GEM Germplasm – Unlocking Maize Diversity for Improving Host Plant Resistance
against West Corn Rootworm and Other Biotic Stresses
Principle Investigator:
Reporting Period:
Martin Bohn
Associate Professor for Maize Breeding and Genetics
University of Illinois, Urbana, Illinois
2011
The Western corn rootworm (WCR) is a serious pest of maize in the U.S. and of
growing importance in Europe (Gray et al. 2009. Ann. Rev. Entomol. 53:303-321). Soil
insecticides, the use of transgenic maize hybrids producing a Bt toxin, and the rotation of
maize with other crops are successfully used to manage WCR populations. However, the
occurrence of WCR populations resistant to insecticides and adapted to cultural practices
raises the question of how sustainable transgenic control measures against WCR are.
Recent reports by Meihls et al. (2008. PNAS 105:19177-19182) and Lefko et al. (2008. J.
Appl. Entomol. 132:189-204) demonstrated the development of Bt toxin resistant WCR
populations under greenhouse conditions. The authors concluded that selection for Bt
resistance in WCR populations will be rapid if resistant WCR beetles do not have the
chance to mate with individuals unexposed to the Bt toxin. In 2011 first reports emerged
describing unexpected and extensive WCR root feeding damage on WCR-Bt hybrids in
Iowa. Gassman et al. (2011. PLoS ONE 6(7): e22629. doi:10.1371/journal.pone.0022629)
studied WCR populations obtained from these damaged fields and found a significantly
increased survival on maize hybrids carrying the Cry3Bb1 gene. Gassman et al. (2011)
explained the development of this field-evolved resistance to a Bt toxin by insufficient
planting of refuges and a potential non-recessive gene action of the resistance. Therefore,
pest management systems integrating well designed refuges and multigenic host plant
resistance will play an important role to effectively manage WCR populations.
A major roadblock towards improved resistance to biotic stresses of maize is the
apparent lack of genetic variation for these traits in the Midwestern maize pools. This lack
of diversity is in stark contrast to the diversity available for improving other quantitative
and agronomically important traits, like grain yield, early maturity, or grain composition
and emphasizes the need for broadening the genetic basis of the Midwestern maize pools
using exotic maize germplasm. The overall goal of this project is the development of maize
cultivars, i.e., inbreds and populations, adapted to the U.S. Corn Belt, with improved host
plant resistance against WCR by broadening the genetic basis for these characteristics
using exotic maize germplasm.
Project Activities
A trap crop strategy was used to ensure high WCR population densities in all fields
(University of Illinois, Crop Sciences Research and Education Center, Urbana) used for
germplasm screening and evaluation in 2011. In 2010, all fields for the summer season
2011 were planted with a corn-pumpkin mix in late June. This ensured that at the end of the
growing season WCR beetles preferentially laid their eggs in these fields. To account for
the spatial variation of egg laying, we use three replications in our field trials, duplicate the
WCR screening nursery, and grow rows of susceptible and resistant checks in regular
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distances throughout the trap crop field. In order to be able to continue the WCR resistance
breeding program in 2012, new WCR trap crops (a total of 4 acres) were established in
Mid-June 2011. Based on the extent of WCR larvae hatch and adult emergence in our 2010
trap crop field and the resulting root damage, we concluded that the trap crop procedure
was successful and provide us with adequate WCR population densities for germplasm
screening.
As in previous years the work in the summer season 2011 was divided in
germplasm development and germplasm evaluation related activities.
Germplasm Development – A WCR nursery with 1,200 rows was planted May 26,
2011. This late planting was caused by cool and wet spring conditions. All materials,
comprised of crosses made in the 2010 nursery with the goal to combine different WCR
resistance genes from diverse exotic backgrounds, segregating F2 populations, genotypes in
different selfing generations as well as double haploids developed in 2010, were exposed to
high WCR pressure. In addition, 41 new GEM based F1 hybrids derived from crosses
between inbreds developed from AR16026:N1210, UR13085:N0204, CUBA117:S1520,
and FS8A(S):S0907 source populations were selfed to from new segregating F2
populations.
