Linkage of RFLP markers to genes controlling resistance to southern corn rust
--J. B. Holland, D. V. Uhr and M. M. Goodman

Inheritance of resistance to southern corn rust (Puccinia polysora) in two maize populations was studied using RFLP markers. Two 100% tropical inbred lines (1416-1 and 1497-2) adapted to temperate environments had been identified as highly resistant to southern rust in a previous study (Uhr, Ph.D. thesis, North Carolina State University, 1991). Two populations segregating for resistance to southern rust were developed by crossing each resistant line to a susceptible hybrid, B73xMo17, and self-pollinating a single (F1) plant from each cross. 145 F2 plants from each population were scored for disease phenotype under natural infestation using a 1-9 scale (1=highly susceptible, 9=highly resistant). Each plant was also selfed to produce F2 families in the F3 generation. The F2 families were then scored for disease phenotype under natural infestation in a replicated field trial using natural inoculum.

Based on these phenotypic scores, the most resistant and most suceptible families in each population were chosen for genotypic evaluation. Specifically, 32 resistant and 13 susceptible families were chosen from the 1416-1 population, and 33 resistant and 20 susceptible families were chosen from the 1497-2 population. Leaf tissue from ten plants of each family was bulked to isolate genomic DNA representing each family. RFLP probes marking regions of the genome known to contain rust resistance loci (either Rpp or Rp genes) were used to determine genotypes of the families. Single factor ANOVA's were used to determine which markers were linked to chromosomal regions that significantly affected disease resistance (Table 1).

Table 1. Additive (a) and dominance (d) effects and coefficients of determination (R2) of RFLP markers on southern rust resistance. (Note that negative a effects indicate B73xMo17-contributed resistance allele.)
 

		1416-1 population			1497-2 population
Chromosome	Marker	a	d	R2	a	d	R2
3	umc10	ns	ns	-	nd	nd	nd
3	umc161	ns	ns	-	-0.38**	1.30**	0.08
3	umc102	nd	nd	nd	-1.24**	-0.73*	0.06
3	umc26	0.74*	-1.04*	0.11	-1.48**	-0.37*	0.11
4	bnl5.46	-0.24**	0.36**	0.01	-1.13**	ns	0.09
10	bnl10.17	1.93**	1.03**	0.52	nd	nd	nd
10	bnl3.04	1.94**	ns	0.57	nd	nd	nd
10	npi285	1.98**	0.64**	0.57	2.11**	ns	0.41
10	umc130	1.54*	ns	0.27	1.75**	ns	0.22

*,** - significant at the p=0.05, p=0.01 levels, respectively.
ns - not significantly different from 0 at p=0.05.
nd - no data available yet.

Markers on the short arm of chromosome 10 showed the largest effects in both populations, indicating that each resistant inbred parent carries a major gene or genes in this region. It is possible that the resistance could be caused by alleles of Rpp9 on chromosome 10. Genes with smaller effects on resistance to southern rust also appear to be located on chromosomes 3 and 4. Although major common rust (P. sorghi) resistance genes are known to be located in these genomic regions, to our knowledge this is the first report indicating that southern rust resistance genes exist on these chromosomes. 

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