Observations have been carried out on the lampbrush chromosomes of Notophthalmus (Triturus) viridescens, the American newt; especially of chromosome II and XI. Chromosome II regularly bears two and occasionally three pairs of giant loops situated close to the centromere, The giant loops are distinguished from the majority of the normal loops by the greater bulk of their matrix and their length, which may vary from 60 mum to 300 mum. The giant loops are usually observed aa single loop pairs arising from a single chromomere but occasionally they occur as multiple loop pairs from a single chromomere. The giant loops also show variation in the distribution of their RNP matrix and more than one polarized matrix unit may be present on a single loop. The size of the loops and the matrix distribution pattern around any one loop pair is constant in oocytes ranging in size in any particular animal. The effect of increasing and decreasing the metabolic rate of the newt, by using hormone injections and cold treatment respectively, on the length and matrix distribution patterns of the giant loops was investigated. Hormone injections may cause changes in loop length, usually an increase but in one case a decrease, or have no effect. This may be due to the initial loop length or the level of gonadotrophin already present in the animal. After hormone injections the matrix distribution pattern, in multipolarized loops, alters with an increase in the proportion of the loop occupied by the first matrix unit. The constancy of loop characteristics over a range of oocyte sizes and the effect of hormone treatment are discussed in terms of matrix roving round a stationary loop axis. Cold treatment causes a dramatic shortening of the majority of lateral loops but leaves the giant loops virtually unaltered. Autoradiographic experiments have shown that the rate of RNA synthesis in the giant loops is half the rate of synthesis on the ordinary loops and that the RNA transcribed by the giant loops contains very little guanine. These results are discussed in terms of differing rates of RNA polymerase movement or attachment, different types of RNA polymerase molecules and the organization of highly repetitive sequences in the genome. The effect of inversion heterozygosity on chiasma distribution in bivalent XI was also studied, 15 out of 94 females studied were heterozygous for an inversion involving almost the whole of the longer arm of bivalent XI and including' the sequentially labelling loops situated close to the end which are transferred, by the inversion, to a position close to the centromere. The chiasma distribution in normal bivalents XI was compared with that of normal bivalents II and inversion heterozygote bivalents XI. Normal bivalents XI have chiasmata restricted to the chromosome ends while normal bivalents II have unrestricted distribution of chiasmata. In inversion heterozygote bivalents XI no chiasmata at all are formed in the longer inverted arm pair and chiasmata become distributed throughout the length of the shorter non-inverted am pair. Chiasma distribution was found to be similar in both sexes. These results are discussed in terms of the availability of recombination nodules and the time of their association with the synaptonemal complex.

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1. DNA fibre autoradiography has been used to study the replication of chromosomal DNA from chick somatic cells in tissue culture at 37°C, with or without FUdR pre-treatment, and from FUdR-treated chick blastoderm cells. 2. Replication proceeds as first described by Huberman and Riggs (1968). Tandemly arranged units replicate bidirectionally from fork-like growing points, sister strands replicating concurrently. Evidence against the existence of defined termini is presented. 3. The rate of replication in these chicken cells is some 25 to 30mum/hr one-way. The mean interval between adjacent initiation sites for replication is similar in all three cell types but is most accurately determined from FUdR-treated chick somatic cells, being 63 mum. 4. This mean value is either within the range or as much as twice as long as estimates obtained by other authors for various mammals. C-values in the chicken and mammalian species are 1.45 and about 3 pg respectively. Clearly in this comparison the mean initiation interval is not related to C-value. 5. The S-Phase is estimated to be 7.5 hr in chick somatic cells and 5 hr in the blastoderm. The replication pattern is much the same in both and is therefore related to S-phase duration, not cell type. From values of replication rate, mean initiation interval and S-phase, it is clear that only about one-seventh of the DNA is replicated at any one during S in somatic cells. 6. It has been claimed that FUdR does not completely block DNA synthesis, and that it promotes unnatural initiation points from which repair synthesis proceeds. However, FUdR as used in the experiments described in this thesis has neither of these effects. 7. Although initiation intervals and the times of initiations vary, neighbouring intervals tend to be similar in length and neighbouring units tend to initiate together. Clusters of concurrently replicating, neighbouring units are bordered by long stretches of DNA which are replicated at different times. 8. Autoradiographs from unsynchronised somatic cells show "sprays" of closely arranged tandes labelled tracks, in which the track lengths are remarkably uniform. It is suggested that these sprays represent the replication of satellite DNA.

