--Mark Dudley and Scott Poethig
Teopod2 (Tp2) is a semi-dominant, neomorphic mutation that causes the inappropriate expression of juvenile characteristics (such as the presence of leaf wax and prop roots, and a characteristic leaf shape) in normally adult phytomers of the plant. One explanation for this heterochronic phenotype is that a "juvenile program" of development , which is turned off at the transition to an adult phase of growth in normal plants, is expressed constitutively in Tp2 plants. To test this hypothesis we have been isolating and characterizing new alleles of Tp2.
To date we have isolated five new alleles of Tp2 (Table 1) by screening for loss of the semi-dominant Teopod phenotype in progeny of crosses in which Tp2 was the pollen parent: +/+ X Tp2/Tp2 or +/+ X Tp2/--.
Table 1. New Tp2 alleles.
These five alleles were recovered in three separate experiments. Tp2-E1 and Tp2-E2 were isolated in 1987 from EMS-mutagenized Tp2 pollen; Tp2-Mum1 was recovered in 1986 in a Robertson's mutator (Mu) background. Tp2-Mum1 has a stable and consistent phenotype, and no Mu-homologous band cosegregates with it, suggesting that the insertion of a Mu-like element is not directly responsible for this allele. Tp2-x2 and Tp2-x16 were recovered from x-ray mutagenized Tp2 pollen. In each of these experiments the progenitor Tp2 allele was linked to the recessive golden (g) marker, 2 map units distal to Tp2 on 10L, to control against contamination.
Partial Revertants. Three revertants, Tp2-E1, Tp2-E2, and Tp2-Mum1, display a partial or attenuated teopod phenotype. The order of severity of teopod expression of these alleles is:
Tp2 > Tp2-E2 = Tp2-Mum1 > Tp2-E1 > +
Tassel morphology of these alleles is a characteristic aspect of the revertant phenotype. Similarly to Tp2, tassel branching in the revertants is reduced; however, in contrast to Tp2, spikelet morphology is normal. The revertant alleles are fertile even in homozygous condition. The Tp2 progenitor allele and all partial revertants are highly pleiotropic and dependent on genetic background. In a background that enhances their expression, such as A632, the severely reduced number of tassel branches and partial vegetative transformation of the tassel characterize these revertants as teopod-like. In mild genetic background such as W23, the range of expression in + and Tp2-E1 plants overlaps, so that it is not always possible to differentiate them.
Knock-out Revertants. Preliminary investigations of Tp2-x2 and Tp2-x16 suggest that they are large deletions. Plants heterozygous for either Tp2-x2 or Tp2-x16 are indistinguishable from wild type siblings, suggesting complete loss of Tp2 function. Transmission of either allele through the male gametophyte has not been observed. In heterozygous plants, transmission of both alleles through the female gametophyte is reduced; Tp2-x2 is transmitted maternally at a frequency of approximately 33%; Tp2-x16 is transmitted at a frequency of approximately 16%. We attempted to use TB-10L translocations to uncover the Tp2-x2 and Tp2-x16 alleles in hemizygous plants using the following cross:
Tp2-x g r-/+ + r- x TB-10L19 (R-scm2)/+ r-
For Tp2-x2, approximately 1/3 of the seeds with colored endosperm (potentially hypoploid embryos) had no detectable embryo. For Tp2-x16 approximately 1/6 of the colored endosperm kernels had no embryo. This is consistent with the frequency with which these alleles are transmitted and suggests that these alleles result in embryonic lethality in hemizygous condition. Together with their relatively poor transmission through both the microgametophyte and megagametophyte, this result suggests that these alleles are large deletions. Further physical and genetic characterization of these alleles is underway.
These new alleles of Tp2 are potentially useful in several ways.
For instance, the partially revertant Tp2 alleles may allow us to
isolate genetic enhancers and suppressors of the teopod phenotype. Knock-out
revertants will be useful in examining the effects of gene dosage on Tp2
phenotype, as well as elucidating the function of the wild type allele
of Tp2. Thus, these alleles greatly enhance our prospects for understanding
the genetic regulation of phase change in maize.
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