Backcrossing is used extensively to either transfer a specific gene from one genotype to another or to improve an inbred line for some quantitative trait (Hallauer and Miranda, Quantitative Genetics in Maize Breeding, ISU Press, Ames, IA, 1981). If the parent used for backcrossing is homozygous, the percentage recurrent parent after N generations of backcrossing is 1-(1/2)N + 1 x 100. Six backcross generations are thus required to obtain greater than 99% recurrent parent. Analysis of backcross progeny with Restriction Fragment Length Polymorphism (RFLP) markers should allow one to compare the theoretical amount of inbreeding with actual levels of inbreeding observed.
Two lines, designated A and B, have been backcrossed 5 and 6 times, respectively, with a source of rust resistance (Rp1-d), A632. RFLP markers were utilized to quantitate the differences between the converted lines A-Rp1-d and B-Rp1-d, the original lines A and B, and the Rp1-d source, A632. Figure 1 shows the location of the 40 markers polymorphic for the lines tested and the chromosomal location of bands from the recurrent parent not found in the converted line. The chromosomal locations of the 10 recurrent parent (line B) alleles not incorporated into B-Rp1-d are spread over 5 chromosomes. The 2 unincorporated recurrent parent (line A) alleles are located on 2 chromosomes. A summary of allelic differences between the converted lines, the original lines and the Rp1-d source is shown in Table 1. The percentage recurrent parent observed for B-Rp1-d is significantly lower than expected.
The deviation of the B-Rp1-d results from expectations might indicate that selection was inefficient for recovery of the desired recurrent parent plant characteristics or that the non-recovered chromsomal regions have little effect on overall B-line plant structure. Although lines A and B are highly inbred, quantitative genetic theory states that a lower original parental level of homozygosity would lead to a lower percentage of recurrent parent. The B-line would simply be less homozygous than the A-line. The general availability of molecular markers should allow further testing of quantitative genetic theories at the molecular level.
Figure 1. Location of RFLP markers utilized to compare lines A, B, A-Rp1-d, and B-Rp1-d
Table 1. Percentage recurrent parent for converted lines A-Rp1-d and B-Rp1-d
Diana Beckman and Edward Weck
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