3. Comparison of ultraviolet and X-ray deficiencies. Earlier examinations of ultraviolet induced deficiencies in maize indicated that they were terminal whereas X-ray deficiencies appeared to be usually, perhaps always, internal. Since non-homologous pairing of pachytene chromosomes frequently occurs, this point could be settled only by a study of a chromosome arm with a terminal cytological marker. In order to select plants with breaks in this arm, a gene affecting a seedling character was essential. Enoades reported bronze (bz) in the short arm of chromosome 9 (corn letter 1943). It was found that in the presence of certain Rr alleles, distinct color developed at the tip of seedling leaves with Bz but failed to develop with bz. Deficiencies of the bronze locus were induced by irradiation of mature pollen from a knobbed-9 stock, wx-Bz-knob. Pollinations were made on a homozygous or heterozygous bz stock, Wx-bz. The colorless-tip F1 plants which subsequently developed bronze pigment instead of anthocyanin furnished the cytological material. The usual acetocarmine smear technique was employed.
In the ultraviolet group, 3513 seedlings were examined of which 9 possibly tipless died in early seedling stage and 9 were bronze plants. In the X-ray group, 1670 seedlings were examined, of which 7 possibly tipless died in the early seedling stage and 11 were bronze plants. The cytological study of the bronze plants is summarized in the table.
Kinds of Chromosomal Change
|1 break||2 break rearrangement|
In the ultraviolet material single breaks in the short arm of chromosome 9 gave terminal deficiencies (with the loss of the knob) in 4 plants. The shortest deficiency, about one-third of the arm, removed the bronze locus and gave less than 1% crossing over between the break and the wx locus. In two cases breaks in different chromosomes were followed by rearrangement in such a way that parts of both chromosomes were lost and only one translocation chromosome survived. These have been called deficiency translocations. In pachytene the translocation chromosome pairs homologously with parts of the two normal chromosomes, and the two single strands usually pair non-homologously to give a three-armed translocation figure. At diakinesis and metaphase I, this association appears as a chain of three chromosomes or, less frequently, as a pair and a univalent. Anaphase I shows 9-10 separations or occasionally 9-9 with a lagging univalent. Pachytene preparations were not clear enough to determine exact points of breakage in the chromosomes.
All X-ray deficiencies resulted from rearrangements involving two breaks within the same cell. In one case both breaks were in the short arm of chromosome 9. giving an internal deficiency. In 3 cases breaks occurred in both arms of 9, a ring fragment which included the centromere being formed. Five deficiency translocations were found. In the case giving the best cytological preparations (involving chromosomes 9 and 5) both breaks appeared to be at or very near the spindle fiber regions. There were no cases of terminal deficiency.
Many plants with deficiency translocations (in this and other material) show a higher percentage of normal pollen than can be accounted for by random distribution of the three associated chromosomes at the first meiotic division.
Katherine O. De Boer