A study of a new source of Bg
--V. V. Koterniak
Kernels with variegated endosperm structure in which flint sectors were alternating with opaque sectors were found in a selfed generation of hybrid Zpl2077/54-14 o2 x SP168 o2. In 1988 S2 forms of this hybrid (further referred to as 3449 o2) originating from variegated seeds were crossed with 346 o2 and 502 o2 lines, which possessed standard opaque endosperm. Most of the F1 kernels were variegated, a small part of them were phenotypically normal. Similar results were obtained when the original source of variegation was selfed. This made it possible to suppose the presence in 3449 o2 of a regulatory factor, in the presence of which the responding recessive o2 allele reverts to normal type.
Further investigations show that this regulatory factor is not either of the known regulatory elements Ac and Spm. Thus F2 seeds from crosses between strains with the responding o2 allele and wx-m7 and pg14-m-En showed no somatic instability at the o2 locus. In 1990 it became possible to carry out the test for allelism with the regulatory element of the Bg-rbg system described by Salamini (MGG 179:497, 1980). The F1 of 3449 o2 with o2-m(r) bg gave fully variegated ears and ears with variegated and a few normal kernels (0.3-4.1%). Plants originating from variegated seeds gave in the F2 normal, variegated and opaque kernels. The ratio of sum of normal and variegated kernels to opaque did not differ significantly from 3:1 (Table 1), indicating allelism of the regulatory element present in 3449 o2 to the Bg element. Taking into account a positive test for allelism, the regulatory element present in 3449 o2 was designated as Bg-3449.
Table 1. Endosperm phenotypes on ears derived from variegated seeds
of the cross 3449 o2 x o2-m(r)bg.
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| 91-5260-1 | 15 | 257 | 86 | 0.18 |
| 91-5260-2 | 4 | 297 | 86 | 1.59 |
| 91-5260-3 | 2 | 182 | 63 | 0.03 |
While studying (346 o2 x 3449 o2) x 346 O2 and (502 o2 x 3449 o2) x 502 O2 crosses it was established that Bg-3449 is not linked to the responding o2 allele. All plants with this genotype were selfed and crossed (as pollinator parents) with opaque testers 346 o2 and 502 o2. As was expected one half of the progenies obtained (57 of 112) segregated normal, variegated and opaque kernels. For 30 of 57 selfed ears the ratio of normal, variegated and opaque kernels did not differ significantly from 12:3:1 (a nonsignificant deviation of the ratio of sum of normal and variegated to opaque kernels from 15:1 was observed on 51 ears). When the same plants were crossed to opaque testers the ratio of normal, variegated and opaque kernels equal to 2:1:1 was significant for 42 ears. Besides ears segregating normal, variegated and opaque kernels, and ears segregating normal and opaque kernels, two ears were found with normal kernels only. The possibility of formation of them will be discussed below.
When selfed on plants originating from variegated seeds, there were no ears observed with clusters of revertant kernels, which may indicate that reversion of the receptive o2 allele in the presence of Bg-3449 takes place in postmeiotic mitotic divisions during mega- or microsporogenesis, which is a distinctive feature of the o2-m(r)-Bg system of controlling elements (Montanelli et al., MGG 197:209, 1984). At the same time the possibility of reversions of the responding o2 allele during sporophyte development seems not to be excluded. First of all it is necessary to mention the complication of classification of kernels on ears of the original source of instability because variegated kernels were mostly of a coarse type of variegation and had a flint crown. Besides this, as was mentioned above selfed progenies of genotypes (346 o2 x 3449 o2) x 346 O2 gave 2 ears with normal kernels only. Further analysis of these kernels showed that they were homozygous for normal allele O2 and did not display Bg activity. The formation of these ears may be explained by reversion of the responding o2 allele to normal during development of the sporophyte before meiosis. Another possible explanation of this phenomenon may be the reversion of the receptive o2 allele in the presence of Bg-3449 in postmeiotic mitotic divisions during mega- or microsporogenesis in combination with at least one of the following events: i) inactivation of the regulatory element, the possibility of which was reported by Salamini et al. (Heredity 49:111, 1982), or ii) nonreplicative transposition of Bg-3449.
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