B. Leigh

Concentration-effect studies with MMS, TEB, 2,4,6-triCl-PDMT, and Den on the induction of dominant and recessive lethals, chromosome loss and translocations in drosophila sperm

Comparative tests were made with four mutagens, treating male germ cells, particularly mature sperm, of Drosophila melanogaster. Dominant lethals, sex-linked recessive lethals, sex-chromosome loss and partial loss, and in one test translocations were used as genetic and points. The four mutagens, methanesulphonate (MMS), 2,3,5,6-tetraethyleneimino-I,4-benzoquinone (TEB), I(2,4,6-trichlorophenyl)-3,3-dimethyltriazene (2,4,6-triC1-PDMT), and diethyl nitrosamine (DEN) are known to differ in their chemical properties and mode of mutagenic action. An apparent relationship between dominant lethal induction and other genetic damage was found only with TEB. All four mutagens are efficient inducers of sex-linked recessive lethals. At low concentrations there was no direct concentration-frequency relationships. The two direct mutagens, MMS and TEB were effective in the chromosome loss tests. DEN does not induce translocations or any of the other types of damage studied which can be attributed to chromosome breakage. It is concluded that the sex-linked recessive lethal test is a simple and efficient way of preliminary screening chemical mutagens with Drosophila melanogaster.

Induction by x-rays of isochrosomes in the germ cells of Drosophila melanogaster males evidence for nuclear selection in embryogenesis

Abstract Drosophila melanogaster males, heterozygous for markers on either the second or third chromosomes, were irradiated and mated to females in which the corresponding homologous pair were present as isochromosomes. Gametes at different stages of development at irradiation were sampled by serially mating the males in 5 2-day broods. All normal zygotes were inviable; most of the viable progeny carried newly induced isochromosomes or were triploid. The new isochromosomes were either heterozygous or homozygous for the paternal markers. The former, heteroisos, were only induced in diploid cells and the latter, homoisos, were induced in all of the germ-cell stages which were tested. The rates of induction could not be measured directly but were estimated from the results of some calibration tests. Spermatocytes and spermatogonia appear to be about 30 times more sensitive than spermatozoa and mature spermatids for the induction of homoisos. The unexpected finding is that homoisos, which presumably result from a chromatid-type exchange, can be recovered from irradiated spermatozoa and late spermatids. The survival of such induced homoisos is explained by taking into account the process of gametic fusion of Drosophila. Only one of the first two fusion nuclei will be genetically balanced and competent to produce an adult. Triploids resulted from the production of diploid female gametes. The data provide a strong indication that this is not a random process.

Radiation-induced loss of ring-X chromosomes in the germ cells of Drosophila melanogaster males.

Abstract A comparison was made between teh sensitivities of spermatocytes and postmeiotic germ cells, for the induction of XO males and sex-ratio shift. Adult males were irradiated and the treated germ cells were samples in a brooding scheme. In post-meiotic germ cells there was a uniform sensitivity for the induction of XO males. Within the range of exposured given, teh frequencies of XO males increased linearly with increasing exposure. In spermatocytes, which were themost sensitive germ-cell stage for the induction of XO males, the frequencies of XO males increased nonlinearly with increasing exposure. Significant sex-ratio shifts were found only when germ cells were samples on the first six days after irradiation. It was not possible to establishe a common relationship between the inductino of XO males and sec-ratio shift when post-meiotic germ cells were irradated. The highest frequenceis of XO males were found in the progeny from sperm samples on the eight day after irradiation. This was also the period when the fertlity of the irradiated males was most reduced. It is suggested that induced crossing-over may be the mechanism which is largely responsible for the production of high frequencies of XO males when spermatocytes are irradiated.

Neutron- and X-ray-induced mutations at the yellow, white, forked and vermilion loci of Drosophila melanogaster; a preliminary analysis

Neutrons and X-rays were used to induce mutations at the yellow, white, vermilion and forked loci of Drosophila melanogaster by irradiation of spermatozoa in males. The mutations were characterized for the presence and location of simultaneously induced rearrangements and recessive lethal mutations. F1 females carrying induced visible mutations were identified, described and tested for fertility. The data are given in this paper. The proportions of mutants at the 4 loci, the ratios of whole-body: mosaic mutations, and the fertility of the mutant-carrying F1 females were similar for both types of radiation. Differences were observed between the frequencies of induced visible mutations and the rates of coincident visible and lethal induction. Although the analysis of the mutant chromosomes has not yet been completed, our data can be interpreted as providing confirmation that there are qualitative differences between the genetic effects of neutrons and X-rays.

The distribution of X-ray induced chromosomal abnormalities in rat kangaroo (Potorous tridactylis) cells in vitro

Abstract Synchronized cells of Potorous tridactylis (female origin) were irradiated with X-rays in vitro at different times during the cell cycle. Chromosome aberrations were scored in the first metaphase after irradiation. It is shown that the distribution of breaks and gaps in the chromosome arms is different at different cell-cycle times, and is non-random.

