As has been previously reported (MNL 60:6-7, 1986), brown-1 (brn1) is a recessive brown-kerneled seedling lethal mutant located on the short arm of chromosome 3. We have now collected linkage data with respect to d1, Lg3, and cl1, which would suggest that brn1 is located in the vicinity of cr1 on chromosome 3. Our linkage data are reported in Tables 1 and 2. All crosses were set up to test linkage in repulsion.
In order to test linkage with d1 and Lg3, we crossed a d1 Lg3 stock onto the first ears of heterozygous brn1 plants whose second ears had been selfed the day before. Kernels from the first ears of plants whose selfed second ears segregated for brn1 were planted the next season, and the Lg3 plants were selfed and outcrossed to a standard line carrying the wildtype alleles. Kernels from the outcrosses of plants which were liguleless, and whose selfs segregated for d1 as well as brn1, were planted in our selfing block, scored for Lg3, and self-pollinated. The resulting ears were scored for brn1, and kernels from each ear were planted in sand benches and the seedlings scored for d1. The data collected from this linkage test are presented in Table 1.
To test linkage with cl1, heterozygous brn1 plants were selfed and outcrossed to homozygous cl1 cl1 Cl3 Cl3 plants. Kernels from these crosses were planted, and the plants selfed and outcrossed to either (1) cl1 cl1 Cl3 Cl3 plants, or (2) a standard line carrying the wildtype alleles. Kernels from the outcrosses of plants whose selfs segregated for brn1 and cl1 were planted in our selfing block and the plants were self-pollinated. For cross (1), the kemels planted in the selfing block were first separated on the basis of whether they were yellow (Cll cl1), or pale yellow (cl1 cl1), before they were planted. This is the basis of the data reported in the top half of Table 2. It should be noted that a fair amount of heterofertilization occurred (about 5%), but this was readily recognized in the selfed ears, and did not influence the linkage results. For cross (2), the kernels were simply planted in rows, since all kernels were yellow. The data for cross (2) are reported in the bottom half of Table 2.
The data from the three point cross with d1 and Lg3 (Table 1) would place brn1 at around 19 units from d1, and 40 units from Lg3 on chromosome 3. The data from the two-point cross with cl1 (Table 2) place brn1 at around 36 units from cl1. Thus, on the linkage map of chromosome 3, brn1 would be placed at position 13 (with respect to d1), position 17 (with respect to Lg3), or position 16 (with respect to cl1). These positions map very closely to cr1, which is located at position 14. Since d1 is located closer to brn1 on chromosome 3 than are Lg3 and cl1, the placement of brn1 at 13 units is probably the most accurate. We will be collecting more linkage data with respect to d1, as well as g2 and possibly ra2 within the next year.
We attempted to map brn1 with respect to cr1 and d1 in a three point cross according to the following scheme: Homozygous cr1 cr1 d1 d1 plants were crossed onto the first ear of a heterozygous brn1 plant whose second ear had been selfed the day before. Kernels from the first ear of plants whose selfed second ears segregated for brn1 were planted the next season, the second ears selfed, and the first ears pollinated by a homozygous cr1 cr1 stock obtained from the Maize Genetics Stock Center, which was indicated as being homozygous wildtype for d1. The kernels from the first ears of plants whose second ears segregated for brn1 and d1 (cr1 could not be scored in the seedling bench) were planted in our selfing block, the plants grown to maturity and scored for cr1, and the plants self-pollinated. The selfed ears were to be scored for brn1, and planted in the seedling bench to be scored for d1. As it turned out, the plants in the selfing block segregated for a dwarf, as well as crinkly, indicating that the cr1 stock used in the second generation cross was heterozygous for d1. In addition, the crinkly trait was nearly impossible to score because there was a continuous range of variation in the plants from very crinkly to very smooth leaves. The cr1 stock also was in a purple background, which made the selfs difficult to classify for brn1. We will try to obtain a different cr1 stock, hopefully in a colorless aleurone background, in order to complete the linkage tests. We would be very grateful for advice on classifying for the crinkly trait.
The following diagram indicates the location of brn1 on chromosome 3, based on our present linkage data:
g2 ---------------- cr1 ---------------d1 ----------------- c1l --Lg3 -------
0 14 32 52 57
Table 1. Linkage data for brn1 d1 Lg3, from the testcross + d1 Lg3 / brn1 + + x + + +.
Table 2. Linkage data for brn] - c1l.
Philip S. Stinard
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