| On the mechanism of appearance of gigas-plants
from nullisomic dwarf wheat T.OHTA and S. MATSUMURA Kihara Institute for Biological Research, Yokohama & National Institute of Genetics, Misima, Japan In the offspring of the pentaploid wheat hybrid between Triticum Spelta and T. polonicm dwarf plants possessing 40 chromosomes (20II) are found. They are nullisomics, deficient in a chromosome pair of the D-genome. As it is the a-g-chromosome in the D-genome which is missing, they are called a-g-dwarfs. So-called giant plants of normal height and vigor appear unexpectedly in their selfed progeny. These are respectively called a-g-gigas, according to the original seven different dwarfs, in which the missing D-pair is replaced by the homoeologous A- or B-chromosome pair. Gigas-plants are assumed to appear from nullisomic dwarf wheat as a result of chromosome aberrations in MI. Various chromosome aberrations in MI and unequal distribution in AI of a-g-dwarf lines were observed and chromosome aberration rate, theoretical and observed appearance rates of gigas were compared for seven dwarf lines. Except the g-dwarf (Dwl. 5), theoretical appearance rates of gigas are too small against the observation, amounting to 5-10%. There are many factors involved, such as competitive fertilizatian, elimination of dwarfs and others. Still more important is the fact that the special chromosomes which are homoeologous to the deficient one, become easily aberrant. In the case of b-warf, the special chromosome is a Sat-chromosome which becomes very often a univalent. |