New inr1 and inr2 alleles

--Stinard, PS

inr1 and inr2 are loci with dominant alleles that suppress aleurone color in crosses to specific r1 haplotypes (Stinard and Sachs, J Hered 93:421–428, 2002). To date, two dominant inr1 alleles, Inr1-Ref and Inr1-JD, and one dominant inr2 allele, Inr2-JD, have been described. We report the identification of two likely inr1 alleles and one additional inr2 allele from diverse sources.

Dil is a dominant inhibitor of aleurone color originally described as one of the factors in the Spotted-dilute controlling element system (Sastry and Kurmi, MNL 44:101–105, 1970). Crosses to various r1 haplotypes give a similar pattern of response as Inr1 and Inr2 alleles, so mapping crosses of Dil were set up with Inr1-Ref, Inr1-JD, and Inr2-JD to test whether Dil maps to the same chromosomal location as these factors. Testcrosses involving Dil and Inr2-JD indicated independent assortment of these two factors (data not shown). However, the testcrosses of Dil involving Inr1-Ref and Inr1-JD (Testcross: [Inr1 R1-Randolph X Dil R1-r(sd2)] × inr1 R1-Randolph) showed very tight linkage (Inr1-ref crosses yielded 3,756 pale and colorless kernels, and no full colored exceptions; Inr1-JD crosses yielded 3,355 pale and colorless kernels and 1 full colored exception that could be a contaminant). Combining the data from both sets of Inr1 crosses yields an Inr1 Dil linkage distance of less than 0.03 centiMorgans. It is likely that Inr1 and Dil map to the same location, and that they are allelic. The full colored exception will be tested for contamination markers next summer.

It was reported by Stinard (MNL 77:77–79, 2003) that an Fcu line obtained from Peter Peterson of Iowa State University (74-1033-8@) elicits dark purple sectoring on a pale purple background in crosses to R1-ch(Stadler) and other r1 haplotypes that are susceptible to aleurone color inhibition by dominant Inr1 and Inr2 alleles. It was not known whether the suppression of background aleurone color was an inherent property of Fcu, or whether the Fcu line carried dominant inhibitors of aleurone color. In order to test for inhibitors, the Fcu line was crossed to Inr1-JD and Inr2-JD, followed by a backcross by R1-ch(Stadler). The Inr2-JD backcrosses showed independent segregation of two dominant aleurone color inhibitors (data not shown). However, the Inr1-JD backcrosses indicated tight linkage between Inr1 and a dominant inhibitor carried by the Fcu line (Table 1; full colored putative crossovers were counted twice in linkage calculations to account for the lack of ability to score the Inr1 Inr* crossover class). The full colored kernels that were observed could be crossovers, contaminants, or they could represent reversion events at Fcu. The frequency of full colored kernels in these crosses (1.7 × 10⁻³) is close to the observed reversion rate for Fcu (3.0 × 10⁻³; reported elsewhere in this MNL). Additional linkage tests will be conducted in a background free of Fcu. It is likely that the inhibitor present in the Fcu line is an allele of inr1, but a more definitive answer will have to wait until these additional tests have been completed.

During the course of our studies of inr2, which maps to the long arm of chromosome 9, we performed crosses to determine whether inr2 lies proximally or distally to the TB-9Lc breakpoint, 9L.10. We were surprised when reciprocal crosses of Jack Beckett’s TB-9Lc Wc1 line to a N9 R1-Randolph line yielded colorless kernels when the TB-9Lc line was used as a female, but segregated for large pale and colorless kernels and smaller full colored kernels when the TB-9Lc line was used as a male. These results are indicative of a dominant inhibitor of R1-Randolph carried on the B-9Lc chromosome. The two kernel color classes are due to nondisjunction of the B centromeres at the mitotic division of the generative nucleus during pollen grain development (Beckett, J Hered 69:27–36, 1978). The larger pale and colorless kernels presumably have hyperploid endosperms carrying two B-9Lc chromosomes and hypoploid embryos carrying no B-9Lc chromosome. Conversely, the smaller full colored kernels presumably have hypoploid endosperms carrying no B-9Lc chromosome and hyperploid embryos carrying two B-9Lc chromosomes. The chromosomal makeup of these progeny kernels remains to be confirmed. The inhibitor in the TB-9Lc stock was tested for linkage with the inr2 locus by the following testcross: [r1 TB-9Lc Inr* × R1-Randolph N9 Inr2-JD] × R1-Randolph N9 inr2. Of a total of 4,648 progeny kernels, all were pale purple or colorless, and none were full colored. Since it maps to the same location as inr2 and shares the same properties as Inr2-JD, it is very likely that the inhibitor in the TB-9Lc stock is an allele of inr2, and we have named it Inr2-9Lc.

Percentage crossing over between Inr1 and Inr* = 0.33 ± 0.14.