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Soil salinity is a major cause of plant stress leading to economic losses in world agriculture. The aim of this study is to provide information about the effects of salinity on the growth of young olive trees (Olea europaea L. ‘Barnea’ and ‘Arbequina’) and to identify some of the underlying physiological mechanisms. One-year-old plants were planted in 30-L pots, using a sand, perlite, peat (1:1:1) soil substrate. Each treatment was drip irrigated with a NaCl solution: 4 dS/m (25 mg NaCl); 6 dS/m (37.8 mg NaCl); 8 dS/m (50 mg NaCl) and a control treatment of 2 dS/m (no NaCl added). Growth in both cultivars decreased after a period of exposure to salinity, ‘Barnea’ being the most affected cultivar. In both cultivars leaf water potentials were reduced when NaCl concentrations were increased. The osmotic potential became more negative when salt concentrations increased, thus indicating that both cultivars perform osmotic adjustment. The relative water content (RWC) did not change with the different saline treatments, suggesting that both cultivars present mechanisms to maintain water content even under stress conditions. On the other hand, high salinity levels induce ionic imbalance given higher Na+ and Cl- concentration in leaves and roots. As a result of the accumulation of these ions, the K+ concentration decreased resulting in a low ratio of K+/Na+. Both cultivars showed symptoms of toxicity in leaves and shoots which indicates that they accumulate toxic ions in the youngest leaves. ‘Arbequina’ showed higher Cl- concentrations in their root tissues over the concentrations in leaves, suggesting that there is a mechanism of Cl- compartmentalization at stem or root level that prevents entry and accumulation of Cl- in the leaf. ‘Barnea’ leaves were more damaged than those of ‘Arbequina’ and accumulated higher Cl- concentration, indicating that ‘Barnea’ is less resistant to salinity.

We used cDNA microarray hybridization technology to monitor the transcriptional response of Human Umbilical Vein Endothelial (HUVEC) cells to x-rays doses ranging from 2 to 200 cGy. An early time window from irradiation (4h) was selected in order to minimize the effects of the cell cycle blockage eventually induced at high doses of irradiation. Three different gene-clustering algorithms have been used to group the 4134 monitored ORF based on their transcriptional response in function of the irradiation dose. The results show that while few genes exhibit a typical dose-dependent modulation with a variable threshold, most of them have a different modulation pattern, peaking at the two intermediate doses. Strikingly even the lowest dose used (2 cGy) seems to be very effective in transcriptional modulation. These results confirm the physiological relevance of sublethal-dose exposures of endothelial cells and strengthens the hypothesis that alternative dose-specific pathways of radioadaptive response exist in the mammalian cells. 111 genes were found to be modulated at all doses of irradiation. These genes were functionally classified by cellular process or by molecular function. Genes involved in coagulation and peroxidase activity and structural constituent of ribosomes were over-represented among the up-regulated genes as compared with their expected statistical occurrence. Three genes coding for regulatory kinase activities (CDK6; PRCKB1 and TIE) are found down-regulated at all doses of irradiation.

Using human lymphocytes from a group of 20 donors, we investigated (i) the X-ray-induced adaptive response (AR) after four different conditioning treatments of 1, 2, 4 and 6 cGy, (ii) chromosomal sensitivity to X-irradiation during G(2) and (iii) the G(2)/M checkpoint response. An AR was found in 11 of the 18 donors (approximately 60%). No correlation was found between the presence of AR and G(2) chromosomal radiosensitivity, or with donor sensitivity to the activation of the G(2)/M checkpoint by 30 cGy or a dose as low as 2 cGy. The AR was not related to any particular conditioning treatment, but was consistently present or absent in any one donor under all conditioning treatments used. As far as chromosomal breakage and induced mitotic delay in G(2) are concerned, large variability between individuals was observed, together with a close correlation in the same donor between mitotic delay induced by a low dose (2 cGy) and the frequency of aberrations.

