Correlation of tassel and ear reversion events in cms-S --Susan Gabay-Laughnan and J. R. Laughnan In maize the first vertical division of the embryo initial determines the right and left halves of the plant, a plant that is bilaterally symmetrical through the midribs of the leaves. This pattern of development was deduced by studying gamma- and X-ray induced losses of chromosome segments in dry seeds whose embryos were heterozygous for various marker genes (Steffensen, Amer. J. Bot. 55:354, 1968; Coe and Neuffer, in The Clonal Basis of Development, pp. 113-129, 1978).

Plants in the M825 inbred background have an average of over 35 tassel branches and ten percent of cms-S plants exhibit spontaneous cytoplasmic reversion to fertility, expressed as fertile-sterile tassel sectors or totally fertile tassels in families of plants expected to be all male sterile. Only plants with large tassel sectors, in which the main rachis is included in the reversion event, or those with totally-fertile tassels have been observed to have correlated reversion events in their ears. We have carried out a study of plants with these two types of events in stocks of cms-VG, cms-I, cms-ML and cms-RD WF9, cms-S 38-11, cms-VG N6, cms-VG K55 and cms-ML M14, all of which have been converted to the M825 nuclear background by ten recurrent backcrosses using M825 inbred line as the male parent. Ears on such plants were pollinated to determine whether they were included in the reversion events; the progeny were scored as male-fertile, male-sterile or mixed (both fertile and sterile). Tassels of plants with large sectors were mapped to determine the positional relationship of the sector with the ears borne on these plants. Ears subtending the fertile tassel sectors were considered to be "in", those subtending the sterile portion of a sectored tassel were considered to be "out", and in a third class ears were judged near enough to the predicted fertile-sterile tassel interface to be considered "on the border".

The studies by Steffensen and by Coe and Neuffer referred to above predict that fertile tassel sectors larger than one-half of the tassel are not likely to occur. Such fertile sectors were observed, however, in about 25% (12 cases) of the 52 plants with large tassel sectors that were studied intensively. Tassels of these 52 plants with large fertile sectors were mapped and ears on these plants were pollinated by M825 maintainer (nonrestoring) plants:
 
Position of ear
Male sterile
Male fertile
Mixed
Out
21
0
0
Border
13
1
1
In
5
8
3

The result from the 16 "in" cases was unexpected but, as it turned out, not inconsistent with the results obtained from ears borne on plants with totally fertile tassels. These were also expected to produce entirely fertile progeny. Of 36 such plants analyzed, only 22 produced entirely male-fertile progeny. Four produced mixed progeny and ten produced male-sterile progeny. A total of 31% of the ears considered to be "in" large fertile sectors, and 28% of the ears borne on plants with totally fertile tassels gave male-sterile progeny.

The above observations indicate a correspondence between male-fertile "in" tassel sectors or entirely fertile tassels and male-fertile progeny from ears on such plants crossed with nonrestoring pollen; in other words, in these cases the male-fertile products of the cytoplasmic reversion event are found in both tassel and ear of the same plant. Even so, in about 30% of plants with this type of tassel fertility the progeny from their crossed ears were male-sterile. Why is there not perfect correspondence between tassel and ear fertility in these cases? We believe the answer lies in the timing of the primary event that leads to fertility. If it occurs in the mother cell that divides to produce the two daughter cells that define right and left halves of the mature plant, or in one of those two daughter cells, both tassel and ear initials should carry the reversion, and correspondence of male-fertile elements in tassel and subtending ear is expected. On the other hand, if the reversion event occurs much later, after cells ancestral to tassel primordia and those ancestral to ear initials are defined, noncorrespondence between male-fertile elements in tassel and ear is expected. This model is consistent with numbers of instances in which ears on male-sterile plants crossed by maintainer plants have produced male-fertile progeny.

The theory of midrib symmetry for the corn plant is not consistent with the position of all fertile tassel sectors, numbers of which, including large ones, have been found to overlap the midrib borderline. It might be argued that "sorting out" of mitochondria can explain these observations since the theory was developed as a result of studies of sectors that occurred following the loss of nuclear genes. We think this is unlikely since such "sorting out" of organelles should still exhibit correspondence with cell lineage patterns; it is more likely that these "nonconforming" fertile tassel sectors result from discrepant or noncoincidental patterns of cell division during very early stages of embryo development.


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