FORT COLLINS, COLORADO

Colorado State University

VICTORIA, BRITISH COLUMBIA

University of Victoria

Linkage distance between whp1 and ch1 --Byrne, PF, Styles, ED Although the loci whp1 (white pollen1) and ch1 (chocolate pericarp1) have both been mapped to the distal end of the long arm of maize chromosome 2 (bin 2.09), the loci apparently had not been mapped in the same population. This region of chromosome 2 became of interest after large QTLs for silk maysin (a C-glycosyl flavone) concentration and corn earworm antibiosis were detected there (Byrne et al., Crop Sci. 38:461-471, 1998). whp1 is a plausible candidate for the QTLs because it encodes chalcone synthase, which catalyzes the first step in the flavonoid pathway. Chalcone synthase activity is required for maysin synthesis, but it is not clear whether whp1, c2 (which also encodes chalcone synthase), or both contribute to maysin synthesis in silks. Plants that are double homozygous recessive at whp1 and c2 produce white, infertile pollen.

Ch1 is a dominant factor for chocolate (tan to dark brown) pericarp and acts independently of p1, the transcription activator that controls expression of most other pericarp pigments. The biochemical basis of ch1 is not known, but because its pigmentation is similar to that produced by other flavonoid pathway mutants (e.g., bp1, A1-b), we felt that it too should be considered a candidate locus for the maysin and antibiosis QTLs.

Independent of the interest in maysin, Derek Styles had developed the following stocks to obtain estimates of the linkage distance between whp1 and ch1:

(whp1/whp1 Ch1/ch1 P-ww/P-wr R-g/(R-r) A3/a3 c2 b1-/pl1)

x (Whp1/Whp1 ch1/ch1 P-rr/(P-ww) R-r c2 Pl1 in1 gl1)

(whp1/whp1 Ch/- (P-wr)/P-ww R c2 b1 -/pl1)

x (Whp1/Whp1 ch1/ch1 P-rr/(P-ww) R-r c2 Pl1 in1 gl1)

(whp1/whp1 Ch/- P-ww/(P-rr) R-g/(R-r) c2 b1 -/pl1)

x (Whp1/Whp1 ch1/ch1 P-rr/(P-ww) R-r c2 Pl1 in1 gl1

Seed of these stocks were planted in Columbia, MO in 1996 and used to pollinate c2 whp1 ch1 plants. At harvest, pollen parents with chocolate pericarps were identified and crosses involving those plants were saved.

In 1997, plants from six ears of the crosses were planted in Fort Collins, CO. They were evaluated for pollen color at anthesis and for pericarp color at harvest. Plants arising from parental-type gametes would be either white pollen plus chocolate pericarp, or yellow pollen plus non-chocolate pericarp, whereas recombinant gametes would give rise to plants that were white pollen plus non-chocolate pericarp, or yellow pollen plus chocolate pericarp. Results were as follows:
 
    Number of plants per classa  
Stock No. of ears evaluated W, Ch W, Cl Y, Ch Y, Cl or R Recombinant fraction
1 1 80 15 16 95 31/206 = 0.150
2 3 46 11 9 49 20/115 = 0.174
3 2 39 14 4 62 18/119 = 0.151
Pooled 6 165 40 29 206 69/440 = 0.157

a W, Ch = white pollen, chocolate pericarp

W, Cl = white pollen, clear pericarp

Y, Ch = yellow pollen, chocolate pericarp

Y, Cl or R = yellow pollen, either clear or red pericarp

The recombinant fraction for the three stocks, ranging from 0.150 to 0.174, and the value for the pooled data (0.157) correspond well to the inferred linkage distance of 14 cM in MaizeDB (Genetic Map of chromosome 2). There may have been some confounding of the Y,Ch and Y,R classes, because chocolate pigmentation is reportedly obscured in red pericarps. However, it was generally possible to distinguish between purely red pericarps and those with both red and chocolate pigments. If some Y,Ch plants were mistakenly classified as Y,R, the result would be an underestimation of the true linkage distance.

Based on these results, the position of the QTLs proximal to whp1, and the position of ch1 distal to whp1, we conclude that whp1 is a better candidate than ch1 for the maysin and antibiosis QTLs. Other strategies are currently being pursued to determine the influence of whp1 and c2 on maysin concentration (M. McMullen, personal communication).
 
 


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