How far does Ac jump on average? "About one foot" (34 cm)!

In order to understand the mechanism of Ac transposition, it is important to know the pattern of transposition. Greenblatt (Genetics 58:585, 1968) selected kernels from either twinned or untwinned red multikernel sectors derived from medium variegated ears (genotype: P-VV/P-WR). The red sectors are the result of a somatic transposition of Ac from P-VV. Greenblatt reported that from a total of 168 red sectors the average recombination distance of tr-Ac from the P locus is 34 CM (centi-morgan). In the course of isolation of Ac induced mutants at P, we collected tr-Ac from the P locus using a different selective screen, thus allowing a comparison of the effect of seed selection methods on the measured recombination results.

From crosses of P-VV/P-WR, r/r X P-WR/P-WR, r-m3/r-m3 (r-m3 is a Ds suppressed R-sc allele, isolated by Dr. Kermicle and here used as an indicator of Ac activity), we selected kernels on these medium variegated ears with the phenotype of red pericarp and variegated colored aleurone. The genotype of these kernels will be either P-RR tr-Ac/PWR or P-WR tr-Ac/P-WR if the pericarp and the pollen mother cell share the same cell lineage. Otherwise, in cases where the pericarp, and the female gametes are discordant, medium variegated will be expected along with 7 percent light variegated. The frequency of tr-Ac red vs. tr-Ac P-WR provides a direct measure of tr-Ac P recombination frequency and is comparable to red sector mapping results. Plants from all kernels were self pollinated (mating of course does not affect pericarp phenotype in this generation).

The seed selections yielded the following data:
312 ears with red pericarp
161 ears with colorless pericarp 1 ear with light variegated pericarp
17 ears with medium variegated pericarp

All ears showed R-sc mutability of the r-m3 allele and hence had tr-Ac activity The recombination frequency is 34% (161/(312 + 161). These results are indistinguishable from those of Greenblatt (1968), confirming an average length of transposition of Ac at 34 CM. While the two selection methods yield the same results, the use of the r-m3 method is much the more efficient in terms of transpositions recordable per ear as well as the work required for confirmation of progeny genotypes.

Among the realized 312 red ears more than 10 percent (46) exhibited somatic instability of the red pericarp somatic lineages. We find this a high rate based on prior observations made by Greenblatt (unpublished) of this phenotypic class. Referring to the reports of Orton and Brink (Genetics 53:7, 1966), many of these unstable red cases illustrate the subsequent return of the tr-Ac back to the P locus. The somatic loss of pericarp pigment might also be due to P inactivation by other transposable element systems. This latter possibility raises the issue of the interactive behavior of different transposable elements and/or they may signal the reactiveness of different element systems to a common cellular stimulation of transposition.

Jychian Chen, Stephen Dellaporta and Irwin M. Greenblatt

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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