The effect of X-irradiation (1 Gy) during G 1 on the transition from G 1 to S and on the length of G 2 over cell cycles subsequent to irradiation was studied in human lymphocytes from six different donors. After irradiation a delay was observed in the onset of S phase, as was an extension of the G 2 phase lasting throughout the three to four subsequent cell divisions. The extension of G 2 and of the cell cycle as a whole is partly related to the presence of chromosome aberrations in the cell. This is demonstrated by: (i) the presence of a larger number of chromosome aberrations in M 1 cells corresponding to sampling times longer after that of irradiation; (ii) the presence of a larger number of chromosome aberrations in cells with a longer G 2 . The most significant chromosome aberrations in this respect are isochromatid fragments. Lastly, we observed that irradiation during G 1 activates another checkpoint governing the way mitosis proceeds. This takes the form of an extension of metaphase; in this case also, in some cells, activation of a possible checkpoint during preanaphase seems to be related to the presence of aberrations.

Experiments were carried out with human lymphocytes to test whether there was any relation between the changes that conditioning treatment can produce in cell progression or in mitotic delay induced by the challenge dose and the presence of an 'adaptive response' (AR). In experiments in which the cells were successively fixed after the challenge dose, the interaction between conditioning treatment and challenge was of the same sign for all the fixation times: therefore it is likely that modifications of the cytogenetic damage in primed cells is not a mere reflection of stage sensitivity. In experiments in which using 1 Gy as conditioning treatment we induced a drastic extension of G2, we did not observe any AR; therefore, even if conditioning treatment can induce modifications in the cell-cycle phases before and/or after challenge, there is probably no link between these modifications and the presence of an AR.

Experiments were carried out with human lymphocytes to test the effect of low-dosage X-ray irradiation (2 cGy) on cell-cycle kinetics and on the mitotic delay induced by the conditioning pretreatment alone or by a subsequent high dose of X-ray. All the tests were performed using lymphocytes from two donors who had previously displayed considerable differences in the interaction between a low and a high dose of ionizing radiation. A dose of 2 cGy led to significant variations in mitotic indices (MI) which differed for the two donors in relation to variations in the times of irradiation and fixation after stimulation with PHA. Moreover in one of the two donors 4–6 h after challenge the pretreated cultures have a higher MI that the controls; on the other hand, conditioning treatment alone induces in the other donor an extension of both G2 and of the time taken by cells in S at the time of challenge to reach mitosis. These findings could in the future provide some insight into the problem of the variability of the adaptive response in human lymphocytes.

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease in humans and is characterized by neuronal loss, neurofibrillary tangles and beta-amyloid deposition. The interaction between neurotrophins and their tyrosine kinase (trk) receptors is important for cellular differentiation and survival. Interestingly, marked reductions in neurotrophins and receptors have been reported in AD. The cause of the decrease in these molecules remains unclear. However, the role of beta-amyloid (A beta) appears central in understanding the mechanisms controlling neurotrophin/trk expression. In this study we exposed SHSY5Y neuroblastoma cells to A beta or hydrogen peroxide and measured the expression of trk B/truncated trk B, and brain-derived neurotrophic factor (BDNF)/NT4 at the protein and molecular level. We show that A beta or hydrogen peroxide (H(2)O(2)) induces oxidative stress and cell cytotoxicity. The exposure of cells to A beta results in an increased trk B expression with a concurrent reduction in truncated trk B levels. H(2)O(2) exposure decreased both trk B and truncated trk B levels at the cell surface. At the molecular level trk B RNA increased in the presence of A beta and was unaffected by H(2)O(2). Similarly, BDNF and NT4 levels increased in the presence of A beta. Pre-treatment of cells with the anti-oxidant melatonin returns trk receptor expression, mRNA and BDNF/NT4 secretion to normal levels. These results are significant as they can help in the planning and implementation of AD treatment strategies involving neurotrophins.

Experiments were carried out using human lymphocytes in order to test the effect of pH shifts on radiosensitivity of cells irradiated in the G 2 stage. In our culture conditions constant variations in medium pH over the range of 7.4–6.8 were observed as a function of incubation time after PHA stimulation; in addition the pH of the medium was adjusted in parallel cultures over the range of 5.9–8.2. Then we exposed the cultures with different pH to a treatment of X-rays delivered 2 h before fixation. The pH of the medium was found to greatly affect the yield of induced chromatid aberrations.