The term, heterochromatin, refers to those portions of a chromosome which stain very darkly early in the division cycle at a time when most of the chromosomal material stains very lightly. After the discovery of the mutagenic effects of X-rays, large numbers of mutants were recovered in Drosophila. These mutants were genetically mapped but none of them seemed to be located in the heterochromatic regions of the chromosomes. This observation led to the hypothesis that heterochromatin was genetically inert. Since then the problem of the function of heterochromatin has engaged the attention of many different researchers. One of the few mutants known to reside in the proximal heterochromatin of the X chromosome is bobbed. Ritossa and Spiegelman (1965) have presented evidence that bobbed is the nucleolus organizer, the site of ribosomal RKA synthesis. Their results suggest that this is a highly redundant region of the chromosome. If heterochromatin in general is greatly redundant, this would offer one explanation for the apparent lack of genetic loci. A mutant phenotype might result only after large blocks of the chromosome have been deleted. A scheme involving the use of attached-autosomes to select for those third chromosomes which have sustained breaks in the proximal heterochromatin was devised. In general, virgin females were irradiated and attached-third chromosomes were selected for by mating these females to attached-third males. Female progeny carrying these newly synthesized attached-autosomes are, in turn, irradiated and newly reconstituted chromosomes were selected for by mating the females to males carrying normal third chromosomes. Thus, each newly reconstituted chromosome has sustained a number of breaks at different positions in the proximal heterochromatin. Using this procedure, we recovered sixty-six homozygous lethal chromosomes. Fifty-three of these sixty-six lethals may be placed in one of the six complementation groups, two of which appear to be multisite deletions. In addition, these fifty-three lethals appear to be located, by the results of genetic mapping, in the region spanning the centromere. These two pieces of evidence suggest that the method used here does, indeed, select for deletions in the proximal heterochromatin. / Science, Faculty of / Zoology, Department of / Graduate

The frequency of chromosome breaks was increased in replicate cultures from each of ten individuals when lysergic acid diethylamide at a concentration of 1 ug/ml of culture was added 24 hours prior to the harvest of the cells. The differences between the control and treated cultures ranged from +3.00 to +7.93 with a mean of +4.63, indicating no variation in response between individuals. The breaks were randomly distributed among the seven groups of chromosomes of the complement. No significant difference in either the number of cells with aberrations, or the number of breakage events was observed between cells cultured from a patient with Fanconi's anaemia before and 24 hours after treatment with 250 ug. of growth hormone. Both were significantly increased over the control. After treatment with growth hormone, the number of breaks per aberrant cell was decreased, and the distribution of frequencies of specific types of aberrations was changed. Non-homologous exchange figures were the only two break events observed in cultures from the patient. None were observed in control cells. The distribution of breaks among the seven groups of chromosomes was random. The frequency of chromosome aberrations was increased in cultures from a single individual when treated with 1 ug/ml of mitomycin-C for one hour at the beginning of the culture period. In the treated cultures, 181 breaks were observed in 64 of the 100 cells examined, whereas only 5 breaks were observed in three of the 100 cells scored in the control samples. Forty-seven exchange configurations were observed in the treated cultures, 42.56% of these being non-homologous exchanges. No marker or dicentric chromosomes were observed. Breaks were randomly distributed among the seven groups of chromosomes of the complement. / Medicine, Faculty of / Medical Genetics, Department of / Graduate

The monofunctional ethylating agent EMS has been found to induce ts lethal mutations in Drosophila melanoqaster. Temperature sensitive lethals can be induced on both the X and the second chromosomes. The frequencies of ts lethal induction with respect to the estimated number of "single hit" lethal mutations are not significantly different (for the X chromosome 12.5% and for the second, 10.9%). The ts lethals from each chromosome have similar visibilities at high and low temperatures. As with the ts lethals on the X chromosome, Suzuki et al, 1967) those on the second behaved in a manner compatible with the suggestion that Drosophila ts lethals are similar to those found in micro-organisms. These mutants, particularly dominant ts lethals, may provide powerful tools for Drosophila genetic investigation. / Science, Faculty of / Zoology, Department of / Graduate

