Biology Reference
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Some of the known components of resistance
are subject to additive gene action (Sharief et al.
1978; Kornegay et al. 1980). Anderson et al.
(1986) found significant differences among F
1
hybrids between relatively resistant and suscep-
tible parents and argued that resistance to leaf
spot may be controlled not only by recessive
genes, but also by epistatic and additive alle-
les. In early leaf spot resistance studies, stabil-
ity of resistance components has been found to
vary across growing regions due to environmen-
tal interactions (Waliyar et al. 1993; Chiteka et al.
1997) as well as to differences in pathogen pop-
ulations (Waliyar et al. 1993) or to both (Chiteka
et al. 1997).
Heritability values for both diseases are
reported to range from low to high depending
on the resistance level of the parents used in the
study, making selection in early generations inef-
fective in crosses resulting from parents with low
heritabilities (Jogloy et al. 1987). Anderson et al.
(1991) reported that values for broad-sense heri-
tability for lesion number, sporulation, and defo-
liation rating for early leaf spot were 0.57, 0.16,
and 0.56 while those for late leaf spot were 0.74,
0.54, and 0.88, respectively. For narrow-sense
heritability for early leaf spot, the values were
0.18 and 0.53 for lesion number and sporulation.
The corresponding values for late leaf spot were
0.74 and 0.26. Values for a second cross were
generally lower in greenhouse studies, suggest-
ing that dominance and epistatic genetic vari-
ance are substantial. In field studies, Iroume and
Knauft (1987) obtained values of 0.16 to 0.26 for
necrotic area and defoliation from segregating
materials and attributed the variation between
different crosses to relative differences in sus-
ceptibility levels of the parents used for each
cross. Broad-sense (H
bs
) and narrow-sense (h
2
)
heritability estimates for pod yield in peanut are
reported to range from 28% to 82% and from
16% to 79%, respectively (Wynne and Gregory
1981; Wynne and Coffelt 1982). Combining abil-
ity estimates from a diallel cross indicated that
GPBD-4 and ICG (FDRS) 79 were among the
best parents for this trait. GPBD-4 was derived
from a cross KRG 1
ICGV 86855; the latter
is an interspecific line (CS 16) developed previ-
ously from a cross between
A. hypogaea
and
A.
cardenasii
(see Gowda et al. 2002; Stalker et al.
2002a).
High levels of resistance have also been asso-
ciated with low yield, suggesting linkage or
pleitropic effects (Iroume and Knauft, 1987),
which means that breeding for high-yielding
cultivars with resistance requires this linkage
to be broken. Iroume and Knauft (1987) sug-
gested this can be done in early generations
under high disease pressure using an index that
combines yield and disease severity traits. This
was corroborated by Anderson et al. (1986) who
reported effectiveness of selection in F
2
plants.
Recently selection for leaf spot resistance, yield,
and cultivated phenotype over four generations
of progeny from a panel of BC
1
s derived from
A.
hypogaea
cv. IAC-Runner-886 x (
A. ipaensis
x
A.
duranensis
)
4x
yielded 12 agronomically adapted
lines with improved disease resistance compared
to the recurrent parent (Galhardo et al. 2011).
In this latter breeding scheme the apparently
normal genetics of the progeny from this cul-
tivated x wild cross was very helpful in obtain-
ing the desired lines. In general, tapping useful
alleles from wild species is hampered by link-
age drag between desired and unadapted wild
alleles requiring several cycles of backcross-
ing to recover most of the desirable agronomic
traits. This is not only difficult but can be costly
and time-consuming. Use of molecular mark-
ers can facilitate the identification, localization,
and genetic dissection of loci that control quan-
titatively inherited traits such as yield (Tanksley
and Nelson 1996) to speed up utilization of wild
alleles.
×
Markers
Markers for additional traits have been devel-
oped. Stalker and Mozingo (2001) identified
three RAPD markers associated with early
leafspot lesion diameter in a peanut population
derived from a cross between an
A. hypogaea
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