R-locus imprinting is tissue-specific and not positional

--Joe Colasanti, Amar J. S. Klar and V. Sundaresan

The R locus of maize is involved in the regulation of anthocyanin pigment production in the seed and plant. A functional R allele allows the expression of purple pigmentation in the seed aleurone. In the R-mottling phenomenon, the expression of R in seeds of R/r heterozygotes depends on the direction of the cross. Certain alleles of R (R-r, for example) exhibit normal seed pigmentation when transmitted through the female gametophyte; however, when transmitted through the male gametophyte, R is expressed incompletely and the aleurone pigmentation is mottled. Kermicle (Genetics 66: 69-85,1970) has shown, using B-chromosome translocations, that the absence of mottling when R is transmitted through the female is not due to an extra dose of the R allele in the triploid endosperm, suggesting that during the development of the male gametophyte, the R allele is "imprinted" or altered in an epigenetic fashion such that its expression is suppressed in the next generation. The molecular basis of this temporary change in the activity of this allele is presently unknown.

In an attempt to determine whether R-allele imprinting in the pollen occurs as a result of the position of the tassel in the plant or whether the imprinting is gametophyte-specific, we have utilized the tassel seed mutant Ts6. Ts6 is a dominant mutation that results in the formation of a female flower at the position where the male flower would normally form. The tassel seed ear is a large multi-spike, tassel-like structure with extensive silks; the ear produces no pollen but it can be cross-pollinated to yield mature kernels capable of expressing full color.

The question we asked was: "Do the kernels of a tassel seed ear carrying the R-r allele exhibit the R-mottling phenotype?", i.e., will R-r be imprinted in female gametophytes derived from what is normally the cell lineage leading to male gametophytes. The Ts6 stock obtained from the maize co-op carried the recessive r-g allele. This was crossed to a R-r stock, and the progeny that inherited Ts6 were crossed to pollen from r-g and R-g testers. Both the tassel seed ear and the normal ear of the Ts6 plants were pollinated. From several such crosses it is apparent that the tassel seed ear behaves much like a normal ear in this respect. That is, when the R-r allele is transmitted through kernels of the tassel seed ear they express full color, and mottling occurs when R-r pollen is crossed to a tassel seed kernel with an r-g genotype. From these observations we conclude that the position of the flower has no effect on imprinting; and that R-locus imprinting is male gametophyte-specific, probably occurring late in the development of the tassel.
 
Cross Ear Phenotype
Ts6/+, R-r/r-g x +/+, r-g/r-g tassel seed full color: colorless, 1:1
Ts6/+, R-r/r-g x +/+, r-g/r-g normal full color: colorless, 1:1
     
Ts6/+, R-r/r-g x +/+, R-g/R-g tassel seed full color: mottled, 1:1
Ts6/+, R-r/r-g x +/+, R-g/R-g normal full color: mottled, 1:1


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