Aneuploidy in the inbred line W23 is probably due to nondisjunction at the second division of the microspore

In 1980, I reported (MNL 54:26) that small kernels on W23 ears often produced aneuploid plants. This year reciprocal crosses between W23 and Mangelsdorf tester and between C R-sc W23 and M. G. Neuffer's "down tester" (bz2 a1 c2 a2 pr c1 bz1 wx r) were attempted. Pollen, but not ears, was obtained from Mangelsdorf tester, and (Mangelsdorf tester X normal) crossed by W23 set only scattered kernels, so that part of the experiment may be a failure. Reciprocal crosses of R-sc W23 and down tester were successful, however, and good seed set was often obtained. When R-sc W23 was pollinated by down tester, all 12 ears had colored kernels. From the reciprocal cross, most of the 14 ears obtained had colored kernels with one to five (or perhaps six) colorless kernels, many of which had colored scutella.

If aneuploid eggs and polar nuclei were produced by R-sc W23, then colorless kernels should have appeared on the R-sc ears pollinated by down tester. Because colorless kernels appeared only on the reciprocal cross, the events leading to the production of the colorless kernels must have occurred on the male side. And because the colorless kernels have colored scutella, it is probable that nondisjunction occurred at the second division of the microspore, producing pollen grains with one sperm carrying an extra chromosome and one sperm lacking it entirely. Then, when the deficient sperm fertilizes the polar nuclei, a kernel with colorless endosperm and trisomic embryo should be produced. If the deficient sperm fertilizes the embryo, the endosperm should receive both chromosomes carrying the color factor involved. Several kernels were observed to be unusually dark in color, so these may represent the class that will produce monosomic seedlings.

Kernels from all classes of seed will be analyzed by taking root tip counts. Although it seems most likely that nondisjunction at the second pollen division is involved, the possibility of heterofertilization has not been ruled out.

J. B. Beckett

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