Genotypes with adequate root lodging resistance and row performance scores were
selfed. New GEM germplasm derived inbreds selected based on their exceptional
combination of biotic stress resistance and agronomic performance in 2010 were crossed to
form new base populations. In addition, we utilized DH lines that showed improved
resistance in our QTL studies as new source materials in the WCR breeding program. A set
of 50 GEM derived S4 lines with improved WCR resistance was testcrossed with one tester
line and a set of 10 S6 lines was crossed with three testers. Cycle 1 of the Illinois WCR
Synthetic was grown and 100 randomly selected plants were selfed to continue the recurrent
selection program using S1 progeny.
Germplasm Evaluation – A set of 41 new GEM based F1 hybrids derived from
crosses between inbreds developed from AR16026:N1210, UR13085:N0204,
CUBA117:S1520, and FS8A(S):S0907 source populations were evaluated for their levels
of WCR resistance (Tab. 1). Root damage ratings (RDR) varied between 0.69 and 3.00
with an overall mean of 1.97. As expected for F1 hybrids root sizes were large with root
size ratings (RSZ) ranging from 1.10 to 3.55. All genotypes showed moderate root
regrowth (RRG) varying between 2.47 and 5.46. Root damage ratings were significantly
correlated with RSZ (rp=0.65). We observed no significant relationship between RRG and
the other root characteristics. A subset of 17 F1 hybrids were as resistant as resistant check
PHZ51× NGSDCRW1 (RDR = 1.52). In order to test the hypothesis that teosinte carries
resistance genes against WCR feeding, we tested a set of testcrossed teosinte introgression
lines developed by Dr. Flint-Garcia (USDA-ARS, Columbia, Missouri) for their level of
resistance to WCR feeding.
Bioassay Development - In order to continue the selection of WCR resistant
germplasm during the winter season, we started to develop a bioassay. Plants germinated in
growth pouches (Fig. 1) were infested with a defined number of WCR larvae. The
following insect traits were evaluated seven days after WCR infestation: (1) head capsule
size [µm], (2) larvae weight [mg], and (3) number of live larvae recovered. The number
2
and length [cm] of primary, lateral, and seminal roots were evaluated and for each root type
the amount of WCR larvae feeding damage was recorded. In addition, we collected root
tissue from infested and non-infested control plants for a subsequent metabolite analysis.
Previous studies in our laboratory identified a set of metabolites that distinguish WCR
infested roots from mechanically wounded and untreated control plants (Hauck et al. 2011,
submitted). In order to relate our results with previous findings in the literature, we selected
for the bioassay development a subset of maize genotypes used by Khishen et al. (2009, see
2009 report). GEM-derived inbred AR17056-16, which showed in field experiments
improved levels of WCR resistance, is a member of this subset. A first analysis of insect
and plant traits revealed significant differences between genotypes. A preliminary
multivariate analysis indicates to separate maize genotypes according to their previously
known field-level resistance to WCR. The growth pouch method allows for evaluating not
only the damage caused by WCR larvae, but we were also able to observe larvae feeding
behavior and development over time (Fig. 2). We envision that this bioassay will help us to
(1) pre-select germplasm in the winter season before field tests are conducted with selected
materials in the summer season and (2) to study WCR resistance mechanisms in selected
maize cultivars.
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TABLES
Table 1 Means for 41 maize hybrids developed from crosses
between inbreds derived from GEM breeding crosses evaluated for
their resistance to western corn rootworm larvae feeding in a field
experiment conducted in Urbana, IL, in 2011.
