A case of possible unequal sister chromatid exchange in Mu stocks

In a large isolation block set up to isolate Mu-induced a2 mutants, the male parent was a Mu A2 Bt stock that carried the self-color R-scm2 allele. This allele also results in colored scutellum. The female parent was a2 bt. In this test 4 seeds were found that were yellow and had purple scutellum. It was assumed that these seeds resulted from pollen grains in which one sperm nucleus had an A2 to a2 mutation, while there was no mutation in the other sperm. To confirm this, the seeds were planted and self-pollinated. If these seeds were indeed due to a mutation in only the sperm fertilizing the polar fusion nucleus, the selfed ears should segregate 3 A2 Bt:1 a2 bt (with a few rare crossover classes). This was not observed! In all cases, about half of the seeds were A2 Bt.

An unequal sister chromatid exchange during the division of the generative nucleus could result in a duplicate (A2 A2) chromosome that would be selected against when the F1 plants were selfed, and the a2 bt pollen only (or predominantly) would function through the male. No such selection would occur on the female side, and thus the selfed ears would be expected to have the observed 1:1 ratios. (See Figure 1.)

To test this hypothesis, plants from A2 Bt seeds from the selfed ears were reciprocally crossed with a2 bt. The ears on the a2 bt plants would be expected to have predominantly a2 bt seeds since the duplicate chromosome would not be transmitted through the pollen. The A2 Bt ears should have A2 Bt and a2 bt seeds in a 1:1 ratio. Basically, these results were observed. Only an occasional Bt and/or A2 seed was observed on the a2 bt ears (= crossovers?).

If the a2 bt stock had a fifth chromosome Ga factor, and the A2 Bt stock ga, similar results might be observed. But if that were the case, how would the original seeds have been produced in the first place? This is possible if the Ga system allows the functioning of ga pollen on silks, and if no Ga pollen was available to compete with that carrying ga. However, such a possibility does not explain the original yellow endosperm-purple scutellum seeds.

The a2 bt stock was obtained many years ago from the Coop. When I first tried using it, I ran into all kinds of difficulty because it had a very strong Ga factor. Pollen with the ga allele would not function on the silks of homozygous Ga plants, even if ga was the only type of pollen available. Such crosses failed to set seeds. Before I could use this stock, I had to eliminate the Ga factor. There has been no evidence of Ga in this stock for years, during which time it has been used in a variety of different crosses. A few years ago I carried out an extensive test of crossing over in the a2-bt region using reciprocal tests. The purple stock used in this cross was the same one used to produce the A2 Bt Mu line, and the a2 bt was the same as the one used in these tests. There was no evidence of a Ga factor being involved in these crossover tests!

It seems very likely that unequal sister chromatid exchange was responsible for producing the original seeds. If this is true, and if the duplications are long enough, it may be possible to demonstrate them cytologically.

Figure 1.

Donald S. Robertson

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