Whereas seedling striping that is due to nuclear events (e.g., wd with unstable ring Wd) is uniform and random, that occurring in ij seedlings is conspicuously not uniform and not random. The first leaf tends to have a regular pattern in which the opposite margins are symmetrically white, with little or no striping elsewhere in the leaf. The second leaf often shows the inverse pattern, but also often repeats the first-leaf pattern; subsequent leaves may continue patterns leaf-after-leaf, or may change, but the impression to the eye is that margin patterns are most common (in some backgrounds the rule) throughout the life of the plant. We are seeking an explanation of the development of such patterns, which contrast with the well-defined sequence of cellular morphogenesis. No doubt they reflect aspects of chloroplast transmission, variation and maturation in concert with cell and tissue proliferation (studies described in a current manuscript by Walbot and Coe show that iojap conditions a programmed change in the ability of the plastid to differentiate, without notable changes in the chloroplast genome).
To examine the relationships of patterns and sectors in Wd ring and iojap seedlings, we have recorded white vs. green in successive millimeters across the width of the first and second leaves at their midpoint. The average percent of millimeters white across the width ("Area"), in the edge millimeter ("Margins"), adjacent to the midrib ("Midribs"), and in the center of the half-leaf ("Centers") gives numerical estimates for the visual impression:
As expected from visual impressions, ij seedlings have white margins on leaf 1 twice as often as on leaf 2, and the inverse is true for white midribs. The Wd ring events, however, show the same tendencies, though not to the same degree. A larger sample has not been grown because the backgrounds are not very similar between the Wd ring and ij sources; in fact, the above ij data are pooled from very variable progenies segregating in F2 or BC1 with inbred lines. Appropriate data will be possible to obtain when uniform comparable materials are derived.
The patterns of symmetry across leaves (margin = margin) and from leaf 1 to leaf 2 can be evaluated from the frequency of agreement vs. disagreement in phenotype of the segments in each seedling. For example, the opposite margins of leaf 1 may agree in phenotype; if leaf 2 is the inverse, then margins of leaf 1 and leaf 2 will disagree, while if leaf 2 is the same as leaf 1, they will agree. If the course of cellular morphogenesis is followed, in which the midrib is in the plane of the first vertical division in the formation of the embryo (D. M. Steffensen, Am. J. Bot. 55:354, 1968), then the margin of the first leaf should agree with the midrib of the second on the same side of that plane, and vice versa.
A few rough numbers have been collected to examine these agree/disagree alternatives, with the same reservations mentioned above regarding uniformity and comparability of the materials:
Iojap seedlings definitely show symmetry in both leaf 1 and leaf 2, as expected. However, it does not appear that the symmetry pattern of leaf 1 is reflected in leaf 2 of the same seedlings, since these margins neither strongly disagree nor strongly agree. Cellular morphogenetic lineages also may be reflected in iojap seedlings, since the agreement between margins and midribs in leaves 1 and 2 in the same plane is equal to or greater than that for Wd losses. These observations suggest that iojap striping in seedlings is not simply a sorting-out phenomenon, nor a random mutational event at the cell level.
E. H. Coe, Jr., and V. Walbot*
*Dept. of Biology, Washington Univ., St. Louis, Mo.
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