| In the X2 - X5-progenies were often found various
abnormal characters, e.g., dwarfness, short spike, lax spike, speltoid,
compactoid, square head, thick culm, slendef culm, bent culm, narrow leaf,
sterility, etc. Most of these abnormalities appeared to be associated
with various chromosome aberrations. For instance, speltoid plants had always
42 or less chromosomes while compactoid plants had always 42 or more. On
the other hand there were some mutants showing only normal 21 bivalents.
They were probably induced through gene mutations. It was generally observed that the plants with 42 chromosomes had high fertilities but those with chromosome numbers deviating from 42 showed decreased fertilities. Moreover, it should be noted that wide variations in the fertility were often observed in the plants with the same chromosome number, probably due to the presence of multivalent pairings of chromosomes or other chromosome aberrations (Tab. 2). Many monosomic plants were found in the progenies of 15 different X2-lines. They were expected to segregate nulli-, mono- and disomic (normal) plants in the following generations, and nullisomic plants were actually obtained in the progenies of seven X2-lines. All nullisomic plants in these lines were dwarf and feeble, and had 40-100% sterility. Mono- and disomic (normal) segregates were usually vigorous and highly fertile. Nullisomic segregates could easliy be distinguished from the related mono- and disomic ones by the differences in their morphological characteristics. Seven nullisomic types were also distinguished from each other by their different morphological characters. Nullisomic plants bred true usually in the following generation if they were fertile. However three giant plants were found in the progeny of a nullisomic strain (N4). They had 41 somatic chromosomes and showed a specific meiotic configuration of 1III + 19II in most of the pollen mother cells. The formation of a trivalent could be due to a substitutional increase of a homoeologous chromosome in place of the chromosome which was lost in the nullisomics. Their seed fertilities and plant growth recovered as in usual monosomics, but their major characteristics were rather similar to those of the original nullisomic plants. In the progenies of three different X2-lines were found some segregates which showed partial asynapsis of chromosomes at MI of PMC's. That is, only one and three asynaptic plants were found in two strains, PA1 and PA2 respectively. In the remaining strain (PA3), however, three partially asynaptic X3-plants were found and all of their X4-X5 plants were partially asynaptic. The chromosome numbers of these segregates ranged from 40 to 43. The number of univalents in the plants with 40 chromosomes varied from 0 to 20 per cell, 5.63 being the average. A range from 1 to 19 univalants and an average of 5.55 univalents were counted in the plants possessing 41 chromosomes. Considering the meiotic abnormalities, the low fertilities in the asynaptic plants could easily be explained. However, the partially asynaptic plants with 41-43 chromosomes usually showed higher seed fertilities than those with 40 chromosomes most of which were completely sterile. And it was further noted that asynaptic plants with 40 chromosomes were generally accompanied with dwarfness with a few exceptions. The high sterility and dwarfness might mainly be caused by the nullisomic nature of chromosome constitution. An experiment is now under way to know the genetic behavior of the partial asynapsis in relation to the asynaptic effect of chromosome V of Chinese Spring wheat (OKAMOTO 1957). |
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