DEFIANCE, OHIO

Defiance College

 Temperature programming of paramutant R-gene expression --Bernard C. Mikula
  Transposable elements offer the opportunity to witness genetic change through modulation of gene expression. A transposable element system which permits high frequency genetic changes directed at a specific allele becomes interesting from an evolutionary point of view since sources of heritable change which can serve evolutionary purposes have evaded experimental methods. Paramutation, a two element system explored by the late R. A. Brink and his students, generates a heterogeneity of R-gene expression which could model evolutionary mechanisms for directed, high-frequency (100%) change at the single gene level. This paramutation system has been shown to be subject to environmental modulation and becomes even more interesting when it is recalled that the paramutated R-gene can undergo incremental change from one generation to another. This system with its long-term, incremental memory makes it possible to search environmental and developmental variables for more efficient methods of programming gene expression.

Our efforts have been directed at the light and temperature environments during early development, the period tassels are determined. In the present experiment, seedlings were kept under continuous light for 21 days except for the two to four-day, 12 hr light and 12 hr dark treatments. Each set of seedlings was held either at 28 or 22 C during the 21-day treatment period. For the rest of the life cycle treated seedlings were transplanted and matured under field conditions. Other growth chamber conditions were those reported in previous MNL reports, Vols. 40-45. Our objective is to find allelic systems most responsive to our variables.

Table 1. Comparison of pigment expressions of paramutated R-alleles from plants grown at temperatures of 28 and 22 C during the first 21 days of seedling development. Several pollen samples were taken from the same plant on different days and are indicated as No. of ears scored. All testcrosses from a single tassel were scored and represented as an average.
 
R-g ex R R-lst
R-g ex R R-st
No. ears scored
28 C R-g score
22 C R-g score
No. ears scored
No. ears scored
28 C R-g score
22 C R-g score
No. ears scored
4
0.7
2.5
1
3
0.4
1.8
6
3
0.9
2.7
1
3
1.6
1.8
2
4
1.3
4.6
1
3
1.8
2.4
3
3
1.4
5.2
5
2
1.8
2.5
5
2
1.4
5.2
3
2
1.8
2.9
5
3
1.5
7.0
6
5
2.0
3.2
3
1
1.7
7.3
1
2
2.3
3.6
3
3
1.9
7.5
1
3
2.5
3.8
4
5
1.9
8.4
2
4
2.5
4.1
3
4
2.0
8.7
2
4
2.6
5.0
6
2
2.0
8.9
2
3
4.1
5.1
3
3
2.9
9.4
4
3
4.6
5.2
4
4
3.0
10.1
1
2
6.3
5.6
2
2
4.8
10.1
5
    
5.9
2
Mean
2.0
7.0
      
5.9
2
           
6.6
3
       
Mean
2.6
4.1
  

An R-g allele, obtained from Native Seeds/SEARCH, Tucson, AZ, was crossed as female to R-st and R-lst obtained from the Corn Coop. Testcrosses of these heterozygotes were made onto W23. Scoring of pigment from resulting paramutated R alleles was done by matching 50-kernel samples against a set of 20 standard kernels. Values for these 50-kernel samples are reported as ear means. Since we have reported significant variation from pollen samples of a single tassel, where possible, several pollinations over a seven day period were made and the average of all is reported.

Our results in Table 1 show that R R-lst heterozygotes produced pigment scores related to the temperature the seedlings received. Yet, the same R allele from the R R-st combination showed no such temperature relationship though treated under the same growth chamber conditions. We have no explanation for the difference; many variables remain to be tested at this early stage of development. It is quite possible that for the R R-st combination developmental timing is different and will require more careful attention.

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

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