A Tale of Two Necrotics

I reported in the last Newsletter (MNL 58:84) that nec3 and nec*-493 were non-allelic. However, in the same issue, Shadley and Weber (MNL 58:160) reported that nec*-493 is allelic to nec*-409 (nec*-409 is the original lab designation of nec3). In order to verify my allelism test, I planted 30 seeds from the ear of the F1 between two plants, each heterozygous for one of the necrotic mutants. The resulting 27 plants were selfed and 24 ears were recovered. Approximately 100 seeds from each ear were planted in the sandbench and the necrotic seedlings were scored. Out of the 24 ears, 5 segregated all normal seedlings, 12 segregated 3 normal: 1 necrotic seedlings, and 7 segregated 1 normal:1 necrotic seedling. The 1:2:1 ratio of the above types of segregating ears and the 1:1 necrotic-seedling segregating ears verify that the original ear had a genotype of (+ nec3)/(nec*-493 +) and that nec3 and nec*-493 are nonallelic. As reported, both mutants are linked to a2 and btl on 5S. The order and exact map distances are not known and may prove difficult to ascertain with two identical lethal mutants.

The story could end here; however, a second plant phenotype was seen in this study which is intriguing at best. Eleven of the above 24 ears also segregated for necrotic-banded seedlings. These seedlings have a single necrotic band or patch (approx. 1-2 cm wide) on the first three leaves. The band appears to have formed at the same time on all leaves in that it was located in the middle of the first leaf, close to the tip on the second and at the tip on the third. All ears segregating for necrotic bands also segregated for necrotic seedlings. The exact numbers are shown below.
 
Segregation # Ears
All normal 5
All normal + nec-banded 0
3: 1 nec 5
3:1 nec + nec-banded 7
1:1 nec 3
1:1 nec + nec-banded 4
  24

From previous families, I had an indication that the necrotic-banding was linked to nec3 and these data support the idea that only one of the necrotic mutants is involved. I thought it might be possible that nec3 was a semi-dominant or there was a dominant modifier affecting nec3, such that +/nec3 resulted in the necrotic-banded phenotype. Unfortunately, the segregation data presented below do not really support either case.
 
Segregation of 3 normal : 1 necrotic ears
Normal Banded Necrotic
25 54 27
28 55 31
30 64 27
25 42 30
34 53 21
16 35 26
47 45 34
Segregation of 1 normal : 1 necrotic ears
Normal Banded Necrotic
15 50 75
7 50 41
12 32 48
18 44 56

All but the last ear of the 3:1 ears fits a 1:2:1 ratio, which is in line with nec3 being a semi-dominant (but why are only 4/7 1:1 ears segregating necrotic-banded and the presence of normal seedlings in the 1:1 ears?) or a linked heterozygous dominant modifier of nec3 or unlinked homozygous dominant modifier (but again why the segregation in the 1:1 ears?). The 1:1 ears fit a 1:3:4 ratio as expected for a double heterozygote of linked necrotic genes and an unlinked heterozygous dominant modifier. However, this is in conflict with the 3:1 ear data. I have tried several additional schemes, but none appear to satisfy all the data. My plan is simply to outcross a necrotic-banded plant in an attempt to study the linkage of the "modifier" and nec3. According to Coe (pers. comm.), 5S has often given unusual segregation ratios and this certainly fits that picture.

Dave Hoisington


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