Mutagenesis research has been focused for a long period on the interaction between mutagens (mostly X-rays) and the cell structure modified by them. Subsequently, the emphasis was shifted to mutation as a cellular process in which the primary lesion was extensively modified by enzymatic DNA-repair systems constitutively present in the cell (Auerbach 1976). Finally, over the past 15 years, considerable interest has been focused on inducible cellular functions (possibly DNA repair activities) occurring in the cell after exposure to very low doses of mutagens, probably through DNA lesions (for reviews, see Walker 1985; Elespuru 1987). DNA-damage-inducible processes are probably part of a set of general biological phenomena by means of which the cells and the organism respond to various types of stress (Luckey 1980; McClintock 1984; Lindquist 1986; Morgan et al. 1986). When stress is represented by a mutagenic agent one of the most interesting responses is the adaptive response (AR; Samson and Cairns 1977). The mechanisms underlying the AR are reasonably well known in prokaryotes in which they were discovered (Sekiguchi and Nakabeppu 1987; Rebeck et al. 1988). In plant (Rieger et al. 1982; Heindorff et al. 1987) and animal cells (Samson and Schwartz 1980, 1983) investigation is focused mainly on the reduction of the yield of induced chromatid aberrations by various pretreatment procedures (clastogenic adaptation).

SUMMARYThe authors have examined the « banding pattern », revealed by the Seabright method (1972), of chromosomes of an aneuploid-diploid line (C-125) and of certain lines of various degrees of ploidy derived from it. It was found that the line C-125 has preserved a large proportion of normal chromosomes, similar to those of the species, some in a diploid and others in a haploid condition.Analysis of the degree of homology of the chromosomes confirmed that, in the derived polyploid lines, part of the genome is in a triploid condition (Oliveri et al. 1972) even if in some cases only parts of the chromosome are in a triploid condition as a result of deletions. This last type of chromosome rearrangement is the one that has had the greatest importance in the evolution of the karyogram of the induced polyploid lines. The results are discussed in relation to problems of « dosage compensation ».

It is widely known that ceils irradiated with X-rays undergo a slowing of the cell cycle. This occurs at any stage of the irradiated cell cycle, and, in addition to the irradiated stage, always involves the G2 phase. The mitotic delay observed when G2 cells are irradiated has been studied extensively and it has been observed that there is a point in G2 called the transition point (TP) beyond which cells are practically refractory to the induction of mitotic delay (Hodge and Nachtwey, 1972; Highfield and Dewey, 1975). It is particularly interesting that the TP shifts closer to mitosis with increasing X-ray doses (Dewey and Highfield, 1976; Schneiderman et al., 1977; Tomasovic and Dewey, 1978). Thus if a population of cells in G2 is irradiated, only that part which has not gone beyond the TP is appreciably delayed. This fraction increases with increased dose. In recent years there has been a great interest in G2 repair of chromosome aberrations (CA) and lethal effects (Hartley-Asp et al., 1980; Kihlman et al., 1982; Preston, 1980; Andersson, 1982; Hansson et al., 1982; Natarajan et al., 1982). An attempt has been made to determine the relation between repair phenomena and mitotic delay (Painter and Young, 1980; Scott and Zampetti-Bosseler, 1980; Zampetti-Bosseler and Scott, 1981; Ching and Pardee, 1982; Lucke-Huhle, 1982). The present experiments were carried out in human lymphocytes to study X-rayinduced CA in G2 cells that had and had not gone beyond the TP. Since the TP shifts closer to mitosis with increasing doses, by using different doses of radiation it was possible to compare the damage induced in cells which, even though in the same stage at the time of irradiation, do or do not undergo mitotic delay.

Cultures of blood from healthy adults were irradiated 48 h after stimulation with 240 R of X-rays and fixed after various time intervals (0–2 h, 2–4 h, 4–6 h). 3 HTdR was added to several cultures after irradiation. Mitotic and labelling indices were used to distinguish between two cell samples inside the irradiated G 2 population: D − cells reaching mitosis without mitotic delay and a high frequency of chromatic breaks and D + cells with mitotic delay and which, during the delay, repair most of the damage produced. After R banding 450 chromatid deletions were located in each of the two cell samples. The D + cells showed a higher frequency of breaks than the D − cells with decreasing chromosome size, in the telomeric and centromeric region and in the junction between the R + and R − bands. These results can be interpreted as indicative of a non-random distribution of repair processes both between and within chromosomes.

