D.R. Parker

Observations concerning the effects of radiations on the segregation of chromosomes

Abstract Nondisjunction of X and of fourth chromosomes was observed following the exposure of immature oocytes of Drosophila melanogaster to doses of X-radiation of from 1000 to 4000 R. No evidence for a threshold was found in this range for either kind of trisomy; this evidence alone does not exclude the possibility that one might be found at some lower dose. The mating of the treated females with males having an attached-XY chromosome permitted the recovery of fertile males that would otherwise have been XO and sterile. Testing of these showed some 22% to be triplo-4, having two maternal fourth chromosomes. Marking the left arm of chromosome 4 with a small duplication made it possible to score marker losses such as might result from interchange with another acrocentric (e.g., the X). There is a high coincidence of marker loss from chromosome 4 and both the XO and triplo-4 conditions, with the highest incidence of marker loss being when these have occurred together. The interpretation that the altered 4s are half-translocations resulting from X-4 interchange is further supported by the finding that they also show altered assortative behavior in compound-X females lacking a Y, when in combination with a standard fourth chromosome. A few show regular segregation from the attached-XY in the male, supporting the interpretation that they have the base of the X capped by the right arm of chromosome 4. It is argued that other trisomies may come about by mechanisms similar to that responsible for the triplo-4 condition. Furthermore, if rearrangement plays a part in the origin of trisomy, operating by altering division-I orientation as a result of heterologous conjunction maintained by chromatid interchange, it is unlikely that there will be a threshold for its induction.

Heterologous interchange at meiosis in drosophila III. Interchange-mediated nondisjunction

Abstract A model of directed disjunction of the components of quasi-bivalents formed by heterologous interchange predicts that interchange should also lead to the recovery of nondisjunctional gametes. When compound-X females lacking a Y chromosome are irradiated, 2 classes of exceptions are predicted by the model, detachments of the compound-X and triplo-4 flies, with the latter expected to be limited to the male offspring. The expectation that the triplo-4 exceptions should be male is very nearly realized, the actual ratios being 269 ♂♂: 9 ♀♀ and 42 ♂♂: 2 ♀♀ in the 2 experiments reported. The frequency of triplo-4 males increases exponentially with dose, in agreement with the increase in recovery of detachments of the compound X. Daily transfers of treated females reveals a close similarity in brood patterns for the 2 kinds of exceptions. This similarity is expected if the 2 results from a common cellular event that is followed by one or the other of a variety of different segregations. Some possible implications of a model linking the origins of trisomy and translocation are briefly discussed.

Coordinate nondisjunction of Y and fourth chromosomes in irradiated compound-X female drosophila

Abstract Irradiated compound-X females having a Y chromosome produce triplo-4 offspring in higher frequency than did similar females lacking a Y chromosome, and the ratio of the sexes among the triplo-4 exceptions is quite different in the two cases, being approx. 4 ♂♂: 1 ♀ in the former and 30 ♂♂: 1 ♀ in the latter. The majority of triplo-4 males were found to have no free Y, hence the nondisjunctional diplo-4 eggs were most often nullo-X-nullo-Y. This condition was predicted by a model of Y-4 interchange as the initiating event of radiation-induced nondisjunction of chromosomes that are not ordinarily conjoined in bivalents by chiasmata. The analysis of a number of exceptions bearing Y fragments capped by a marked arm of chromosome 4 confirms that the segregations of the homologue of the broken 4 and of the compound-X, following Y-4 interchange, are such as to give the observed excess of males among the triplo-4 offspring.

Induced heterologous exchange at meiosis in Drosophila. I. Exchanges between Y and fourth chromosomes.

Abstract Break points have been mapped for a group of 51 Y chromosome fragments induced in irradiated Drosophila melanogaster females. Fragments arising by exchange with the right arm of chromosome 4(4R) have the breakages clustered in the long arm of the Y (Y L ) proximal to the most distal male-fertility locus, and proximal to the cubitus interruptus locus in 4R. Fragments arising by other exchanges have the breakages clustered toward the ends of the Y, usually distal to the most distal male-fertility loci of either arm of the Y. It is suggested that specific, relatively short regions are regularly associated to form “sites” within which the induction of exchange is possible.

Chromosome pairing and induced exchange in Drosophila

Abstract Fragments of Y chromosomes were induced by irradiating attached-X Drosophila melanogaster females carrying a doubly marked Y chromosome, and detected by loss of one of the Y markers. Slightly more than half of the 55 fragments recovered showed linkage of fourth chromosome markers, implying a high frequency of association of Y and fourth chromosomes in oocytes. Where such fragments were shown to have Y centromeres, a high incidence of non-disjunction from the attached X was found, suggesting that exchange between heterologues leads to their separation at Anaphase I, and frequent non-disjunction from their regular pairing partners. Lucchesi s 9 assumption that fragments are not pairing-dependent for their formation is thus invalidated. It is suggested that non-reciprocal exchanges induced between the two Y arms may account for other fragments, these likewise being pairing-dependent. There is as yet no evidence that any exchange process occurs with appreciable frequency in oocytes in stage 7 or earlier that is not dependent on regular, frequent associations of the chromosomes involved.

