Biology Reference
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NemaTAM had the same markers for nema-
tode resistance as were present in COAN, but
it and other selected breeding lines had mean
yields under disease-free conditions that were
135% to 160% higher than COAN had (Church
et al. 2000). It was concluded that the link-
age between resistance and low yield had been
broken. However, scores of flanking markers
were unavailable, and as such, it was never
demonstrated whether the difference between
COAN and NemaTAM resided on the chromo-
some containing the resistance gene or on a dif-
ferent chromosome.
Several benefits were observed in use of mark-
ers for development of the nematode-resistant
variety NemaTAM (Choi et al. 1999; Church
et al. 2000; Simpson et al. 2003; Cason et al.
2010). It was demonstrated that use of mark-
ers was more efficient than phenotypic selec-
tion, because plants selected by markers for the
homozygous resistance gene bred true, unlike
materials selected based on phenotype (which
included heterozygous plants). Also, markers
were more accurate, because phenotypic selec-
tion was accompanied by a certain amount of
escapes. Although MAS would be affected by
recombination between marker and the trait
being scored, this rate of crossing over was less
than the rate of assigning incorrect phenotypes.
In addition, scoring could be performed on col-
lected leaf tissue, eliminating the need to harvest
the plant to perform nematode egg counts. How-
ever, the use of the RFLP marker was costly and
required a large amount of DNA, radioisotope,
and a long time (one to four weeks) before devel-
oping the X-ray film. Church et al. (2000) were
able to determine the genotype of only 65-86%
of the individuals attempted because of techni-
cal difficulties, such as the low quality or quan-
tity of DNA, incomplete digestion of DNA, or
poor hybridization or background on Southern
blots. A nonisotopic method was used by Muitia
et al. (2006), but this was more cumbersome and
expensive than the use of radioisotope.
MAS was also used in development of a
nematode-resistant, high-oleic variety from Tif-
guard. Tifguard is a nematode-resistant cultivar
that also has resistance to tomato spotted wilt
virus (Holbrook et al. 2008). Because of the cost,
and difficulties associated with the RFLP marker
technology, Tifguard was developed using stan-
dard phenotypic selection methods, using COAN
as donor parent for nematode resistance (Hol-
brook et al. 2008). It would be desirable to
have a high oleic peanut cultivar with the
disease-resistant package available in Tifguard.
Based on the development of improved molec-
ular markers, it was decided to use MAS to
develop Tifguard High O/L.
Improved markers were developed for the
root-knot nematode resistance gene, and were
used in development of Tifguard High O/L. Nagy
et al. (2010) performed high-resolution mapping
of nematode resistance with breeding material
derived from the synthetic tetraploid pathway in
comparison with an A-genome diploid species
map. Twelve polymorphic markers and a previ-
ously published sequence characterized ampli-
fied region marker S197 (Chu et al. 2007a),
developed from the published sequence of RAPD
marker RKN440 (Burow et al. 1996), were found
to be tightly linked with Rma in populations from
two tetraploid crosses. During the breeding pro-
cedure to develop Tifguard High O/L, a domi-
nant marker S197 (resistant allele), a dominant
CAPS marker 1169/1170 (susceptible allele),
and a codominant simple sequence repeat (SSR)
marker GM565 (Nagy et al. 2010) were used
(Chu et al. 2011). This allowed for the identi-
fication of homozygous resistant, homozygous
susceptible, and heterozygous individuals.
Two homoeologous genes (
ahFAD2A
and
ahFAD2B
) encode for the key enzyme regulating
the O/L ratio in peanut (Ray et al. 1993). Dou-
ble recessive mutants are needed for the expres-
sion of the high O/L trait. A mutation in the
ahFAD2A
is prevalent in
A. hypogaea
subsp.
hypogaea
(Chu et al. 2007b), and all parents used
in the development of Tifguard High O/L carry
this mutant allele. A cleaved amplified polymor-
phic sequence (CAPS) marker 1101/1048 (Chu
et al. 2009) was used to identify breeding lines
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