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UBC Theses and Dissertations

Autosomal products of meiosis arising from radiation-induced interchange in female Drosophila Melanogaster Gibson, William Glen


The present study was initiated with the view of achieving two goals: 1) to establish a suitable genetic assay system for measuring the frequency of spontaneous and induced structural and numerical aberrations of autosomes during meiosis in females and 2) to provide a better understanding of the mechanisms responsible for the production of the aberrant classes recovered. By selective exclusion of all regular meiotic products this system enabled the recovery of large numbers of aberrant products. The multiplier system served as an internal dosimeter and provided an estimate of the population size from which the aberrancies arose which in turn provided a measure of the frequencies of each event. The four different classes of exceptional meiotic products were named according to the source or the structural nature of the chromosomes: reductional nondisjunction as "matroclinous"; equational nondisjunction as "equationals"; loss of chromosome 2 as "patroclinous"; and the attachment of homologous arms as "compounds". The results suggest that two main factors affect the recovery of induced aberrations: of most importance is isosequentiality and of lesser importance is genetic background. The three classes of simultaneously recovered progeny (excluding equational nondisjunctions) arise from a common mechanism of induction; a mechanism which also accounts for free arm formation. The location of the breaks, the position of the chromatids and the method of reconstitution determine the type of aberration produced. The reconstitution of these breaks in aberrant ways are referred to as interchanges. Furthermore, it would appear that the reconstitutions are restricted in that euchromatic breaks attach to euchromatic breaks and heterochromatic to heterochromatic. Interchanges resulting from breaks on opposite sides of the centromeres of homologues result in the formation of non-sister compound chromosomes and from breaks on opposite sides of the centromeres of sister chromatids result in the formation of sister compound chromosomes. The interchange, if between heterologues, could lead to the nondisjunction of a pair of chromosomes and be recovered, as in the present study, as matroclinous progeny. The reciprocal product of the interchange between heterologues would produce an equal number of nullo eggs observed as patroclinous progeny, but if the dyad so formed is heteromorphic, i.e. chromatids of different length, it would result in the greater recovery of patroclinous progeny because of the preferential inclusion of the shorter chromatid. The evidence for interchange mediated aberrations is provided by the recovery of free arms of chromosome 2. Experimental support for these events is provided by the unequivocal identification of the centromeres involved, which, as in this study, is made possible through the use of metacentric autosomes.

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