2. Distribution and recombination of X‑ray induced chromosome breaks.
A general survey was made on the apparent breaks of maize chromosomes induced by X‑rays of about 2000 r units with special reference to their distribution and recombination among individual chromosomes. The cytological location of the breaks was determined with the precision possible at the pachytene stage in the progeny obtained from treated pollen, and was expressed as a decimal fraction of the distance from the centromere to the end of each arm. Statistical analysis of 647 breaks on the basis of the breakage hypothesis showed that, excluding the breaks resulting in deficiencies, the breaks that produced viable rearrangements such as translocations and inversions were distributed at random among the arms of the chromosomes. There was close agreement between the frequencies of observed breaks and that of expected according to the relative proportion of the arm length of all chromosomes measured at meiotic prophase. However, significant deviation from chance occurrence of breaks was noted in the heterochromatic regions where the knobs and the nucleolar organizer are located.
It was attempted to determine whether the frequency distribution of number of breaks in each plant agreed with the expected frequencies calculated on the basis of Poisson formula if randomness. The test gave a very poor fit. Greater departure from random distribution was noted in classes with odd number of breaks, and the observed values for classes with 2 and 4 breaks far exceeded the theoretical. Such discrepancies are due to the fact that the samples included only types that survived to the observable stage, thus representing only part of the potential breaks.
About 65% of the treated pollen that functioned transmitted structural changes of chromosomes detectable, at mid‑prophase or later stages of the first meiotic division. Of the total aberrations observed there were 68.5% translocations, 4.8% inversions, 24.2% deficiencies and 2.5% others.
The data on the relative frequencies of different types of translocations agreed fairly well with those reported by previous workers (cf. Catcheside, 1938. Jour. Genetics 36:321‑328). Nevertheless, the results of the present investigation were not in agreement with expectations based on the breakage hypothesis, without certain modifications of the hypothesis including a proximity effect. The distribution of translocation breaks among arms and chromosomes and of the number of translocations per plant were found to be at random. This suggested that a great majority of the induced translocations must have been viable. However, some degree of differential recombination concerning translocations was noted between the chromosomes and also between the arm, since the possible recombinations were not evenly spread among the chromosomes.
Comparison of the relative frequencies of intra‑arm, inter-arm and inter‑chromosome rearrangements on the basis of the same number of breaks per cell or plant showed that there was no agreement between the observed and the expected values. The experimental frequencies of intra‑arm changes, the majority of which were deletion types, were consistently higher than the calculated values while those of translocations were lower than the theoretical. This preferential exchange within an arm seemed to be induced by failure of re‑fusion of the broken ends, which would result, in the production of more deletions than inversions. More deletions occurred in the chromosomes of small size. A plausible explanation supposed that shorter chromosomes were perhaps free to move in the nucleus during or after irradiation so that rejoining may occur between two breaks in an arm at greater distance than in longer chromosomes that are less free to move. In this respect, the observations in Zea mays seem to be in line with those in Drosophila, Tradescantia and Tulip.
(This note is abstracted from part of a thesis presented to the Graduate School of Cornell University in 1948.)
C. H. Li