resources across
species.
(3) Analysis at the DNA sequence level of
regions that are orthologous at the map
level has shown that marker orders are
highly conserved between species that
diverged as much as 60 million years ago.
Small rearrangements such as inversions,
translocations, duplication, and deletion of
one or a few genes are, however, common
evolutionary events.
(4) Map-based gene isolation in wheat has
become a reality. In addition to the wheat
resources, comparative information, in
particular between wheat and sequenced
grass genomes, is an integral part of map-
based cloning strategies in wheat. The
