QTLs for degree of pollen-silk discordance, expression of disease lesion mimic, and leaf curl response to drought
--B.E. Zehr, J.W. Dudley and G.K. Rufener

QTL identification has been carried out using a set of 224 S2 progeny derived from the cross of inbred line Mo17 with population BS11(R)C7. Specific RFLP markers and statistical approaches used for QTL analysis are as previously described in a related study by Zehr et al. (TAG 83:903, 1992). Replicated S2 progeny rows were grown at two Illinois locations in both the summers of 1988 and 1989, the former year being a drought environment and the latter year having relatively normal rainfall. Measurement of discordance between average silk emergence and pollen shed was made in 1989 only; data were taken as days from planting. Four RFLP markers showed greatest significance of association with the difference between average pollen and silking dates (Table 1). These data are in partial agreement with those of Phillips et al. (Proc. 47th Annu. Corn Sorgh. Res. Conf., 1992), who described umc12 as having close association with a gene conferring major influence on maturity in corn, as determined by relative date of pollen shed and silk emergence. However, our data did not indicate significance of umc12 for these two maturity measures directly, only in their relative degree of separation.

Table 1. RFLP markers having greatest association with degree of discordance between pollen and silking dates.
 

Chrom. Arm	Marker	Prob. > F
2S	npi239	0.0001
4L	umc66	0.0093
6L	npi223	0.0016
8L	umc12	0.0064

Expression of a disease lesion mimic is characteristic of Mo17 and material derived from this inbred line. In the S2 progeny described above, lesions were evident as brown necrotic spots with clear centers and chlorotic halos on leaf blades. Two levels of expression were generally seen; either numerous small lesions, or relatively fewer yet large oblong lesions due to expanded halo width. Data were taken in the drought stress environment of 1988, which seemed to enhance lesion expression. Plants showing lesion mimic were segregating both within and among progeny rows, and data were recorded as the number of plants per row with easily identifiable lesion phenotype at the time of flowering. Four markers representing three chromosome arms showed highest degree of association (Table 2). Two of the chromosome arms represented contain previously identified disease lesion mimic mutations: Les14 and Les17 on chromosome arm 3L, and Les8 on chromosome arm 9S. The small lesion phenotype observed in this study is very similar to that described for Les14 (Neuffer, MG, MNL 66:39, 1992), suggesting that these two mutations may be allelic. The second phenotype of this lesion mimic (few but big lesions) could be due to the effect of genetic modifiers present in other genomic regions detected in this study (i.e., 3L and 9S). It is well documented that expression of lesions in almost all mimic mutations is highly dependent on genetic background (Walbot et al., Disease lesion mimic mutations, in: Genetic Engineering of Plants, Plenum Pub. Corp., New York, 1983).

Table 2. RFLP markers having greatest association with disease lesion mimic phenotype.
 

Chrom. Arm	Marker	Prob. > F
3L	umc96	0.0088
4L	umc66	0.0094
4L	npi451	0.0003
9S	bz1	0.0013

In the drought environment of 1988, pre-flowering stress response was characterized by leaf curling. Differences in degree of leaf curl were apparent among progeny rows. Data were taken pre-flowering at approximately the eight leaf stage using a rating scale from 1 to 5 on a row average basis; a rating of 1 indicating little or no leaf curl for all plants within a progeny row, and a rating of 5 indicating extreme curling for all plants of a row. Major associations with leaf curl response were found for the 8 markers (Table 3). Mapping studies using maize RFLP probes have shown a large degree of colinearity between the genomes of corn and sorghum (Whitkus et al., Genetics 132:1119, 1992; Melake Berhan et al., TAG 86:598, 1993), sorghum being a related crop with good drought tolerance characteristics. Three RFLP markers with significance in this study (umc27, bnl16.06, npi114) have been mapped directly in sorghum, making possible any future transfer of such information between the two crops.

Table 3. RFLP markers having greatest association with leaf curl response to drought.
 

Chrom. Arm	Marker	Prob. > F
2S	npi239	0.0005
4L	bnl15.07	0.0023
4L	npi451	0.0061
5S	umc27	0.0085
6L	umc38	0.0051
7L	bnl16.06	0.0031
7L	umc80	0.0011
8S	npi114	0.0078

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