SUMMARYEndoreduplicated cells appear in sub-cultures prepared by trypsinising cultures of Chinese Hamster in the plateau phase, which after a change of medium are grown for at least two days, in stoppered bottles. E-cells perform the first DNA synthesis before trypsinisation and the second after it; therefore these cells were blocked or slowed down at the moment of trypsinisation in a stage of the cycle with 4C of DNA. This stage is probably comparable to a G. stage that follows a first cell cycle that has concluded without a normal mitosis.

SUMMARYExperiments have been performed in order to study the spontaneous appearance of cells with diplochromosomes (endoreduplicated cells) in Chinese Hamster cultures. Endoreduplicated cells are present in sub-cultures (sub-M-cultures) prepared by trypsinising cultures (M cultures) in the plateau phase of growth. Two conditions are particularly important for the appearance of endoreduplicated cells: a change of medium 3 days before trypsinisation and the growth of the M-cultures in a closed atmosphere from that moment on. The maximum frequency of endoreduplicated cells occurs when the M-cultures are grown in bottles that have been stoppered also during the 6–9 days prior to the change of medium. In these conditions the frequency of endoreduplicated cells found in the sub-M-cultures 26–30 hours after trypsinisation varies around 8%. In the crowed M-cultures, which grow in closed atmospheres, the change of medium is followed by an unbalanced cell growth that leads to a greater protein content per cell. On ...

SUMMARYA study has been made, by means of autoradiographic methods, of the final part of the period of DNA synthesis in aneuploid diploid cells of the Chinese hamster, cultivated in vitro, and in stabilized polyploid cells derived from them. The results have shown that, in the polyploid lines, chromosomes or parts of chromosomes have not become late-replicating if they were not so in the diploid cells. The reverse, however, remains an open possibility: namely, the reversion of the heterochromatin of an arm of the presumed X-chromosome. It has also been observed that, in the polyploid lines, the large and medium metacentric chromosomes end their synthesis earlier than in the diploid line.

Males of different genetic constitutions but having the common characteristic of producing two classes of sperm, differing in the presence or absence of a normally functioning nucleolar organizer (NO) region, were mated with females of different genotypes. After mating, the males were discarded and the females were transferred to successive culture vessels until they no longer produced any progeny. In all experiments, the proportion of NO-bearing sperm which accomplish fertilization, relative to sperm which lack this region, is higher in early than in later broods.

In order to study the relationship between dosage and frequency of X-ray induced crossing-over in males of Drosophila melanogaster , four samples of about 130 males each were irradiated with doses of 500, 1000, 2500 and 4000 R respectively. On the 10th–11th day after irradiation, the two classes of complementary crossovers were found to occur independently of each other in the progeny of the individual males, and the distribution of crossovers per irradiated male followed a Poisson distribution. In consequence, the clusters in this brood were considered as pseudo-clusters, originating from different crossover events in the latest gonial cells. It is this stage that is characterized by the highest sensitivity to the induction of crossing-over. The number of crossovers found on these days has therefore been used as a direct estimate of the number of crossing-overs produced, taking into account the probable induction of crossing-over at the four-strand stage. After logarithmic transformation, a linear relationship was observed between the dose and the percentage of crossovers produced in the late gonial cells in the centromeric region of the second chromosome (regression coefficient = 1.768 ± 0.030). Crossing-over induced in this chromosome region therefore is most probably a “two-hit” phenomenon, to be explained in terms of two distinct and independent breaks. This conclusion is further supported by the observation that fractionation of the dose significantly reduced the frequency of induced crossing-over. When N 2 was given between the fractions the crossing-over frequency was restored to that of unfractionated radiation. It is thought that N 2 acts by preventing restitution of the breaks required for the pseudo-crossing-over. Post-treatment with N 2 after unfractionated radiation resulted in a significant enhancement of the crossing-over frequency. Irradiation in N 2 reduced the frequency of induced crossing-over to about one fifth of that observed in air, whereas radiation exposure in oxygen approximately doubled the crossing-over frequency as compared to that induced in air. The ratio-sensitivity in O 2 and N 2 differed by a factor of about 10. This is considered to offer further support for the two-hit origin of induced centromeric crossing-over in this stage.