The nature and time of occurrence of radiation-induced nondisjuction iof the arocentric X for fourth chromosomes in Drosomphila melanogaster females

Abstract Radiation-induced nondisjunction in Drosophila melanogaster females usually-possibly invariably-involves the participation of chromosomes other than the pair in which the numerical aberration is noted, with one of the two acrocentric pairs frequently being involved in the assortative error of the other. Nearly one-half of all diplo-X eggs produced following the irradiation of immature oocytes of females having free Xs are found to be nullo-4, and, in agreement with earlier reports 9,11 , about one-fourth of all nullo-X eggs are diplo-4. The incidence of structural alterations is markedly higher in chromosomes involved in nondisjunctions than in those recovered from normal segregations, with the structural changes being those expected from interchange between X and fourth chromosomes where only one of the two interchange products (a “half-translocation”) is recovered. X chromosomes may acquire an arm of chromosome 4, and fourth chromosomes may lose the marker from the left arm, as if the short, heterochromatic right arm of the X had been substituted. Homozygosis of markers near the centromere of the X chromosome shows that nearly all failures of segregation must occur at division I. While the data do not require that there be some division II nondisjunction, neither do they categorically deny the possiblility of its occurring at a very low level. The findings are as expected on the model of heterologous conjunction via chromatid interchange as the major and perhaps exclusive cause of radiation-induced nondisjunction.

X-ray-induced recombination in the fourth chromosome of Drosophila melanogaster females I. Kinetics and brood patterns ☆

Abstract X-ray-induced recombination in the fourth chromosome exhibited a greater than linear dose-response and occured predominantly in the heterochromatic regions spanning the centromere. In brooding experiments recombinant fourth chromosomes were recovered in all broods with no apparent relationship between the recovery of recombinant chromosomes and the recovery of triplo-4 and detachment-bearing progeny. There was no evidence of clustering in either early or late broods. We have concluded that X-ray-induced recombination is a translocation-like process involving homologues.

X-ray-induced recombination in the fourth chromosome of Drosophila melanogaster females II. Segregational properties of recombinant fourth chromosomes

Abstract X-ray-induced recombinant fourth chromosomes exhibited a wide range of nonrandom associations with the reversed metacentric compound-X chromosome in C( i )RM/o females. The addition of a Y chromosome altered the degree of association, often randomizing segregation of the recombinant fourth chromosome and the compound-X. In some cases, however, the addition of a Y chromosome caused the recombinant fourth chromosome and the compound-X chromosome to segregate together less often than random. We have concluded that X-ray-induced recombination is an interchange process involving breakage and rejoining of homologues. These breaks need not be precisely between homologous loci and may lead to small duplications and deficiencies. Therefore an “induced crossover” chromosomes is in reality a kind of “half-translocation”. Whether the observed variations in nonrandom associations are due to differences in mere size or differences in genetic content of the centric regions of the recombinant chromosome is problematical.

Radiation-induced nondisjunction and Robertsonian translocation in drosophila

Abstract In Drosophila melanogaster, gametes formed by oocytes in which Robertsonian translocations were induced in an immature stage usually show chromosomal imbalance. It is estimated that fewer than 20% of the gametes bearing newly induced Robertsonian translocations “fusing” X and fourth chromosomes are of balanced constitution. In contrast, when the two acrocentric pairs, X and fourth chromosomes, are replaced by an X-4 Robertsonian translocation, treatment of immature oocytes of homozygotes produces some 5–6-fold fewer sex-chromosome trisomics than do females of normal karyotype. In the place of such trisomics (having separate sex chromosomes), there is a much smaller number of compound-X chromosomes formed and a number of compound-fourth chromosomes as well. However, the production of “XO” males is not appreciably smaller in the translocation homozygotes. A number of possible mechanisms to account for this are suggested. The findings are consistent with the expectations of the hypothesis that radiation-induced nondisjunction results from improper conjunctions of heterologues, brought about by chromatid interchange7–12, 16.

The effect of exposure fraction on the yield of nondisjunctional segregation involving compound X and free 4-chromosomes in drosophila

This paper describes the effect of fractionation of an exposure of 2000-R X-rays into two 1000-R exposures separated by a 3-h interval on the frequency of various segregation products following chromatoid interchange between a compound X and the 4th chromosome. It was observed that fractionation leads to a decrease of all exceptional progeny, the result being significant at the 0.05-probability level for both classes of detachments; i.e., with or without a 4th chromosome. The finding that the same effect of fractionation was observed for numerical aberration (non-disjunction) and detachment supports the idea that these reflect different consequences of the same primary effect, radiation-induced rearrangement. Furthermore, alternative recoveries were observed to occur at equal frequencies. Following C(1)-4 interchange, the univalent 4 does not segregate at random, but assorts apart from its homologue 1.5 as frequently as it moves to the same pole.