Two experiments that we have conducted provide information about the first division of the zygote. The results of both are consistent with the morphological work of L. F. Randolph (J. Agr. Res. 53:881, 1936), and the conclusions of D. M. Steffensen (AJB 55:354, 1968) from x-ray induced events, that the first division is cross-planar (separating the embryo proper from the suspensor) rather than longitudinal. They make the observations of sectoring in plants from EMS-treated pollen (see article by Bird and Neuffer in this issue) difficult to explain.
In the first experiment, developing seeds from the cross b a pl R-r x B A Pl R-g were x-rayed 21-33 hours after pollination. Plants grown from these seeds were observed for losses of the markers (see table). Among 571 plants, 31 losses affected the whole plant and only 3 losses were fractional. The first division would be expected to be taking place at about the time of the irradiations in this experiment, according to Randolph's data, and divisions that were longitudinal would yield fractional individuals. Even if the 3 fractional events represent longitudinal divisions rather than delayed losses, they are infrequent. As an aside, the morphologies of the hemizygotes were variable, but each arm appeared to have a characteristic effect on plant form.
In the second experiment, pollen of B Pl Wd R-r was exposed to ultraviolet light and crossed onto b pl wd Ring-Wd R-g ear parents. Among 465 plants none were sectorial, while 9 were whole-plant exceptions (3 Wd losses, not validated for other markers before death; 2 B losses, validated; 1 pl loss, not validated; and 3 R-r losses, validated). L. J. Stadler (Proc. VII Int. Cong. Genetics, p. 269, 1939) has shown that UV induces fractional events in the endosperm, while x-rays do so only rarely. If the first division, separating strands with UV lesions from those without, were longitudinal, sectorial plants would have been expected.
E. H. Coe and R. S. Poethig
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