Genotypes
CUBA117:S1520_C× FS8A (S):S0907_A
CUBA117:S1520_C× FS8A (S):S0907_B
CUBA117:S1520_C× FS8A (S):S0907_C
CUBA117:S1520_C× FS8A (S):S0907_D
CUBA117:S1520_G× FS8A (S):S0907_A
CUBA117:S1520_G× FS8A (S):S0907_B
CUBA117:S1520_G× FS8A (S):S0907_C
CUBA117:S1520_G× FS8A (S):S0907_D
CUBA117:S1520_H× FS8A (S):S0907_A
CUBA117:S1520_H× FS8A (S):S0907_B
CUBA117:S1520_H× FS8A (S):S0907_C
CUBA117:S1520_H× FS8A (S):S0907_D
CUBA117:S1520_N× FS8A (S):S0907_A
CUBA117:S1520_N× FS8A (S):S0907_B
CUBA117:S1520_N× FS8A (S):S0907_C
CUBA117:S1520_N× FS8A (S):S0907_D
CUBA117:S1520_O× FS8A (S):S0907_A
CUBA117:S1520_O× FS8A (S):S0907_B
CUBA117:S1520_O× FS8A (S):S0907_C
CUBA117:S1520_O× FS8A (S):S0907_D
UR13085N0204_A× AR16026:N1210_H
UR13085N0204_A× AR16026:N1210_L
UR13085N0204_A× AR16026:N1210_M
UR13085:N0204_B×AR16026:N1210_H
UR13085:N0204_B×AR16026:N1210_L
UR13085:N0204_B×AR16026:N1210_M
UR13085:N0204_C× AR16026:N1210_H
UR13085:N0204_C× AR16026:N1210_L
UR13085:N0204_C× AR16026:N1210_M
UR13085:N0204_D× AR16026:N1210_H
UR13085:N0204_D× AR16026:N1210_L
UR13085:N0204_D× AR16026:N1210_M
UR13085:N0204_E× AR16026:N1210_H
UR13085:N0204_E× AR16026:N1210_L
UR13085:N0204_E× AR16026:N1210_M
UR13085:N0204_F× AR16026:N1210_H
UR13085:N0204_F× AR16026:N1210_L
UR13085:N0204_F× AR16026:N1210_M
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RDR
0-3
1.13
2.85
2.58
0.88
1.20
2.07
2.87
2.94
2.81
3.00
2.04
2.58
2.18
1.46
1.57
2.96
2.66
2.87
1.10
1.24
2.66
1.38
1.08
1.16
2.19
2.82
2.78
2.03
1.09
1.24
1.99
1.63
2.87
2.90
1.75
1.41
1.61
2.89
Traits1
RSZ
RRG
--------- 1-6 ----2.19
3.60
2.49
3.70
3.01
4.29
1.68
3.53
2.21
3.41
2.47
2.68
3.04
3.66
2.90
5.46
2.50
3.22
2.62
3.61
2.25
3.34
2.30
3.78
1.10
3.44
1.43
4.15
2.34
3.93
3.18
3.42
2.69
3.95
1.50
3.92
2.48
3.35
1.58
2.65
2.58
3.10
2.93
3.45
1.52
2.47
1.40
2.64
2.10
3.32
2.69
4.40
2.71
3.39
3.55
3.96
1.80
3.88
1.37
3.69
1.42
3.58
1.65
2.50
2.91
4.40
2.89
2.94
2.36
3.77
1.42
2.92
2.13
3.54
3.23
3.07
UR13085:N0204_G× AR16026:N1210_H
UR13085:N0204_G× AR16026:N1210_L
UR13085:N0204_G× AR16026:N1210_M
RDR
0-3
0.84
0.69
0.72
Traits1
RSZ
RRG
--------- 1-6 ----1.30
4.06
1.38
4.15
1.49
3.26
PHZ51× NGSDCRW1
B37×H84
Bt-Check
Bt-Check Isoline (non-Bt)
1.52
2.60
0.16
2.54
-/3.86
2.03
2.94
Genotypes
-/5.90
5.13
3.71
Mean (without Checks)
1.97
2.21
3.55
Standard Deviation
0.81
0.66
0.59
Repeatability (%)
0.47
0.42
0.40
1
RDR, root damage rating, 0-3 node injury scale; RSZ, root size, 1=large root
system, 6=small root system; RRG, root re-growth, 1=intensive re-growth, 6=
no regrowth).
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FIGURES
Fig. 1 Growth pouch system used to screen maize genotypes for their level of WCR
resistance.
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Fig. 2 WCR larvae feeding on embryogenic maize root systems observed in growth
pouches. A. Second instar WCR larva probing a root. B. Circles indicate WCR
larvae feeding damage. C. WCR larvae tunneling in a root as indicated by arrows.
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Invited Presentations
Bohn, M. 2011. Native Resistance of Maize Against the Western Corn Rootworm
(Diabrotica virgifera virgifera LeConte). XXII Eucarpia Maize and Sorghum
Conference. June 19-22, 2011, Opatija, Croatia.
Bohn M. 2011. What do we know about the genetic basis of native resistance in maize to
the western corn rootworm? P-IE Section Symposium: The Future is Now: Blended
Refuge, Resistance, and Rootworm Options for Tomorrow. The 59th Annual Meeting
of the Entomological Society of America. November 13-16, 2011, Reno, Nevada.
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