Biology Reference
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populations as race 1 on the basis of reaction
on host differential plants. However, these pop-
ulations differed significantly in their parasitism
on tomato (Canto-Saenz and Brodie 1986). In
another study, Luzzi et al. (1987) used Mi race
3, established by combining three highly virulent
populations from the southeastern United States,
and successfully identified many new sources of
resistance to Mi nematode.
Peanut Ma nematode has been shown to be
very diverse in its morphology and cytological
profiles (Cliff and Hirschmann 1985). Moreover,
based on the host pathogenicity in peanut, two
races of this nematode species were identified:
race 1, which infects peanut, and race 2, which
does not (Sasser 1972). In soybean, both Ma
races can produce eggs, but race 2 is more aggres-
sive, damaging, and fecund than race 1 (Pedrosa
et al. 1994).
of Ma resistance in soybean. Recently, among
new exotic soybean germplasm identified to be
highly resistant to multiple nematode species,
PI 438489B was demonstrated to be resistant to
three species, SCN, RKN, and RN (Shannon,
per. comm.), and was subsequently employed
in genetic mapping of resistance to RKN and
RN (Vuong et al. in preparation; Xu et al.
2013).
In efforts to genetically locate genomic
regions conferring resistance to RKN species,
Tamulonis et al. (1997a) analyzed RFLP mark-
ers in an F2 mapping population developed from
PI 96354, mapping two QTL for resistance to Mi
nematode, a major one on Chr. 10 and a minor
one on Chr. 18. These QTL explained 31% and
14% of the phenotypic variation, respectively.
These two QTL were later confirmed by Li et al.
(2001) by analyzing SSR markers in a mapping
population derived from the same cross of PI
96354. Ha et al. (2004) reported that the flank-
ing SSR markers on Chr. 10 also co-segregated
with
Rmi1
, a gene conferring Mi nematode resis-
tance. Subsequently, Ha et al. (2007) developed
two SNP markers that have been shown to be
highly effective in marker-assisted selection for
Mi nematode resistance. In a separate genetic
mapping study, Tamulonis et al. (1997b) ana-
lyzed an F2:3 population developed from PI
200538 and identified two genomic locations for
resistance to Ma nematode, a major one on Chr.
13 and a minor one on Chr. 15. These QTL in
combination accounted for 51% of the pheno-
typic variation in gall number.
Analyzing the 1,536 soybean SNP array (the
USLP 1.0) (Hyten et al. 2010) in an F7 recombi-
nant inbred line (RIL) population derived from
PI 438489B, Vuong et al. (unpublished data)
detected and mapped QTL for Mi resistance on
Chrs. 8, 10, and 13, consistent with previously
reported QTL (Tamulonis et al. 1997a; Li et al.
2001). Lately, with whole genome sequencing
(WGS) technology applied to the same genetic
population, these QTL were also detected and
consistently mapped to the same genomic loca-
tions. Of these, a major QTL on Chr. 10 was
SourcesofResistancetoRKNandQTL
Mapping
In an evaluation of a subset of plant instructions
from the USDA Soybean Germplasm Collection,
Luzzi et al. (1987) identified many new sources
of resistance to RKN species, in which PI 96354
was highly resistant to Mi nematode, while PI
200538 and PI 230977 were highly resistant to
both Ma (race 2) and Mj nematodes. In a green-
house test, although not different in gall index
and Ma reproduction, PI 200538 was less dam-
aged by Ma under field conditions than was PI
230977 (Pedrosa et al. 1994). In another eval-
uation, Harris et al. (2003) reported additional
sources of resistance to Ma. Of these, PI 594403,
PI 594427C, and PI 594651B were shown to
be potentially unique resistance accessions to
this nematode species. Recently, Yates et al.
(2010) utilized three F2 populations developed
from crosses of previously reported PI 200538
and newly identified PI lines, PI 594403 and PI
594651B, to characterize Ma resistance sources.
The authors concluded that these PIs contained
unique resistance genes that, when combined
with known PI 200538, could improve the level
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