RFLP analysis of isogenic lines B14 and B14A --E.A. Lee, M. Lee, and K.R. Lamkey
 
B14 and B14A are isogenic lines distinguished from each other by the greater degree of resistance to corn leaf rust (Puccinia sorghi) for B14A. B14A was developed through eight generations of backcrossing and selection for rust resistance using B14 and Cuzco as recurrent and donor parents, respectively. The objective was to transfer an allele for rust resistance at the Rp1 locus from Cuzco to B14 and recover the agronomically desirable features of B14 (Russell, Crop Sci 5:95-96, 1965). Morphologically, B14 and B14A appear to be identical. Theoretically, B14A should contain 99.8% of the nuclear genome of B14.

The two lines were examined for RFLPs with 58 mapped clones in combination with three restriction enzymes, EcoRI, HindIII, and EcoRV. With EcoRI digests, 98.3% of the clones detected identical banding patterns for B14 and B14A; however, only 87.9% of the clones revealed identical patterns with HindIII digests of the two inbreds. Similar observations have been reported for other pairs of inbred lines isogenic for rust resistance (Beckman and Weck, MNL 62:107, 1988).

Nine RFLP clones detected polymorphisms between the two inbreds (Table 1). The clones have been mapped to chromosome one (3 clones), three (2), six (2), seven (1), and ten (1). The Rp1 locus has been mapped to the short arm of chromosome ten (Russell and Hooker, Crop Sci 2:477-480, 1965); however, RFLP clones used in the study did not detect differences between B14 and B14A in that region.

Table 1. RFLP clone-enzyme combinations that detected differences between B14 and B14A.
 
Enzyme
Locus
Chromosome arm
HindIII
UMC084
1L
HindIII
UMC106
1L
HindIII
UMC011
1S
EcoRV
UMC017
3L
EcoRI
UMC050
3S
HindIII
UMC046
6L
HindIII
UMC085
6S
HindIII
UMC012
7S
HindIII
UMC064
10L

The presence of donor parent (or non-recurrent parent) HindIII RFLP patterns at a level greater than theoretical expectations may be attributable to several factors. The unexpected RFLP patterns could be linked to donor chromosome segments selected during backcrossing; possibly, they contain genes influencing the response to rust infection or the segments contain genes which enhance the agronomic performance of B14. Russell and Hooker (Crop Sci. 5:95-96, 1965) reported significant differences for grain yield between B14 and B14A single-cross hybrids in rust-free environments. Mutations in B14 or B14A seem to be an unlikely source of the variation given the number of clones detecting polymorphisms. Pollen or seed contamination cannot be eliminated as sources of the variation until the donor parent has been examined for polymorphisms.


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