The effects of X-radiation on chromosomes in the microspores of trillium erectum linn

Abstract

Microspores were irradiated in the homoeotypic telophase or early "resting stage" of the pollen tetrads. The 6 anthers in a bud are usually almost synchronous in their division stages. One anther was removed immediately after irradiation and the remainder at 2-3-day intervals. It was thus possible to determine with fair accuracy the stage of division at the time of irradiation, and to be certain that the observations were made in the 1st division cycle after irradiation. The chromosomes were stained to show their internal, chromatid structure. Xo morphological effects were observed immediately following irradiation, but mitosis was delayed. X-radiation causes breaks and translocations of either whole chromosomes or of their constituent chromatids. This applies equally to the region of the "attachment constriction." Lateral translocations of chromosome fragments on to a broken chromatid were found with high frequency in the 1st mitotic prophase following irradiation. In the succeeding nuclear generation these will appear to be terminal translocations. "Constrictions" apparently similar to those reported by others as arising after irradiation, were found upon de-staining to be merely chromatid breaks. Breaks occurring in chromatids will not be obvious before anaphase in preparations so stained that the chromo-nemata are not differentiated from the surrounding matrix, and they may not be seen until the next division. This explains, in part, the general observed "delayed action" of x-rays in causing fragmentation. The somatic chromosomes of Trillium microsporocytes, including the "attachment constriction," are longitudinally double at all stages except just prior to anaphase separation, when they are 4-partite. "Trabants" have been found to arise de novo. One example of ring formation was observed. The observations are discussed in relation to certain genetic evidence on the nature of chromosome rearrangements and mutations.