--Virginia Walbot

This is chapter 2 of the story of I216-3, an exceptional plant carrying bz2-mu2 with an early excision pattern. The sectoring is radically different from the typical Mu-induced allele in which few sectors larger than 250 cells are found (A. A. Levy and V. Walbot, Science 248:1534, 1990). In bz2-mu1, the highest excision frequency occurs near the end of aleurone development, resulting in sectors with 32 (25) or 64 (26) cells. In contrast, among the 714 large + fine spotted bz2-mu2/bz2 kernels in the derivatives of I216-3 examined from an outcross population, early somatic events were common.
 

Sector	Cell Number	Number of Events	Frequency (x 10-3)
1/2 kernel	217	7	4.90
1/4	216	30	10.5
1/8	215	57	9.98
1/16	214	88	7.70
1/32	213	148	6.48

Frequency was calculated by dividing the number of events by the number of possible events (product of 714 and the reciprocal of sector size). These frequencies are similar to those recorded previously for bz2-Ds and c2-Spm.

The ratio of large:fine spotted kernels varied widely among the ears examined, and is hypothesized to reflect the activity state of Mutator rather than segregation of a single factor regulating excision timing. Virtually the entire population of bz2-mu2 individuals (those described above and two additional groups with 31 more plants) show signs of losing Mutator activity: fewer than expected spotted kernels, some kernels with just a few spots per kernel. Further support for the hypothesis that Mutator activity state is affecting the large spotting phenotype comes from examination of the progeny of fine spotted I216-3 kernels with the typical Mutator phenotype. When fully active, these gave rise to ears with the full spectrum of phenotypes: germinal revertants, large spots and fine spotted kernels. Kernels with rare spotting indicative of initial loss of Mutator activity, on the other hand, gave kernels with no or small sectors only.

By Southern blotting the reporter alleles in large spot stocks appear unchanged. In progress are trans-activation tests to determine whether the early excision bz1-mu1 and bz2-mu2 stocks can change the sector size in c2-mu1 and a2 mutable alleles. I want to determine whether the early excision pattern is dominant and whether such lines can reactivate cryptic Mutator reporter alleles efficiently. Using a bz2-mu2 R-r stock I plan to determine whether early excision also applies to tassel development. Also in progress is a test of the transmission of exceptional "large spots" found in typical Mutator material. Twelve exceptional bz2-mu1 kernels were found among 38,000 in the reversion test described in an accompanying note. Eight of these kernels had one spot each of ~256-512 cells, and the remaining 4 kernels each had one very large sector (estimated sizes: ~2,000, 5,000, 10,000 and 16,000 cells). If these exceptional kernels represent heritable changes in Mutator activity, then this change is relatively common (3 x 10-4).

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