--Rob Martienssen, Alice Barkan, William C. Taylor and Michael Freeling
The high chlorophyll fluorescence maize mutant hcf*-106 is a recessive, pale green, seedling lethal that arose in a Robertson's Mutator line and shows somatic instability (namely small, late dark green sectors) typical of a Mu insertion. Molecular analysis and cloning has shown that a 3.7kb SstI fragment containing a Mu1-like element probably corresponds to part of the hcf*-106 locus (Martienssen, Barkan, Freeling, and Taylor, submitted).
The mutant hcf*-106 phenotype is only observed in seedlings with Mu1 elements that are active and unmodified at the HinfI sites in their terminal inverted repeats. Seedlings homozygous for the mutant allele that have modified elements are normal in appearance and survive to maturity. This suppression of the mutant phenotype results in its disappearance from pedigrees that are losing Mu activity in successive generations of selfing and outcrossing. However, suppressed plants can be backcrossed to Mu-active plants that are heterozygous for hcf*-106 and, typically, these crosses re-activate inactive Mu1 elements. Consequently, backcross progeny that are homozygous for hcf*-106 adopt a mutant phenotype and are seedling lethal. We have termed hcf*-106 "Mu-suppressible", by analogy with McClintock's Spm-suppressible mutants in maize (Masson et al., Genetics 177:117-137, 1987).
Occasionally, mutant seedlings are observed that have large sectors of dark green, low-fluorescent leaf tissue on pale green high-fluorescent mutant leaves. These sectors are much larger than those typical of Mu excision, and Southern analysis shows no sign of the revertant allele diagnostic of revertant sectors. However, these wild-type sectors contain hypermodified Mu1 elements relative to adjacent mutant leaf tissue (Martienssen et al., submitted). This shows that leaf sectors containing modified Mu1 elements can arise from single somatic cells during development.
Sectored plants display an interesting relationship between the size of inactive sectors and leaf position. While lower leaves are mainly mutant, the proportion of "wild-type" tissue increases in successive leaves until the upper leaves are entirely normal in appearance. Apparently, as plant development proceeds, dividing cells within the meristem "turn-off" progressively to generate larger sectors of "wild-type" tissue until, in the upper leaves, most cells contain inactive Mu elements. The more meristematic divisions a cell undergoes, or the longer a given cell lineage spends in the meristematic condition, the more likely it is to adopt an inactive phase.
This developmental pattern may provide an explanation for the reciprocal
effect observed in crosses between Mu-active and Mu -inactive
plants (Walbot, Genetics 114:1293-1312, 1986; Bennetzen, Mol.Gen. Genet.
208:45-51, 1987; our unpublished results ). That is, some active plants
(but not all) are more likely to lose Mu activity through the male
rather than the female flower. Because of the relationship between developmental
position and size of inactive sector, the male flower at the apex of the
plant may contain larger inactive sectors than the female flower, resulting
in a higher proportion of gametes carrying inactive Mu. According
to this model, crosses involving unsectored plants would show no reciprocal
effect. Reciprocal effects observed in the cycling of some Spm elements
(Fedoroff and Banks, Genetics 120:559-577, 1988), and in the inheritance
of paramutable phenotypes of B (booster) (Coe, Genetics 53:1035-1063,
1966) could reflect a similar mechanism.
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