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compared with greenhouse screening. It is likely
that the cost of MAS will further decrease as
SNP markers become more widely available.
As marker technologies continue to improve,
the cost will be further reduced as new geno-
typing platforms become more available (Con-
cibido et al. 2004). Pioneer Hi-Bred International
and Monsanto Corporation routinely use marker-
assisted breeding for SCN resistance.
There are 118 known sources of
G. max
acces-
sions resistant to SCN that have been identi-
fied (Arelli et al. 2000; Rao-Arelli 1997). These
sources are all unadapted exotic germplasm and
have resistance to one or several SCN HG
types. Of these, only eight have been commonly
used to develop soybean cultivars or germplasm.
These include Peking, PI 88788, PI 90763,
PI 437654, PI 209332, PI 89772, PI 87631-1,
and PI 438489B. There have been efforts to
study whether a wild soybean species, Glycine
soja
,
has novel SCN resistance genes (Wang
et al. 2001; Winter et al. 2007). This species is
widely distributed in China, Japan, Korea, Tai-
wan, and eastern Russia and is believed to be the
ancestor of cultivated soybean (Hymowitz and
Singh 1987). Wang et al. (2001) found a novel
SCN resistance QTL from
G. soja
accession PI
468916 for resistance to HG type 0 (race 3). It has
been shown that stacking resistance alleles from
wild and domestic soybean sources results in a
higher level of SCN resistance (Kim et al. 2011).
Developing high-yielding cultivars using PIs
as SCN-resistant sources can be difficult. The
difficulty is due to three or four major genes,
plus minor genes for resistance, and the low
recovery of high-yielding progeny. The process
is complex because of the number of unde-
sirable traits from SCN-resistant PIs, such as
low yield, lodging, shattering, and susceptibil-
ity to other pathogens. Commonly, two or more
breeding cycles are necessary to recover SCN-
resistant genotypes with good productivity and
other desirable agronomic traits (Shannon and
Anand 1997). In addition, SCN resistance can
become diluted up to a point due to the recom-
bination of resistant and susceptible genes, as
crosses and selection become more removed
from the original resistance source (Shannon
et al. 2004). This results in soybean cultivars
or germplasm that lack the level of resistance of
the original PI.
Productive varieties resistant to SCN are
numerous; however, nearly all varieties with
resistance trace to exotic lines Peking and PI
88788 (Concibido et al. 2004; Diers and Arelli
1999). With the widespread deployment of SCN-
resistant genes from these two sources, nematode
populations have responded with changes in par-
asitism causing HG-type shifts, resulting in these
sources of resistance being less effective against
SCN (Niblack et al. 2003). Broad-based resis-
tance to different SCN HG types would be partic-
ularly beneficial in breeding to limit the damag-
ing consequence of potential race shifts in nema-
tode populations in the field. The HG type 1.2.5.7
(race 2), which can attack “PI 88788 type” resis-
tance, is now a prominent SCN population in
southeast Missouri and other U.S. states (Niblack
et al. 2003). Soybean scientists at the Univer-
sity of Missouri screened the USDA Soybean
Germplasm Collection and found that a black
seed exotic accession, PI 437654, had the highest
level of resistance to all HG types of SCN stud-
ied (Arelli et al. 1997). The SCN-resistant variety
Hartwig, with broad resistance to SCN HG types
from PI 437654, was developed and released
(Anand 1992). Productive group V varieties,
such as Anand (Anand et al. 2001), Stoddard,
Jake (Shannon et al. 2007a, 2007b), and LD00-
2719P (Diers et al. 2010), which trace to PI
437654, have been developed with broad resis-
tance to SCN HG types. However, only a few cul-
tivars trace to PI 437654 via cultivar Hartwig. In
spite of this broad resistance, some populations
of the nematode have been found that are capable
of overcoming Hartwig-type resistance (Young
1999). Thus, it is evident that breeding for
SCN resistance will remain a constant challenge
because of parasitic variation in nematodes.
Some soybean germplasms show tolerance to
SCN. A soybean line is considered tolerant to
SCN if infected plants yield almost as well as
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