Agriculture Reference
In-Depth Information
the chromosome and variation in the trait
phenotype (Doerge 2002; Wang et al., 2004).
Further, the additive effects of each allele can
be determined which associates the parental
alleles with a favorable and a nonfavorable
phenotype.
The three main factors affecting QTL analysis
include (i) size of the population, (ii) density of
markers and genetic spacing on the genetic map,
and (iii) quality of the phenotypic data. The size
of the population determines the distribution of
crossover points in the genetic map and thus
directly affects the resolution of the QTL analy-
sis. Likewise, the marker density will directly
affect the resolution of the QTL analysis. Both of
the former attributes are easily controlled and the
genotyping data is often discrete or absolute.
Hence the success of QTL analysis is most
affected by the quality of the phenotypic data.
Poor data can lead to detection of spurious QTLs
or improper estimation of the effect of a real
QTL. It follows that complex traits such as yield,
preharvest sprouting tolerance, or FHB resistance
require a high degree of replication and careful
measurement of traits within each replication.
More simply inherited traits such as leaf rust
resistance, or presence and absence of morpho-
logical traits, can be tested with fewer replica-
tions, but care must still be taken with trait
measurement.
in 1989. The international wheat community
decided to create a single, robust genetic map
from the biparental cross of W7984 (synthetic) ×
Opata. The progeny consisted of 115 RILs and
the cross was genetically wide, revealing >50%
polymorphism across the genome. The cross was
particularly useful for mapping the D genome,
since the cross used a synthetic × bread wheat
design. This cross is still in use today and is dis-
tributed around the world. Recently, the same
ITMI population was re-created and is com-
posed of >200 DH lines and >1,500 RILs
(J.P. Gustafson, M.E. Sorrells, pers. comm., with
D.J. Somers).
Wheat is a polyploid species carrying three
different genomes designated A, B, and D.
Geneticists can employ aneuploid lines, such as
the wheat nullisomic-tetrasomic (NT) lines, to
show from which chromosome each marker DNA
fragment is derived, either through PCR or
hybridization. This technique is still used heavily
today with the development of SNP markers
(Somers et al., 2003; http://wheat.pw.usda.gov/
SNP/project.html) and other genome-specifi c
markers. Therefore wheat genetic maps are com-
posed of correctly labeled chromosomes, since the
NT lines were developed through classical cyto-
logical studies and techniques that could identify
the chromosomes based on mitotic staining
patterns.
A critical milestone reached in wheat research
was establishing early genetic maps of wheat that
identifi ed the chromosomes and provided a set of
DNA markers that could be used to anchor future
genetic maps. The ITMI map was a RFLP-based
map and served to anchor future maps and also
to facilitate the transition between RFLP- and
PCR-based maps that employed either AFLP or
SSR markers. In 1998, the fi rst robust genetic
map of wheat developed with SSRs (GWM) was
released by Dr. Marion Roder and colleagues
(Roder et al., 1998). This was followed by a second
ITMI map with markers developed for the D-
genome (GDM) (Pestsova et al., 2000). These
maps consisted of >220 SSRs and 55 SSRs,
respectively, distributed across the genome of the
ITMI population and anchored to chromosomes
using the available RFLP data on the same cross.
EARLY PROGRESS AND DEVELOPMENTS
Genetic maps
Wheat genetic mapping has always been fortunate
to have a stable research investment and an ever-
growing abundance of genetic markers. This
began with RFLP markers, initially developed at
the John Innes Centre, Norwich, UK, and the
research laboratory headed by Dr. Mike Gale
(Chao et al., 1989; Devos et al., 1993). This was
followed by an international effort to map the
wheat genome, with RFLP markers developed
from many laboratories around the world. This
effort was coordinated by the International Triti-
cae Mapping Initiative (ITMI) that was initiated
