Evidence for location of Mrh-37962 on chromosome 5 --Ellen Dempsey and M. M. Rhoades In our recent publication (Shepherd, Rhoades and Dempsey, Devel. Genet., 1989) we described a second occurrence of Mrh (possibly by transposition). The new allele was tentatively placed on chromosome 5. The new Mrh (Mrh-37962) was found in the progeny of a selfed plant of a-mrh Sh2/a sh2; Mrh/mrh constitution. The sh2 kernels were all dotless, but the Sh2 kernels were of three types: high-dot, low-dot, and dotless. The unexpected "low-dot" phenotype was shown to be the result of segregation of a second Mrh, while the high-dot kernels possessed the original Mrh on chromosome 9 or both Mrh factors. Plants with Mrh-37962 were crossed with the wx-translocation series and the F1 progenies testcrossed with a wx testers to determine its chromosomal location. The F1 plants were selfed and used as male parents in the testcross. Close linkage of wx and Mrh-37962 was observed in only one of the F1's, that involving translocation 5-9c (breakpoints 5S.07-9L.10). The data presented in the table came from three testcrosses of the same male parent.

The colorless seeds all have one dose of the a-mrh responding allele and should show dots when Mrh-37962 is present. The new Mrh specifies a low rate of somatic mutation to A and, when introduced by the male parent and present in one dose, Mrh-37962 is often unexpressed. The colorless kernels without dots were therefore grown and tested for presence of Mrh-37962. Most of the Wx kernels proved to have Mrh-37962 while most of the wx kernels did not (noncrossover (0) classes). The revised data for the colorless kernels gave the following totals: 85 Wx with dots, 1 wx with dots, 3 Wx without dots, and 60 wx without dots. It is clear that Mrh-37962 is closely linked to Wx. The progeny plants were scored for pollen sterility and this permitted identification of the point of exchange in the four crossover plants. Three of them (including the wx dotted individual) were crossovers between the wx gene and the translocation breakpoint. The fourth was either a crossover in the Mrh-breakpoint region or a loss of the Mrh-37962 controlling element. Several additional male testcrosses of the same type were made. Since these have not been completely analyzed for unexpressed Mrh alleles, only data from the positive dotted group are presented. Among the dotted kernels, 291 were Wx and 16 were wx. Again, close linkage between Wx and Mrh-37962 was observed, indicating location of Mrh-37962 on chromosome 5.

The expected 1:1 ratio of Wx:wx was not found in the above crosses. The deficiency in the wx class may be due to presence of a gametophyte factor on chromosome 5, closely linked to wx because of translocation heterozygosity. Chromosome 5 is known to harbor such factors and they often complicate the ratios for genes on chromosome 5 when transmitted through the male parent. We encountered this difficulty in self-pollinations to establish linkage of Mrh-37962 with the bt gene near the centromere of chromosome 5. Two self-pollinations of A/a-mrh; Bt Mrh-37962/Bt mrh plants gave 8.6% and 6.8% bt when 25% was expected. However, among the a-mrh kernels showing dots, 177 were Bt and only 2 were bt in phenotype. The percentage of bt in the group known to possess Mrh-37962 was only 1.1%. Another indirect indication that Mrh-37962 is on chromosome 5 came from an examination of 41 selfed ears in family 41069. Twenty of these were produced by Bt mrh/bt mrh individuals and 21 were from Bt Mrh-37962/bt mrh plants. Among the first group the percentage of bt was 12.7%; among the second group 3.8% bt was found. We conclude the Bt Mrh-37962 and the Bt mrh chromosomes differed in the type of gametophyte factor present, with the stronger factor located on the Bt-Mrh-37962 chromosome. There was some variability in both groups, with a range of 3.7%-22.2% in the dotless group and 1.1%-10.4% in the group exhibiting dots. This may be due to crossovers between Mrh-37962 and the gametophyte factor.

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