The data obtained from these linkage tests agree reasonably well with previously established linkage values based on classical mapping techniques ("1993 map," Neuffer et al., Mutants of Maize, Cold Spring Harbor Laboratory Press, 1997). The sh1 wx1 distance of about 20 cM obtained in our tests (Table 1) is less than that indicated in Mutants of Maize (27 cM), but agrees well with the two-point summary value presented by Emerson et al. (Cornell Univ. Ag. Exp. Sta. Mem. 180, 1935) of 21 cM. Neither our tests, nor those summarized by Emerson et al., would detect double crossovers between sh1 and wx1. The wx1 wc1 distance of about 56 cM obtained in our tests (Table 2) agrees well with that presented in Mutants of Maize (51 cM) and takes into account at least some of the double crossovers.
The wx1 inr2 distances from both tests are nearly identical (about 23 cM) and fall easily within the standard error range. Applying this value to the "1993" chromosome 9 linkage map, one obtains a map coordinate of 79 for the location of inr2, placing it in the vicinity of bk2, which maps at position 82.
Additional linkage tests involving wx1, inr2, bk2, and v30 are in progress.
The placement of inr2 to chromosome 9 raises interesting questions as to whether inr2 is identical to the da1 or da2 dilute aleurone color loci identified by Eyster. da1 (dilute aleurone1) conditions recessively inherited pale aleurone color, and is reported to be on chromosome 9 (Eyster, J. Hered. 22:224-225, 1931). Linkage data presented in Emerson et al. (Cornell Univ. Agric. Exp. Sta. Mem. 180, 1935) indicate that da1 is located 6 cM from pg12 and 21 cM from wx1. The pg12 wx1 distance is about 14 cM. Linkage data for da1 with respect to sh1 are ambiguous (26 cM and 41 cM in two separate tests). These results indicate that da1 is probably located on the long arm of chromosome 9, and that the gene order is wx1 pg12 da1. This would place da1 in the vicinity of inr2. It is conceivable that da1 could be a recessive mutant allele of inr2, but we have not yet identified such an allele. The Stock Center’s da1 stocks were found to carry a dominant r1 haplotype-specific aleurone color inhibitor allele at the inr1 locus (Inr1-Ref), but inr1 is located on the long arm of chromosome 10 (Stinard, MNL 73:89-90, 1999). The da1 stocks did not appear to carry any recessive mutants affecting aleurone color, and the only factor affecting aleurone color that we managed to isolate from these stocks is located on chromosome 10. For that reason, we feel that Eyster’s original da1 mutant may have been lost from these stocks.
We have found that some lines of maize carry dominant inhibitor alleles at both inr1 and inr2 (Stinard, MNL 74:70-71, 2000). If Eyster’s original da1 line carried both factors, then perhaps he mapped one of the factors (inr2) to chromosome 9, and other isolates of the stock received by the Stock Center could have carried only the Inr1-Ref allele. If the da1 line that Eyster originally examined carried both of these dominant inhibitory alleles, and if the line were heterozygous for susceptible and nonsusceptible r1 haplotypes, F2 segregation in such lines would yield a ratio of 49 purple to 15 dilute kernels, which would be confused with a 3:1 ratio. If the inhibitory inr1 allele is lost and one only considers segregation at inr2 and r1, then the ratio becomes 13:3. Eyster (1931) presents kernel counts for only one self-pollinated ear: 258 purple and 84 dilute. This ratio does not differ significantly from a 3:1 ratio (c2 = 0.035), but does differ significantly from a 13:3 ratio (c2 = 7.582, p < 0.01). Eyster could have mapped a dominant inr2 allele thinking that it was recessive, but such an alternative would require a set of circumstances that render this possibility unlikely.
The da2 locus, also reported to be on chromosome 9 (Eyster, Bibliographica Genetica 11:187-392, 1934), has a dominant allele, Da2, reported to inhibit aleurone color in the kernel crown. Eyster’s linkage data, reported in Emerson et al., indicate that da2 is located 7 cM from c1 and 26 cM from wx1. Therefore, da2 is located on the short arm of chromosome 9, perhaps distal to c1, and could not be the same locus as inr2.
Table 1. Three-point linkage data for sh1, wx1, and inr2.
Testcross: (Sh1 Wx1 Inr2-JD R1-Randolph X sh1 wx1 inr2 R1-Randolph)
X sh1 wx1 inr2 R1-Randolph.
|0||Sh Wx Inr||598|
|sh wx inr||610||1208|
|1||Sh wx inr||170|
|sh Wx Inr||166||336|
|2||Sh Wx inr||195|
|sh wx Inr||197||392|
|1 + 2||Sh wx Inr||26|
|sh Wx inr||34||60|
map distance sh1--wx1 = 19.8 ± 0.9 cM
map distance wx1--inr2 = 22.6 ± 0.9 cM
map distance sh1--inr2 = 42.5 ± 1.1 cM
Table 2. Partial three-point linkage data for wx1, inr2, and wc1.
Testcross: (Wx1 Inr2-JD wc1 R1-Randolph X wx1 inr2 Wc1 R1-Randolph)
X wx1 inr2 wc1 R1-Randolph.
|0||Wx Inr wc||641|
|1||wx Inr wc||230|
|2||Wx Inr Wc||370|
|1 + 2||wx Inr Wc||70|
map distance wx1--inr2 = 22.9 ± 1.2 cM
map distance inr2--wc1 = 33.6 ± 1.3 cM
map distance wx1--wc1 = 56.4 ± 1.4 cM
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