Marker-Assisted Selection for Biotic Stress Resistance in Peanut - Translational Genomics for Crop Breeding: Biotic Stress - page 134

1989) and M. javanica (Stalker and Moss 1987).
Since then, low to moderate levels of resistance
were discovered in A. hypogaea (Holbrook and
Noe 1992). Diploid crosses involving A. carde-
nasii demonstrated the presence of at least two
dominant resistance genes (Starr and Simpson
1991). Resistance took the form of a hypersen-
sitive response that inhibited the development
of invading juveniles, and resulted in an almost
total suppression of nematode reproduction. In A.
batizocoi and A. diogoi , the mechanism of resis-
tance could not be determined because of the dif-
ficulty in making interspecific test crosses with
a susceptible parent, but resistance was asso-
ciated with a lengthening in the time for juve-
niles to develop into adults and a decrease in the
percentage of juveniles that reached adulthood
(Nelson et al. 1990). Interestingly, the resistance
of the resistant cultivar COAN, which is due
to a single gene derived from A. cardenasii ,is
expressed as a failure of the invading nematode
to initiate a functional feeding site in the vascu-
lar tissues, and many of the invading nematodes
then emigrate from the roots or remain local-
ized in the cortical tissues (Bendezu and Starr
2003). In the root-knot nematode-resistant A.
stenosperma , penetration and development of the
nematodes was dramatically reduced in compari-
son with that occurring in cultivated peanut. Nei-
ther giant cells nor nematodes developed beyond
the second stage were found. Several cell fea-
tures, including darkly staining cytoplasm and
altered organelle structure, were observed in the
central cylinder, indicating a hypersensitive-like
response (HR) of infested host cells (Proite et al.
2008).

1996) and by crosses among diploids followed by
doubling with colchicine to the tetraploid level
(Simpson 1991). The nematode-resistant culti-
var COAN was the first peanut cultivar that con-
tained a distinct trait donated from wild species
(Simpson and Starr 2001). COAN was devel-
oped from the TxAG-6 amphidiploid, crossed
to Florunner and advanced by five cycles of
backcrossing followed by selfing and selection
for root-knot nematode resistance (Simpson and
Starr 2001).

MarkersandUseinSelection

The first markers for an agronomically useful
trait in peanut were for resistance to root-knot
nematode ( M. arenaria )from A. cardenasii .Two
closely linked sequence characterized amplified
region (SCAR) markers were identified for genes
for reduced galling and egg number (Garcia
et al. 1996). Simultaneously, three RAPD mark-
ers were associated with nematode resistance in
several backcross breeding populations derived
from the interspecific hybrid TxAG-6 [ A. batizo-
coi

A. diogoi )] 4x (Burow et al.
1996); however, these were all for the same gene,
and although these did provide flanking markers,
the one marker opposite the other two did not
appear to be qualitatively inherited, but appeared
to differ quantitatively in amplification, and was
thus deemed too difficult to score accurately for
marker-assisted selection (MAS). Instead, two
(non-flanking) RFLP markers ca. 4cM from the
resistance gene were developed by bulked seg-
regant analysis (Church et al. 2000). The use of
non-flanking markers was in part the result of a
large gap (

× ( A. cardenasii ×

30cM) (Burow et al. 2001) between
markers on the other side of the gene.
MAS was used for the development of
NemaTAM, the second nematode-resistant
peanut cultivar (Simpson et al. 2003). The vari-
ety COAN had superior yield under disease
pressure but had low yield under disease-free
conditions. Two additional generations of back-
crossing accompanied by the use of RFLP mark-
ers were used for the development of NemaTAM.

>

Breeding

Before introgression of resistance alleles from
wild species, no root-knot nematode-resistant
peanut cultivars were released, for lack of known
sources of resistant germplasm. Root-knot nema-
tode resistance was introduced into A. hypogaea
from two crosses, that of A. hypogaea x A. car-
denasii via the hexaploid route (Garcia et al.