2.
Pollen mutations.
During studies of mutable‑waxy, an unstable allele at the waxy locus, some data were obtained which are as yet unexplained. They are summarized here in the hope that an analogous phenomenon might have been encountered by someone with other material, in which case I would appreciate learning about it.
Wx wx
plants produced red‑staining (with I2‑KI) and
blue-staining pollen grains in equal numbers, as was first shown by Demerec and
by Brink and MacGillivray. The red‑staining grains are assumed to carry
the wx allele, and the blue‑staining
grains the Wx allele, judging
from The staining properties of pollen from Wx Wx and from wx wx plants. Similarly, wx wx wx endosperm starch stains red, while endosperm with one or more Wx alleles stains blue. In the case of the unstable
allele, wxm, it has
been determined that endosperm starch is red‑staining before mutation.
The direction of mutation, which results in the occurrence of mosaic kernels,
is from red‑staining to blue-staining. From these data it was assumed
that the pollen carrying wxm would be red‑staining, while pollen carrying a Wx allele, due to mutation from wxm to Wx,
would be blue‑staining. If this were so, it would provide a basis for
studying time and frequency of mutation in the tassel of wxm wxm plants.
In general, wxm wxm plants, whether selfed or crossed with wx wx, give rise in the F1 mostly to mosaic kernels plus a varying per cent of non‑waxy kernels, the latter breeding true in most cases for a stable non‑waxy (Wx) allele. A large per cent of blue‑staining pollen was frequently found in the tassels of wxm wxm plants, but these plants gave rise predominantly to mosaic, rather than to non‑waxy kernels. This observation led to the following study.
Plants which were wxm wxm were grown to maturity. A pollen sample taken from each was used in part to pollinate wx wx plants and the rest was stained with iodine and a sample of 300 grains counted to determine the per cent blue‑staining. The results, summarized below, show clearly a discrepancy between the per cent blue‑staining pollen and the per cent non‑waxy kernels on the resulting ears. Differential pollen viability was ruled out by many previous crosses with these stocks in which the linked marker c was recovered in the ratio of 1:1 from crosses of the type: c wx/c wx x C wxm/c wx.
|
Plant |
% Blue‑staining pollen |
% non‑waxy kernels |
|
|
|
|
|
591‑2 |
30 |
0 |
|
591‑3 |
59 |
7 |
|
591‑4 |
50 |
20 |
|
591‑6 |
35 |
0 |
|
591‑7 |
36 |
8 ‑ one ear |
|
|
|
3 ‑ second ear |
|
590‑1 |
33 |
14 |
|
590‑2 |
44 |
1 |
|
590‑4 |
3 |
0 |
|
590‑5 |
40 |
20 ‑ one ear |
|
|
|
1 ‑ second ear |
|
590‑10 |
11 |
0 |
|
590‑12 |
49 |
2 |
|
590‑16 |
25 |
0 |
From these data it appears that blue‑staining (i.e., mutated) pollen gives rise to some mosaic kernels as well as, in some cases, to non‑waxy kernels. One might suspect that forward‑mutation is the explanation, i.e., that the blue‑staining pollen carried an unstable dominant allele which subsequently mutates to the original unstable recessive. However, the fact that no unstable dominant alleles have been found, and that the extent of blue‑staining tissue increases during endosperm development, have indicated that mutation from blue-staining to red‑staining (from dominant to recessive) occurs rarely if at all in this material.
Ruth Sager