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.
|All normal + nec-banded||0|
|3: 1 nec||5|
|3:1 nec + nec-banded||7|
|1:1 nec + nec-banded||4|
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
|Segregation of 3 normal : 1 necrotic ears|
|Segregation of 1 normal : 1 necrotic ears|
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.
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