Effects of X-Rays on Chromosomes of Lilium Longiflorum during Meiosis.

TUDIES of the types of aberrations induced in chromosomes by irradiation S during the meiotic prophase have been infrequent. The complications introduced by synapsis and crossing over make the analysis much more difficult than is the case for mitotic divisions on which most of the work has been concentrated. However, in the assessment of radiation damage to genetic material, the differential sensitivity of the meiotic stages and a knowledge of the types of aberrations expected may be of considerable importance. SINGLETON and CASPAR (1954) have shown that the mutants recovered in Zea mays after irradiation vary strikingly with the stage of gametogenesis irradiated. Earlier, SPARROW ( 1944, 195 1 ) had observed remarkable changes during meiosis in the sensitivity of Trillium chromosomes to the production of aberrations by irradiation. GLASS (1955) has reviewed the variations in mutants recovered after irradiating Drosophila at various stages of gametogenesis. One of the difficulties encountered in most material is the inability to predict with accuracy the stage of nuclear development at the time of irradiation. In periods where rapid shifts in sensitivity occur, determination of stage irradiated becomes an important consideration. By employing the anthers of Lilium longiflorum, in which a close relation exists between the stage of gametogenesis and bud length, predictions of stage within narrow limits can be made. The large anthers provide thousands of cells at similar stages of development. Although there are twelve pairs of chromosomes, these are large and may be well-spread in squashes of relatively large pollen mother cells. Utilizing this material a study of the differential sensitivity of the chromosomes to’ radiation has been made for premeiotic and meiotic stages. An analysis of the types of aberrations was attempted with particular reference to the types that could be used to distinguish between chromosome, chromatid, and subchromatid breakages. The data are presented with interpretations that relate to such topics as time of chromosome duplication, crossing over and possible causes for variations in sensitivity.