Mosaic pericarp does not result from an Spm insertion
In the last issue of the Newsletter (64:81), I suggested that the unstable P allele, P-mo, could have resulted from the insertion of an autonomous Spm in a P allele. The basis for this suggestion was the observation that three different mosaic pericarp stocks from R. A. Brink's collection of P mutants all contained more than one active Spm. The test was made by crossing mosaic pericarp plants by bz-m13 (a dSpm insertion in a Bz allele), and then crossing the F1 plants (P-mo/P-wr; Bz/bz-m13) by a sh bz wx; no Spm tester. If the F1 plant contains one or more Spm's, then one observes a segregation for bz-variegated kernels. In a more extensive test of one of these F1's, almost every mosaic pericarp plant in the resulting progeny segregated bz-variegated (bz -> Bz) kernels showing that the plants contained an active Spm. The few exceptions--mosaic pericarp plants that were Bz/Bz or Bz/bz--were explicable on the basis of dSpm excision sufficiently early in development that the entire ear was derived from the cell in which that event occurred or by contamination. In addition, many nonmosaic pericarp plants segregated bz-variegated kernels. In all, 72% of the plants in the progeny had some bz-variegated kernels.
The three mosaic pericarp stocks from the Brink collection had each been backcrossed five times to the inbred, 4Co63. Tests made as outlined above of the 4Co63 line that I have been carrying and that also came from the Brink collection indicated that the inbred did not have an active Spm thus suggesting that the Spm's were derived from the mosaic pericarp stocks and were carried along through five BC's to an inbred that lacked the transposable element. Evidence from the past summer shows, however, that Spm is not involved in the P-mo allele. A second approach to testing whether P-mo has resulted from an Spm insertion is to derive plants that are bz/bz; P-mo/P-wr and test whether all such plants activate bz-m13. The answer is that although derived from a P-mo/P-ww line with more than one Spm present via successive outcrosses to stocks (bz-m13 and the sh bz wx tester) not carrying an Spm, not all such plants do. Therefore, P-mo does not result from an Spm insertion in a P allele.
In the same Newsletter in which I reported that 4Co63 does not contain an active Spm, Peter Peterson reported (64:8) that 4Co63 does contain an active En (Spm) in the homozygous condition. Dr. Peterson kindly provided his line of 4Co63, which also came from the Brink collection via J. Kermicle, and his tester stock, c-m(r). Tests here in the summer of 1990 using the Peterson tester substantiated the previous observations. No plant of my 4Co63 line had an active Spm(En), while all plants of the Peterson line were homozygous. It's not clear when these lines of 4Co63 diverged. Nevertheless, it seems that an obvious source of the Spm's present in the mosaic pericarp stocks that I have been testing is the recurrent 4Co63 parent used by Dr. Brink.
It was noted above that in the F1 progeny (P-mo/P-wr; Bz/bz-m13) tested extensively in the summer of 1989 by crossing by a sh bz wx stock 72% of the plants segregated for bz-variegated kernels suggesting that the P-mo/P-ww plant crossed by bz-m13/bz-m13; P-wr/P-wr had two unlinked Spm's. It was also noted that there were a few mosaic pericarp plants on which the kernels were either Sh Bz/Sh Bz or Sh Bz/Sh bz. Since these constituted exceptions to expectations if P-mo resulted from an Spm insertion, the kernels from these plants were tested. In 1989, there were five mosaic pericarp plants that had only Sh Bz/Sh Bz kernels. In 1990, the progeny of each of the five were tested as female parents by crossing times bz-m13/bz-m13 and times a sh bz wx; +Spm tester. The results were the same for the five progenies; no plant had an active Spm as shown by failure to activate bz-m13, and some plants segregated bz-variegated kernels when crossed by the sh bz wx; +Spm tester so these had a responsive bz-m13. Therefore, the failure to produce bz-variegated kernels in 1989 was the absence of Spm from the genome. This constitutes further evidence that P-mo does not result from an Spm insertion.
The plants from the Sh bz/sh bz kernels on the two plants (41128-3 and -4) that had Sh Bz/sh bz and Sh bz/sh bz kernels in 1989 when crossed by a C sh bz wx; no Spm tester were crossed by bz-m13/bz-m13 to ascertain whether the parental plants had an active Spm. In the progeny of 41128-3, 90% of the 112 plants tested had at least one Spm, and many clearly had more than one. In the progeny of 41128-4, 85% of the 88 plants tested had at least one Spm, and again many had more than one. The results suggest that both 1989 parental plants were heterozygous for three unlinked Spm's. It is not possible to exclude definitively the possibility that the Sh bz chromosome came from a contaminating gamete from a distant C Sh bz wx tester in 1985, but it is intriguing that both these plants with a chromosome carrying a presumptive change very early in development from Sh bz-m13 to Sh bz apparently had three Spm's as shown by their progeny tests.
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