WOOSTER, OHIO
OARDC-OSU
HONOLULU, HAWAII
University of Hawaii
COLUMBIA, MISSOURI
USDA-ARS and University of Missouri
HARARE, ZIMBABWE
CIMMYT

Monogenic tolerance to maize streak virus maps to the short arm of chromosome 1
--D. Kyetere, R. Ming, M. McMullen, R. Pratt, J. Brewbaker, T. Musket, K. Pixley, and H. Moon

Maize streak, incited by maize streak geminivirus (MSV), is an important economic disease of maize that occurs in most African countries. Previous genetic studies have reported monogenic (Storey and Howland, Ann. Appl. Biol. 59:429-436, 1967; Soto et al., Ann. Appl. Biol. 100:539-546, 1982) or multigenic inheritance (Kim, Crop Sci. 29:890-894, 1989) of resistance to infection by MSV.

In this study, recombinant inbred lines (RILs) derived from a cross of the tropical inbreds Tzi4 (MSV tolerant) and Hi34 (MSV susceptible) were evaluated for MSV tolerance using controlled leafhopper (Cicadulina mbila, Naude) infestation protocols slightly modified from Leuschner (IITA Res. Briefs 1(2):4-6, 1980). Experiments were conducted at two locations: one glasshouse experiment at Namulonge, Uganda, and two field experiments at the CIMMYT Mid-altitude Maize Research Station in Harare, Zimbabwe. A rating scale of 1 (very few streaks or widely spaced spots) to 5 (severe, systemic streaking and chlorosis) based on the protocol described by Soto et al. (Ann. Appl. Biol. 100:539-546, 1982) was used to assess disease severity of RILs, parental sublines, and the F1 planted in a lattice design with two replications.

Seventy-one RFLP markers provided informative polymorphisms in a total of 87 RILs. The association between marker genotype and MSV tolerance was determined using three analytical approaches: single factor analysis of variance (SFAOV, PROC GLM, SAS), and two interval mapping procedures described by Knapp and Bridges (J. Hered. 81:234-235, 1990) and Lander and Botstein (Genetics 121:185-199, 1989). The recombinant inbred lines were considered as an 'F2 intercross' population for MAPMAKER/EXP 3.0 analysis.

Results of analysis of variance indicated phenotypic tolerance scores of the parents, F1 and RILs were consistent across locations. The F1's reaction was intermediate in the Namulonge experiment and similar to the tolerant parent, TZi4, in the two Harare experiments, suggesting a degree of dominance is associated with MSV tolerance in TZi4.

Results from SFAOV and the two interval mapping approaches showed a significant association of MSV tolerance with four linked RFLP markers, bnl12.06, npi262, umc167, and umc67, found on the short arm of chromosome 1. An R2 of 0.74 was observed for the association between npi262 and MSV mean tolerance score of four ratings (Table 1). The two interval mapping approaches identified the same chromosome 1 region with individual intervals accounting for as much as 90% of the variation in MSV tolerance (Table 2). Analysis using Knapp and Bridges (J. Hered. 81:234-235, 1990) resulted in significance levels of less than 10-17 associated with intervals in the region. Qualitative analysis of data from the final (fourth) rating also was performed. Recombinant inbred lines were classified as tolerant if they were more tolerant, or not significantly less tolerant than, Tzi4; or susceptible if significantly less tolerant than Tzi4. These data fit a chi-square goodness of fit test for a 1:1 Mendelian ratio, further indicating presence of a single gene. Multipoint linkage analysis mapped this gene, designated msv1, between bnl12.06 and npi262 at a genetic distance of 10 cM from bnl12.06 and 3 cM from npi262.

Table1. Results from single factor analysis of variance showing loci significantly associated with MSV tolerance (mean tolerance value based on scores based on four rating intervals) on chromosome 1 in three experiments.
 
Namulonge
Harare (1)
Harare (2)
Locus1 Prob. R2 Prob. R2 Prob. R2
bnl12.0 6 0.009 0.11 0.0001 0.32 0.0001 0.34
npi262 0.0001 0.43 0.0001 0.58 0.0001 0.74
umc167 0.0001 0.32 0.0001 0.46 0.0001 0.56
umc67 0.0001 0.23 0.0001 0.32 0.0001 0.37
php208 55 0.009 0.09 0.010 0.09 0.002 0.12
bnl8.29 0.004 0.10 N.S. --- 0.020 0.06
1Loci listed in relative order on chromosome 1 (Coe, MNL 1993).
Prob. = significant probability level.
N.S. = not significant. R2 = the fraction of phenotypic variation associated with genotypes at respective loci.

Table 2. Results from MAPMAKER/QTL analysis showing marker intervals on chromosome 1 detecting significant variation, peak LOD and percent variation (% var.) for MSV tolerance.
                                            
Namulonge
Harare exp1 Harare exp2
Locus LODb % var.c LOD % var. LOD % var.
bnl5.62-bnl12.06 N.S. --  15.57 90.0 20.99 91.9
bnl12.06-npi262 10.93 52.9 22.33 89.0 27.16 83.6
np1 262-umc167 11.44 56.3 18.93 76.6 26.60 83.5
umc167-umc67 7.96 35.9 10.19 43.5 15.70 72.5
umc67-php20855 N.S. -- 8.25 65.0 10.26 65.0
a Loci in intervals listed in relative order on chromosome 1 (Coe, MNL 1993).
b Maximum LOD between each interval.
c Percent of phenotypic variation explained by respective loci (=R2).
N.S. = not significant.



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