We have classified the commoner aberrations that can be scored in a diplochromosome. This chromosome configuration can be useful for a study of the mechanisms that underlie chromosome aberrations. Three experiments were done in which endoreduplicated Chinese hamster cell (strain CHEF-125) were irradiated at various stages in their cell cycle. The results showed good consistency in the three experiments and confirmed that the spatial arrangement of the chromosomes influences both the type and the frequency of the aberrations produced.

SUMMARYFrom a pseudotetraploid strain of Chinese hamster, induced with colchicine and having an average of 40 chromosomes per cell, two strains having chromosome numbers of around 70–74 were induced with a second treatment. About one year after treatment, the chromosome numbers of these two lines decreased to an average of 52.7 and 54.6.The analysis of the karyotype showed that the chromosomes were not lost at random: of type m or st were mostly lost, while the type t chromosomes remained unchanged.

An account is given of a working hypothesis on the factors underlying symmetric or asymmetric rejoining in chromosome aberrations. According to this hypothesis, throughout the cell cycle the broken chromosome ends rejoin in such a way that the polarity of the two sub-units of the chromatid does not change at the points of rejoining. In phase S of the cycle there are two restrictions to the preceding rule: ( a ) the aberration starts with a break in the parental sub-unit; and ( b ) the broken end of a parental sub-unit interacts preferentially with a sub-unit in process of synthesis. From this it follows that in phase S if there is interaction between chromatids with parental sub-units of the same polarity there will be an asymmetric rejoining; if there is interaction between chromatids with parental sub-units of opposite polarities there will be a symmetric rejoining.

SUMMARYBy autoradiographic methods, a study has been made of the duration of the mitotic cycle in a diploid-aneuploid cell line of the Chinese hamster and in polyploid lines derived from this line after treatment with colchicine. Compared with the diploid line, there is, in the stabilized polyploid lines, a clear lengthening of G2 stage only; the S and M stages, and probably also the G1 stage, undergo no significant variations. Before stabilizing their chromosome number on a modal value that is constant with time, the polyploid lines show a lengthening of the cell cycle in all its stages. Within the diploid-aneuploid line, the cells with a longer G2 stage, on the average reveal a greater number of small late-replicating chromosomes.

The incorporation of [3H]uridine and [3H]thymidine in the Dorophila melanogaster testes has been studied by means of autoradiography. RNA synthesis was observed in spermatogonia and young spermatocytes. At 30 min after [3H]uridine injection only the nucleus and nucleolus appeared labeled; at 2–4 h post injection (p.i), labeling was also observed in the cytoplasm. At 8 h p.i. labeling was very heavy in the cytoplasm and lighter in the nucleolus, while it diappeared altogether in the nucleus. At 4 days p.i., labeling was transmitted to the cytoplasm of spermatids and at 6 days to sperm; it disappeared in the final stages of maturation, when these cells were no longer arranged in bundles. in the post-meiotic stages no RNA synthesis was observed. The non-germ line cells showing RNA synthesis were: testis coat cells, cyst cells, terminal epithelium cells and seminal vesicle wall cells. Direct RNA synthesis was not observed in nutritive cells. DNA synthesis was observed in spermatogonia as early as 30 min after [3H]tymidine injection. By contrast, [3H]thymidine labeling in spermatocytes was rarely observed, even at 8 h after injection, and then only in the very young spermatocytes. An analysis of the data from several other organisms on RNA metabolism in the course of meiosis has shown that RNA synthesis always occurs in the primary spermatocytes; by contrast, RNA synthesis does not always occur in post-meiotic stages and thus does not seem to be an essential step during spermiogenesis.

