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were long maintained on Braxton, a susceptible
cultivar grown in a greenhouse. Recently, when
mapping QTL for resistance to RN in soybean
PI 437654, Ha et al. (2007) employed an RN
population from a culture originating in
Arkansas and maintained on soybean cultivar
Braxton. Wubben et al. (2010a, 2010b) used a
stock culture of an inbred RN population for
their studies of cotton.
Evaluations of host plant reactions to the vir-
ulence of RN nematodes were normally con-
ducted in a greenhouse with ambient tempera-
ture maintained at 28-34
◦
C. Reproduction of RN
indicated by a reproductive index (Pf/Pi) (final
population/initial population) was estimated and
widely used for the identification of resistance
sources (Robbins et al. 2000, 2001, 2002). Alter-
natively, a rating method was also used earlier
(Robbins et al. 1999). Overall, it is likely that
genetic variation for virulence in RN nematode
species is not as complex as other plant-parasitic
nematodes, resulting in the simplicity and effi-
ciency of the evaluation of host plants' reactions
to the virulence of RN.
bean was dependent on the resistance source.
For instance, Peking-derived soybean cultivars
with resistance to SCN were potentially resis-
tant to RN (Caviness and Riggs, 1976), whereas
PI 88788-derived cultivars showed resistance to
SCN but were not resistant to RN (Davis et al.
1996; Robbins et al. 1994; Robbins and Rakes
1996).
Earlier genetic studies showed that RN resis-
tance was controlled by one locus with reces-
sive alleles (Harville et al. 1985; Williams et al.
1981) or by two loci with unequal effects
(Harville et al. 1985). However, in an unpub-
lished study by Pioneer Hi-bred International
(
http://www.pioneer.com)
in an RIL mapping
population derived from PI 437654, two QTL
for effective resistance to RN were identified and
mapped to Chrs. 19 and 11, respectively. Interest-
ingly, this PI was reported to be resistant to both
SCN and RN (Robbins and Rakes 1996). Sub-
sequently, soybean researchers have undertaken
genetic mapping efforts leading to the identifica-
tion and confirmation of many QTL underlying
resistance to multi-races of SCN (Cregan et al.
1999; Webb et al. 1995; Wu et al. 2009). Employ-
ing the same RIL mapping population developed
from a BSR101 x PI 437654 cross, Ha et al.
(2007) refined previously reported QTL regions
with SSR markers on Chr. 19 and identified addi-
tional QTL on Chr. 11 and 18, respectively. These
two new QTL showed an epistatic interaction
and provided the lowest reniform index (RI) in
the presence of both favorable alleles from PI
437654.
In addition to studies of available sources
of resistance to either single or both nematode
species as previously reported, evaluations of
new exotic soybean accessions have been con-
ducted in the NCSB, MU. Among those, PI
438489B, PI 437690, and PI 404198B have been
demonstrated to be highly resistant to both SCN
and RN (Nguyen Lab, unpublished data). QTL
analysis conducted using an F7 RIL population
derived from a Magellan x PI 438489B cross
confirmed a genomic location for resistance to
RN on Chr. 18 (Vuong et al. in preparation),
SourcesofResistancetoRNandQTLMapping
Similar to SCN and RKN management proce-
dures, host plant resistance, in conjunction with
non-host crop rotation, have been the highest
research priorities and the most desirable mea-
sures to control RN damage for efficient soybean
production, in particular following the cancella-
tion of permits for the use of many nematicides
(Boerma and Hussey, 1992). Similar to efforts
being made for SCN and RKN, evaluations of
soybean germplasm for resistance to RK have
been conducted (Davis et al. 1996; Rebois et al.
1968, 1970; Robbins et al. 1999, 2001, 2002).
It was determined that some soybean geno-
types with resistance to SCN could potentially
show levels of resistance to RN, possibly due to
the similarities in the nematode-induced feeding
site formation and parasitic mechanism (Rebois
et al. 1970). However, further investigations also
reported that multi-nematode resistance in soy-
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