Induced nondisjunction in drosophila oocytes

Nondisjunction of the sex chromosomes and compound-second chromosomes was induced in Drosophila melanogaster by irradiation of immature oocytes and cold treatment of mature oocytes. These two treatments cause nondisjunction by different mechanisms. This results in a differential for stage sensitivity and the production of relatively different frequencies of the various exceptional progeny classes.

Neutrons and X-rays, comparative studies with Drosophila melanogaster: 1. the viability and fertility of induced autosomal translocations

Studies on the genetic effects of neutrons and X-rays have produced evidence that may be interpreted as indicating that neutrons induce clusters of closely linked genetic changes. According to this interpretation, it is to be expected that neutron-induced translocations will have a higher rate of associated recessive lethality, compared with translocations induced by low-LET radiation such as X-rays. The experiment reported here was designed to test whether this expectation is fulfilled. The dose--frequency response with neutrons for the induction of autosomal translocation was established by exposing males from the Oregon-K stock and then sampling treated mature sperm. From the data obtained, it was estimated that 10 Gy neutrons should induce about the same frequency of autosomal translocations as 27 Gy X-rays. These 2 doses were used to induce translocations in the spermatozoa of males carrying lethal-free autosomes, derived from the Oregon-K stock. Induced translocations were tested for homozygous viability and fertility. When these criteria were used, no qualitative difference was detected between the translocations induced by neutrons and X-rays.

The formation and recovery of two-break chromosome rearrangements from irradiated spermatozoa of Drosophila melanogaster

Chromosomes and chromatid-type rearrangements can be induced by exposure of spermatozoa of Drosophila to ionising radiation. A model, proposed to explain the formation and recovery of compound autosomes, has been extended to account for the induction of centric fragments capped by a duplication of paternal chromosome material. Three basic assumptions have been used; (1) that the sperm nucleus contains a haploid set of unreplicated chromosomes, (2) that the broken chromosome ends can be joined together before or after replication, and (3) that one of the first two cleavage nuclei may be lost and an adult organism derived from the other. The present paper reports a theoretical application of this combination of assumptions to the general case of the formation and recovery of two-break rearrangements. This has led to an elucidation of the relation between repeats, compounds, fragments, and deficiencies on the one hand and inversions and translocations on the other hand. Dicentric chromosomes and segmental aneuploidy can be simply explained. A selective screen is formed by the segregation of chromatid rearrangements and the aneuploidy tolerance levels of the early cleavage nuclei. Thus there is an alternative way of explaining observations which might indicate preferential breakage or joining.

X-ray induction of autosomal translocations in spermatozoa of Drosophila melanogaster and maternal effects of X.Y-chromosomes.

Wild-type ORK Drosophila melanogaster males were given an exposure of 3000 R X-radiation. Mature sperm were then sampled by mating to X.Y/X.Y, X.Y/X, or X/X females that carried markers on the second and third chromosomes for the detection of induced autosomal translocations. Two pairs of maternal stocks were used and heterozygous X.Y/X females were obtained by making both reciprocal crosses. The highest frequencies of induced translocations were obtained with X/X females. In one series these frequencies are higher than those obtained with either X.Y/X or X.Y/X.Y females. In the other series a uniform frequency of translocations was obtained with all types of female, except for one of the two types of heterozygous female, which gave lower frequencies. The experiments have provided data which show that the addition of Y-chromosomes to the maternal genome does not have a specific effect on the recovery of induced paternal autosomal translocations. Maternal Y-chromosomes increased the proportions of fertile F1 males, this effect being consistent in direction but varying in degree.

Neutrons and X-rays, comparative studies with Drosophila melanogaster: 2. sex-chromosome loss and partial loss, evidence for the induction of chromtid aberrations in spermatozoa

Losses and duplications of BSY y+-chromosome markers were induced by irradiation of spermatozoa with either 0.5-MeV neutrons or 100-kV X-rays. These 2 types of radiation are known to induce significantly different ratios of double:single strand breaks in DNA. Exceptional progeny were grouped into 3 categories; no Y marker, one Y marker, and Y marker duplications + mosaics. The last combination consisted of exceptions derived from only chromatid-type rearrangements. All other classes of exceptions may be derived from either chromatid- or chromosome-type rearrangements. Doses of 15 Gy neutrons and 27 Gy X-rays induced identical frequencies of exceptional progeny, giving an RBE of 1.8. The ratios of the 3 classes of exceptions were similar for both types of radiation. This observation can be interpreted as indicating that, under the conditions used here, chromosome and chromatid rearrangements are not derived directly from double and single DNA strand breaks, respectively.