2 experiments were carried out, following the same scheme. Chinese hamster cells (line CHEF-125) were cultivated for 4 h in the presence of tritiated thymidine ([3H]TdR) (Expt. 1, 0.5 μCi/ml; Expt. 2, 0.1 μCi/ml). After removal of the isotope, some of the cultures were X-irradiated in stage S-G2 preceding M1 (series S-G2 I), others in stage S-G2 preceding M2 (series S-G2 II). Analysis of the colchicine-C-anaphase of M2 cells showed that in both experiments the X-rays produced an increase in the isolabelling regions only if given in the S-G2 stage preceding M1. On the other hand, the X-rays produced an increase in sister chromatid exchanges (SCEs) in both series of the second experiment. In the first experiment, where the controls had a high frequency of SCEs, no increase in SCEs was recorded in either of the two series treated with X-rays. The frequency of the isolabelling regions, in relation to the total labelling (Expt. 2), was 1.4% in the control series, 2.5% in series S-G2 I and 1.3% in series S-G2 II. Analysis of the distribution of the isolabelled regions along the chromosome showed that the regions furthest from the centromere were those most involved in this phenomenon and that, for the most part, the isolabelling was not directly associated with an exchange. The results have been interpreted, in accordance with the theory put forward in a previous work, to mean that SCEs and isolabelling regions are due to different phenomena, the former resulting from an exchange involving the whole chromatid, the latter from subchromatid exchanges probably involving a single polynucleotide strand.

The distribution of chromosomal aberrations between and within chromosomes of male D. melanogaster somatic cells after treatment with UV has been analyzed.—Distribution of the breaks between chromosomes was largely nonrandom since we found a higher aberration frequency than that expected on the Y chromosome. Moreover, within the chromosomes the aberrations are clustered in the pericentromeric heterochromatic regions. The above distribution is compared with that of the breaks induced by X rays and methyl-methane-sulphonate (MMS) which were distributed in a different pattern.

Human lymphocytes exposed to low doses of ionizing radiations from incorporated tritiated thymidine ([ 3 H]dThd) or from X-rays become less susceptible to the induction of chromatid aberrations by high doses of X-rays. This indicates that low doses of ionizing radiation can produced an effect similar to the adaptive response observed with alkylating agents in prokaryotes, animal and plat cells. To determine whether there is individual variability in the adaptive response to ionizing radiations we exposed human lymphocytes from 18 different healhty donors to ‘adapting’ doses of [ 3 H]dThd (0.01 μCi/ml) or X-rays (0.01 Gy) and subsequently to a ‘challenge’ treatment of 0.75 Gy of X-rays delivered 2 h before fixation. Four of the 18 donors did not show an adaptive response; in some cases in these individuals a synergistic response of increased, rather than decreased, damage was found. Two of these 4 donors showed no adaptive response in 3 subsequent experiments separated by 4-month intervals. This suggests that the human population exhibits a heterogeneity in the adaptive response to ionizing radiations which might be, at least in part, genetically determined.

IN Drosophila melanogaster, much information has been collected on chromosome mutations analysed by genetic methods, but there has so far been no cytological analysis of the chromosome mutations induced in somatic cells. By irradiating cells of D. melanogaster, in which we knew that somatic pairing was present, we have also wished to extend our previous studies1, carried out on endoreduplicated Chinese hamster cells, to verify the influence of chromosome pairing on the type and frequency of induced chromosome aberrations.

The relative frequencies of X-ray-induced symmetrical and asymmetrical chromatid interchanges were analysed as a function of the cell cycle in somatic cells of Drosophila melanogaster . In exchanges between autosomes, during the S phase, a clear prevalence of asymmetrical rejoining was observed. Later, this prevalence became reduced as the cell approached mitosis. On the other hand, in the exchanges between X-chromosomes in the females there were no significant variations in the type of rejoining through the cell cycle, there being a slight but consistent preference for symmetrical rejoining. These data are discussed in relation to the hypothesis proposed by Olivieri et al. [12] on the organization of the chromosomal sub-units in the interphase nucleus.

A technique has been perfected for enabling good microscope preparations to be obtained from the larval ganglia of Drosophila melanogaster. This system was then tested with X-rays and an extensive series of data was obtained on the chromosome aberrations induced in the various stages of the cell cycle.—The analysis of the results obtained offers the following points of interest: (1) There exists a difference in radio-sensitivity between the two sexes. The females constantly display a greater frequency of both chromosome and chromatid aberrations. They also display a greater frequency of spontaneous aberrations. (2) In both sexes the overall chromosome damage is greater in cells irradiated in stages G2 and G1. These two peaks of greater radiosensitivity are produced by a high frequency of terminal deletions and chromatid exchanges and by a high frequency of dicentrics, respectively. (3) The aberrations are not distributed at random among the various chromosomes. On the average, the Y chromosome is found to be more resistant and the breaks are preferentially localized in the pericentromeric heterochromatin of the X chromosome and of the autosomes. (4) Somatic pairing influences the frequency and type of the chromosome aberrations induced. In this system, such an arrangement of the chromosomes results in a high frequency of exchanges and dicentrics between homologous chromosomes and a low frequency of scorable translocations. Moreover, somatic pairing, probably by preventing the formation of looped regions in the interphase chromosomes, results in the almost total absence of intrachanges at both chromosome and chromatid level.

