Evidence implicating the Spm (I-En) family of transposable elements in the mosaic pericarp (P-mo) allele --Oliver Nelson Several years ago, a test of the possible involvement of the Spm (I-En) family of transposable elements in the P-mo allele of the P locus was initiated. In common with the extensively studied P-vv allele, P-mo conditions a variegated pericarp phenotype when homozygous or heterozygous with P-ww, but P-mo is known not to result from the insertion of an Ac(Mp) element in a P-rr allele as does P-vv. The investigation was started with three P-mo stocks provided by R. A. Brink. These stocks had been originally received by him from three different sources (Fontaine, St. John, and Weatherwax) and had been crossed and then backcrossed five times to the inbred 4Co63, which is P-ww/P-ww (white pericarp, white cob). 4Co63 is the same inbred that was used as a recurrent parent in investigations of the genetic basis of the variegated pericarp (P-vv) phenotype by R. A. Brink and his students. It should be noted that P-mo/P-ww plants have white cobs while P-vv/P-ww plants have variegated cobs. The backcross progeny from a plant showing the mosaic pericarp phenotype consists of nearly equal numbers of mosaic pericarp (P-mo/P-ww) and white pericarp (P-ww/P-ww) plants although there is an excess of P-ww/P-ww plants over the proportion expected on the assumption that there should be a 1:1 segregation.

Since the stocks that would allow an immediate answer to the question of whether these P-mo/P-ww accessions from Dr. Brink had Spm activity were not available, the question was posed by crossing these accessions by a stock that is P-wr, bz-m13, no Spm. The bz-m13 allele resulted from the insertion of a defective Spm (dSpm) in a Bz allele, and it conditions a nonmutant phenotype in the absence of a transacting Spm. The F1 progenies were then crossed by a C sh bz, no Spm tester stock. Since the mosaic pericarp stocks are Bz/Bz, if they do not contain an active Spm, the resultant kernels on the testcross by the C sh bz, no Spm tester would all display the Bz phenotype. If the P-mo/P-ww accessions do contain one or more active Spm's, then a certain proportion of the kernels would show variegation typical of bz-m13 in the presence of the active Spm. For the Fontaine and Weatherwax accessions where mosaic pericarp plants were used as the parents in the original cross by bz-m13, 80% of the mosaic pericarp plants showed kernels with the variegation typical of bz-m13 in the presence of Spm. No cross onto a mosaic pericarp plant of the St. John accession was made. The same proportion of P-ww/P-ww plants in these same progenies showed variegated bz-m13 kernels. Thus nearly 3/4 of the plants in the F1 progeny had an active Spm whether or not they were mosaic pericarp. On the assumption that the production of a mosaic pericarp plants requires an active Spm, the mosaic pericarp plants that do not have bz-variegated kernels need to be explained. Such could result from contamination (i.e., the F1 plant being tested resulted from pollination by a Bz gamete, not a bz-m13 gamete), from presetting of P-mo before being separated from Spm at meiosis, or from an excision event creating a stable Bz or bz taking place early enough in development that the entire ear is derived from this cell lineage.

Even if the parent plant in the backcross progeny were P-ww/P-ww rather than P-mo/P-ww, the majority of the F1 plants have an active Spm, and this is so for all three accessions. This observation as well as the presence of an Spm in most nonmosaic plants in the progeny of mosaic pericarp parents raised the question of whether the active Spm(s) were contributed by the recurrent parent, 4Co63. Crosses analogous to those described above but crossing the bz-m13, no Spm stock onto 4Co63 instead of P-mo/P-ww and pollinating the F1 by the C sh bz, no Spm tester showed that 4Co63 is not the source of the active Spm(s). Thus three accessions of mosaic pericarp backcrossed five times to a recurrent parent that lacks Spm with selection each generation for plants strongly expressing the mosaic phenotype have retained at least one and possibly two active Spm(s) that are not linked to the P locus.

A test has been made for the presence of a dSpm inserted at the P locus. This was done by pollinating the Weatherwax accession (five times backcrossed to 4Co63 and hence segregating P-mo/P-ww and P-ww/P-ww plants) by a C sh bz wx, + Spm tester. The intent was to ascertain if any of the plants producing ears with white pericarps and white seeds could produce mosaic pericarp plants if an active Spm were introduced. A poor seed set in the summer of 1988 necessitated bulking the seed from a number of ears for planting in Florida. Two ears of the 68 produced were mosaic pericarp. It should be noted that if there were two unlinked Spm's present in mosaic pericarp plants sampled as some of the data seem to suggest, then only 1/5th of the white pericarp, white cob plants could be genotypically P-mo/P-ww minus Spm, and only 1/2 of their progeny would be mosaic pericarp. It's not possible, however, to compare the results derived from a bulked sample such as the one planted to any stated hypothesis. So while there was an indication of a dSpm inserted at P in P-mo, clearly more data are required to be certain that the seeds producing the mosaic pericarp plants did not come from a mosaic pericarp ear that had been misclassified even though this is unlikely.

Stocks that are bz/bz, P-mo/P-wr are now available and will make it possible to test directly the Spm(s) present using bz-m13 as a tester allele. While it is clear that the Spm family of transposable elements is involved, the data are not compatible with the simplest hypotheses, which are that the P-mo allele is P-rw+Spm or P-rw+dSpm with either a linked or unlinked Spm.

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