II. 51. Radiomimetic effect of the short arm of chromosome 1 (Telo 1S) in barley?*
T. Tsuchiya, Department of Agronomy, Colorado State University, Fort Collins, Colorado 80523, U.S.A.
* This work is supported by the Research Grant from National Science Foundation (GB30493) and from USDA (12-14-5001-265).
This author reported the "univalent shift" in the progeny of telotrisomic 1S (Triplo 1S) (Tsuchiya, 1971b). In this case the frequency of the telotrisomics in the F2 population from Triplo 1S x f8 mutant was 58.3% compared to the usual 38% transmission rate for telotrisomic 1S (Singh and Tsuchiya, 1977). In contrast to the usual expectation, 18.3% of the telotrisomic plants had telocentric chromosome for the long arm of chromosome 1 (1L) in addition to 40.0% telocentric chromosome 1S. This fact suggests that misdivisions occurred frequently in the normal chromosome 1 in the parental F1 plants of telotrisomic 1S to produce telocentric chromosome 1L.
The next example is the occurrence of the plant with 2n=13+2 telocentric chromosomes in the progeny of 2x X 2x + 1 telo 1S (Triplo 1S) (Tsuchiya, 1971a). The two telocentric chromosomes were indirectly confirmed to be the telocentric chromosome 1L and 1S (Tsuchiya, unpublished). This incident also suggests the occurrence of misdivision in the normal chromosome 1 in the Triplo 1S.
Azizeh and Tsuchiya (1979) reported the occurrence of a new trisomic type in the progeny of Triplo 1S, the telotrisomic for chromosome 1S. The morphology of the new trisomic plant was similar to the Triplo 1S yet the extra chromosome was not the telocentric chromosome 1S but a kind of submetacentric and smaller than primary, or secondary chromosome for 1S. The meiotic chromosome behavior showed that the extra chromosome was not secondary or even pseudo-isochromosome.
Even though the exact nature of the extra chromosome is not known yet, it is reasonable to believe that the extra chromosome consists mostly of the short arm of chromosome 1.
It is also believed that one kind or another of chromosome breakage has occurred probably in meiosis of the parental Triplo 1S plant and produced the new extra chromosome which transmitted to the progeny instead of the telocentric chromosome 1S (Azizeh and Tsuchiya, 1979).
Tsuchiya (1979) found one more case of new trisomic plant which carried an extra acrocentric chromosome 1. Based on the size and shape of the extra acrocentric chromosome and the morphology of this new trisomic plant, it was assumed that the extra chromosome has almost complete long arm and the proximal part of the short arm with the distal portion of 1S missing. The exact nature of this extra acrocentric chromosome is now under study by Giemsa banding technique and genetic linkage analysis (Azizeh, unpublished).
There may have been some other cases of chromosomal aberrations which escaped from being found in progenies of Triplo 1S.
The peculiar nature of the short arm of the chromosome 1 (1S) was also observed in its genetic constitution and the effects on morphology and physiology as the extra chromosome. With 1S as the extra chromsome in Triplo 1S there is no effect on the morphology and physiology in Triplo 1S, and Triplo 1L showed exactly the same morphology as primary trisomic Triplo 1 (Bush), even though the length of the short and long arms of chromosome 1 is almost the same (Tsuchiya, 1969 1971a; Fedak et al. 1971). Yet the short arm carries a good number of detectable mutant genes as shown in the recent linkage maps (Tsuchiya, 1978). It is therefore obvious that chromosome 1S is not genetically inert but does not have genetic effects on the developmental process in the ontogeny of the Triplo 1S plants.
Considering all those findings on Triplo 1S and/or telocentric chromosome 1S, it is suggested that the short arm of chromosome 1 (1S) may have a kind of radiomimetic effect (Dustin, 1947; Rieger et al., 1976; Eigsti and Dustin, 1955) to produce chromosome breakage in chromsome l.
Even though this is a mere speculation that chromosome 1S may have a radiomimetic effect of chromosome breakage, especially at the centromeric region of chromosome 1, there is a supporting evidence for the specific nature of chromosome 1S. A large number of plants have been studied in the progenies of primary trisomic for chromosome 1 (Triplo 1) and telotrisomic for chromosome 1L (the long arm), yet no univalent shift has so far been observed in Triplo 1L.
The detailed analysis of meiotic behavior of chromosomes in the Triplo 1S will provide at least some clues to the origin and the nature of the suggested radiomimetic effect of chromosome 1S.
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