Experiments have been carried out for the purpose of testing an hypothesis (Olivieri et al., 1970) intended to explain why females of Drosophila melanogaster do not utilise all the sperm transmitted during copulation. According to this hypothesis, the deposition of eggs and the emergence of sperm from the storage organs occur with a certain degree of independence. For this reason the number of sperm that one female utilizes for fertilization out of the total number of sperm stored, for a given intensity of emergence of the sperm, is directly proportional to the intensity of egg deposition. The experiments reported here have shown that: These results seem in agreement with the hypothesis proposed.

The relative frequencies of X-ray induced symmetrical and asymmetrical chromatid interchanges were analysed as a function of the cell cycle. The asymmetrical type of interchange was the most common. When the proportions of symmetrical interchange at all sampling times were plotted, the resulting “best line fit” had a significant negative slope. This finding is interpreted to mean that rejoining to form interchange aberrations is not random and varies through the cell cycle. Further experiments were carried out on the types and frequencies of interchanges produced between paired sister chromosomes of diplochromosomes. There was a highly nonrandom formation of symmetrical and asymmetrical exchanges whether they occurred spontaneously or were induced by X-rays or [ 3 H]-thymidine. The X-type symmetrical exchange was 6–20 times more frequent than the U-type asymmetrical exchange. These data are interpreted to mean that some preferential patterns of rejoining are determined by chromosome proximity and other spatial features that are variable throughout the cell cycle.

Studies were made of the relative frequencies of sister-strand exchanges, isochromatid deletions, and symmetrical and asymmetrical interchanges involving daughter endoreduplicated chromosomes (diplochromosomes) in tritiated thymidine-labeled aneuploid Chinese hamster and diploid human cells. It was found that the reunion of broken ends in the endoreduplicated chromosomes was not random and that the ratio of symmetrical to asymmetrical interchanges was about the same as the ratio of sister-strand exchanges to isochromatid deletions. These data are interpreted to mean that sister-strand exchanges may be radiation-induced exchanges resulting from isolocus breaks that do not rejoin in the expected random fashion. The very high frequency of interchanges in daughter endoreduplicated chromosomes is taken as evidence in support of the site hypothesis of Wolff and Atwood (see ref. 19).

That two points of chromosome damage must exist simultaneously for a twobreak chromosome aberration to arise is a well-documented fact. Sax (see Lea, 1, for review) first demonstrated this phenomenon with his now-classic dose-rate and dose fractionation experiments. The points of damage remain capable of interaction with each other for a measurable period of time. If a dose of sparsely ionizing radiation is administered over a period of time shorter than this "rejoining" time, all the breaks induced by that dose of radiation can interact with one another provided they are within a certain distance of one another. If the radiation is administered over a period of time greater than the rejoining time, breaks induced by the first portion of the dose will close before the entire dose is administered and, therefore, will not be able to interact with subsequently induced breaks. The yield of twobreak aberrations obtained with low-dose-rate irradiations, consequently, will be lower than the yield obtained with high-dose-rate irradiations. In the case where the dose-response curve of a particular aberration type conforms to the equation Y = bo + b1D + b2D2, where Y is the yield, bo the spontaneous rate, bi the coefficient of the linear term, b2 the coefficient of the dose square term, and D the dose, the dose-square term (b2D2) contributes proportionately more to the yield of aberrations at the higher doses than at the lower doses. A lowering of the dose rate will result in a decrease in the coefficient (b2) of the dose-square term. This, of course, is contingent upon the time of irradiation being longer than the rejoining time. Thus, at low dose rates, although the dose-square term would still be present, the dose-response curve would be expected to be dominated by the linear term (blD). This follows from the observation that at low doses the dosesquare term does not contribute as much to the yield of aberrations as the linear