Mapping the HtN resistance gene to the long arm of chromosome 8
--Kevin D. Simcox and Jeffrey L. Bennetzen

Single gene resistance to Setosphaeria turcica, causal agent of the northern corn leaf blight in maize, is conditioned by four dominants, Ht1, Ht2, Ht3, and HtN. Ht1 and Ht2 have been previously mapped to the long arm of chromosome 2 (Hoisington and Coe, in Development and Application of Molecular Markers to Problems in Plant Genetics, pp. 19-24, 1989) and the long arm of chromosome 8 (Zaitlin et al., MNL66:69-70, 1992), respectively. The ht3 gene has not been mapped, but was shown by Hooker (MNL55:87-88, 1981) to segregate independently of both ht1 and ht2. No information is yet available on the map location of the HtN gene.

We are interested in studying interactions between genes that specify resistance to S. turcica. The identification of flanking RFLP markers would be useful in confirming allele status in segregating populations. To determine the map position of HtN, race 1 of S. turcica (avirulent on HtN and virulent on Ht1 genotypes) was used to identify resistant and susceptible backcross individuals (W22 HtN/A619 Ht1 x A619 Ht1) by greenhouse screening. Assignment of HtN to a linkage group involved selecting RFLP markers separated by approximately 50cM from each of the ten homologs and hybridizing RFLP clones to membranes containing DNA's from equal numbers of resistant and susceptible backcross progeny. Linkage to HtN was initially detected with umc16, located on the long arm of chromosome 3. Two duplicate loci were detected, umc16a and umc16b. Hybridization of chromosome 3 markers bnl15.20 and bnl10.24a indicated that HtN was linked to umc16b (4cm, SE=3, n=45), but not linked to umc16a (48cM, SE=9, n=33) on chromosome 3. Helentjaris, T and co-workers (Genetics 118:296-299, 1988) mapped duplicate regions of the maize genome and found that the region around umc16 on the long arm of chromosome 3 was duplicated on chromosome 8, near idh1. Linkage of HtN with umc30a (5cM, SE=3, n=60) and umc117 (7cM, SE=5, n=27) placed HtN on the long arm of chromosome 8 in the region of idh1. An additional 99 backcross progeny were used to generate the following map order; umc48 - 9cM - umc30/umc117 - 1.1cM - HtN. The backcross (W22 HtN/gl18 htN v16 j1 x gl18 htN v16 j1 was used to map HtN with respect to morphological markers on the long arm of chromosome 8. No linkage was observed between HtN and gl18, however HtN1 was found to be 13cM (n=336) proximal to virescent16 and 30cM (n=112) proximal to japonica1.

When W22 HtN was crossed with A619 Ht1 the expected 15:1 segregation was observed in the F2 (Table 1). An absence of independent segregation between HtN and Ht2 was observed (Table 1), as expected, since Zaitlin and co-workers (1992) placed Ht2 within the umc48-umc89 interval, near HtN. The recovery of susceptible F2 progeny in the W22 HtN/A619 Ht2 cross indicates that these genes are not allelic. The absence of linkage between HtN and A619 Ht3 confirms the findings of Hooker (1981) that Ht3 is not linked to the region on the long arm of chromosome 8 containing HtN and Ht2 (Table 1). The markers flanking Ht2 will be useful in confirming whether an inbred contains either Ht2 or Ht3. Problems in identification arise since both of these genes have very similar phenotypes when inoculated with an avirulent race of S. turcica, and differential races have not been reported. Upon attempting to map Ht3 out of RB37 Ht3 using interval mapping we discovered that resistance in RB37 "Ht3" mapped near Ht2 (K. D. Simcox and M. D. McMullen, unpublished data). Since both HtN and Ht2 segregate independently of Ht3, we can only assume that our RB37 "Ht3" source contained Ht2 and not Ht3. The contamination of Ht3 material by Ht2 has been seen by others (Dave Zaitlin, personal communication) and represents a serious problem, especially when inappropriate genotypes are used in disease surveys.

Table 1. F2 segregation data between HtN and the Ht1, Ht2, and Ht3 genes.
 

	Observed
Crosses	R	S	Expected ratio	X2 value	P value
W22 HtN/A619 Ht1 F2	148	11	15:1	0.12	0.50-0.75
W22 HtN/A619 Ht2 F2	176	4	15:1	8.4	>0.005
	390	15	15:1	4.5	0.05-0.025
W22 HtN/A619 Ht3 F2	188	12	15:1	0.02	0.90-0.95
	151	11	15:1	0.08	0.75-0.90
	152	3	15:1	4.64	0.05-0.025

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