Paramutation: a tassel mosaic controlled by temperature
--Bernard C. Mikula
In MNL 64 we reported differences in the level of paramutation of an R-g allele received from Native Seeds/SEARCH resulted from temperature conditions administered to R R-lst seedlings during the first three weeks of development. Temperatures of 28 C produced plants whose R-expressions in testcrosses were significantly lighter than those from plants grown at 22 C for the same period. Seeds of 1989 testcrosses were grown out and testcrossed in 1990 to test for heritability. Fig. 1 shows that the relative differences of the previous year are still apparent in the testcrosses of 1990. Four sets of testcrosses on the left of Fig. 1 represent four different testcross ears from 1989 which scored 10 or above for level of pigmentation (on a scale of 0 to 20). Five sets of testcrosses on the right represent five testcrosses from 1989 which scored 1.0 or less for paramutated R-gene expression.
When R R-lst plants are testcrossed over the seven days that pollen is shed, a tassel mosaic for paramutated R-gene expression can be noted. Seeds from each of the seven testcrosses of the previous year were grown out and testcrossed for heritability of the tassel mosaic; differences can still be observed in paramutated R-gene expression (Figure 2). The three ears on the right are from testcrosses from the earliest pollen collection (8/4); the two sets of three ears to the left represent collections made five and seven days later (8/9 and 8/11). Tassel mosaics were more readily observed with the new R-genes from the Southwest than from the more highly inbred R-genes in W22 background which often tend to be "subliminal" and require tedious scoring procedures.
Does inbreeding reduce the range of tassel mosaicism? In the early days of getting the paramutation phenomenon accepted, it was essential to set aside the arguments for segregating modifiers. The climate for accepting "unstable" genes within the canons of orderly Mendelian behavior demanded an inbred background if paramutation was to be taken seriously. Our results over the past few years seem to be telling us that "unorthodox" genetic behaviors will require careful attention to how conventional procedures may inhibit the investigations of some of the more challenging questions facing genetics. "Inbred" habits are hard to overcome!
Four new R-g alleles from Native Seeds/SEARCH were made heterozygous with an R-lst allele received from the Maize Coop. When plants from R R-lst seedlings, grown at 3l C or 22 C for the first three weeks, are testcrossed a clearly visible difference in the level of paramutation is noted. Fig. 3 shows an example of how one of the alleles responded to these temperature differences to which seedlings were exposed. The bottom half of Figure 3 shows testcrosses of plants whose seedlings received 22 C; the upper half of the photo shows testcrosses of those plants whose seedlings received 31 C. All of the other three alleles tested show similar differences. It must be pointed out that all male-transmitted differences similar to those shown here have been found to be heritable. Heritable responses of a gene to temperature at a critical stage in development may be considered a case of transgenerational, programmable, genetic modification. Jacob summarized the 20th Century dogma when he said the genetic "program does not learn from experience". Is it possible that with this paramutation evidence the dogma can be challenged, using "unstable alleles" such as R and R-st? The transacting paramutagenic alleles are probably but one example of the many two-element transacting systems whose utility for discussions of the most interesting and fundamental biological questions must still be determined.
Before passing judgment on Jacobian dogma, we would advise caution. It must be pointed out as did Rhoades when he first described a two element system in 1941, there are some who would consider unstable genes "sick". The "sick genes", occupying the attention of genetics the last half of this century, may, however, provide the therapy essential for initiating 21st Century genetics!
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