Study on a new male sterile gene and apoptosis
--Zhou, H; Li, J

Dr. Jingxiong Li and his assistant once found a male sterile plant in their sweet breeding material several years ago and studied its genetics. He found the male sterility was controlled by one single recessive gene, temporally named msx. Using the B-A translocation system to locate the gene, he thought he had found a new male sterile gene. The gene was located on chromosome 4 with a recombination value of 15-20 with su1; msx was not the same gene as Ms41 and Ms44, also on chromosome 4, because these two genes were dominant and not at the same site according to crosses (unpublished data). The male sterility of msx was very stable, and no fertile pollen grains were found in anthers in several years. Now we are mapping the gene on the maize RFLP map, and results will be published in our next paper.

We observed microspore development by employing a highly sensitive microfluorescent method this year. The msx msx and Msx msx genotypes were in the same inbred background. We took the anthers from two plants of msx msx and Msx msx each day after meiosis, fixed in Carnoy's solution, and marked nuclear DNA with DAPI overnight at 4C. The results were as follows:

1. Pollen in Msx msx anthers was normal (see Figure 1A-10A, male fertile plant) from quartet to binucleate stage.
2. Pollen in msx msx anthers was abnormal (see Figure 1B-10B), male sterile plant) from the late uninucleate stage (see 3B), when the chromatin began to condense (compare 3A to 3B).
3. The chromatin formed a death ring (4B and 5B) in msx while Msx was in first mitosis (4A and 5A).
4. At the time of anaphase of first mitosis, the male sterile spore only showed a clear DNA digesting ring (6B), not a dividing nucleus (6A).
5. In the binucleate phase, msx showed DNA digesting (7B), but the spore was as big as the fertile one.
6. In 8B, the spore showed a split nucleus, as the normal spore (8A) was in second mitosis.
7. In 9B and 10B, the split nucleus was dissolving, with little DNA left, the fertile spore going into trinucleate phase. The sterile cell (cytoplasm, cell wall et al.) was not digested yet.
From the observations, we also found that DNA dissolution was earlier than cell dissolution. So we think that the microspore abortion of maize genic male sterility is a programmed cell death or apoptosis, a physiological cell death, not a cell necrosis.

Figure 1-10A: Male fertile microspore development of Msx.
Figure 1-10B: male sterile microspore abortion of msx.

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