The Sensitive-Volume of the Meiotic Chromonemata of Gasteria as Determined by Irradiation with X-Rays

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Abstract
The distribution of meiotic and mitotic stages of pollen mother cells in a spike of Gasteria was determined. Cells of one bud were in approximately the same stage. Spikes were exposed to various doses of x-rays and the anthers of each bud examined microscopically when the cells reached the 1st meiotic ana-phase. Three types of chromosome abnormalities were observed: (1) attachment and attenuation of chro-matids; (2) breakage of chromatids; (3) achromatic spots in the chromatids. All these effects are in the chromonema of the chromatid and apparently do not involve the matrix material. Maximum frequencies of abnormalities were observed in buds irradiated in the pachytene stage of meiosis and in the very early pro-phase or end of the resting stage of the preceding mitosis. There is a rectilinear relationship between dosage and frequency of induced abnormalities. The results are taken to indicate that the x-ray sensitive portions of the chromonemata are closely approximated in pairs at pachytene and the very early prophase, that each homologue is at least 2-parted at pachytene, and that a chromonematic division occurs between pachytene and the 1st meiotic anaphase. The diameter of the sensitive volume of the chromatid was calculated and found to be similar to the diameters of proteins such as albumin and trypsin. The linkage mechanism (that portion affected in the production of attachments and fragments and presumably therefore the portion responsible for maintaining the linear cohesion of the chromonema) was found to make up approximately 90% of the total sensitive volume. This was found to agree with the estimated reduction in length of the chromonema between pachytene and metaphase.

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