There has recently been renewed interest concerning the location of the major loci responsible for the Segregation Distortion phenomenon in Drosophila melanogaster. Hartl (1974) has shown that two major sites are involved: Sd and Rsp. Rsp confers insensitivity to SD chromosomes, while Sd is considered to be the major locus that initiates distortion, Sd is located to the left of Rsp and both are located between Tft and cn. Ganetzky (1977) has extended these findings by showing that just distal to pr there is a locus that, if deleted on a SD chromosome, eliminates distortion and he argues that this is the Sd site. Ganetzky (1977) also uncovered another important locus, in or near the heterochromatin of 2L, that, if deleted from a SD chromosome, greatly reduces the ability of that chromosome to distort and he argued that this site is an enhancer of SD, E(SD). Ganetzky (1977) , also suggests that Rsp might be located very close to the centromere in the proximal heterochromatin of 2R. The results presented here demonstrate the presence of an important component of SD located within the proximal heterochromatin of 2L. These results also show that there is another important site located just distal to pr. However, when this site is removed by recombination from a SD chromosome, a certain level of residual distortion remains. It is argued that the site that Ganetzky (1977) called E(SD) is likely responsible for this residual distortion in the absence of the site just distal to pr. Thus the site near pr is called Sd₁ and the site near 1t is called Sd₂,. Loss of either site results in a large reduction, but not complete elimination, of the distorting ability of a SD chromosome. Other data are presented that, on the whole, agree with Ganetzky's (1977) proposal that Rsp is located in the centromeric heterochromatin of 2R, very close to the centromere. Miklos and Smith-White (1971) have suggested that k (the segregation ratio observed from a given mating) is a deceptive measure of the degree of distortion and they have proposed another method of measuring distortion based on their model of sperm dysfunction. Some of the weak assumptions of this model are discussed and a simpler alternative is presented. The alternative model assumes that the potential segregation ratios of a population of SD males follow a truncated normal distribution. Data are presented that are not necessarily inconsistent with this assumption. The same data show that it is likely that certain SJJ chromosomes differ in their susceptibility to modifiers of It is concluded that at present k provides the clearest measure of distortion. / Science, Faculty of / Zoology, Department of / Graduate

The present work deals with several approaches to the study of regions near the centromere of chromosome 3 of Drosophila melanogaster. The goals of this research were: (i) to examine spontaneous crossing over near the centromere in detail; (ii) to locate the Deformed locus genetically and to determine whether this lesion is a recessive lethal; (iii) to test the efficacy of radiation-induced crossing over as a method of producing proximal aberrations; and (iv) to genetically and developmentally characterize a temperature-sensitive (ts) allele of a Minute locus, located near the centromere. CHAPTER 2 describes a study of recombination, which deals with short genetic regions near the centromere of chromosome 3, using the intervals, st-in-ri-eg²-Ki-p[sup P] and Gl-p[sup P]-Sb-H. The following generalizations have emerged: (i) an excess of multiple crossover chromosomes was recovered, and the intervals which immediately span the centromere showed the highest negative interference; (ii) a positive correlation of simultaneous exchange within closely-linked intervals, was noted for many of the multiple crossovers; and (iii) several classes of reciprocal crossover products were not recovered equally. Three possible explanations for these results are: pre-meiotic exchange, chromatid interference and gene conversion. The results of one experiment also indicated that the interchromosomal effects of C(1)M3 are most pronounced within the st-in and Ki-p[sup P] intervals. CHAPTER 3 describes a genetic study of the Deformed locus. The mapping results confirmed that Dfd is closely linked to Ki. Genetic analysis of the crossover chromosomes suggested that Dfd is homozygous viable and this was confirmed by the synthesis of homozygous Dfd stocks. This indicates that the Dfd locus is not located within section 84F in proximal 3R. CHAPTER 4 deals with experiments involving the use of radiation to produce crossovers near the centromere of chromosome 3, in males. Crossovers originating from exchange nearest the centromere, were associated with clusters more frequently than those originating from exchange within other proximally-adjacent segments. Induced exchange was frequently accompanied by mutation and/or chromosome damage, at or near the site of exchange. This was particularly true for crossovers resulting from exchange in wholly euchromatic segments. It is suggested that many of the radiation-induced crossovers arise through asymmetrical exchange, and that this approach will permit the isolation of proximal aberrations. CHAPTER 5 describes the genetic and developmental analysis of a ts Minute. As a ts allele of a proximally-located Minute locus, Q-III exhibits the classical dominant M traits, recessive lethality, and a highly pleiotropic phenotype, at 29°C. This phenotype was analysed in detail through the use of various temperature shift experiments. Q-III possesses a polyphasic temperature-sensitive period (TSP) for lethality extending from the first larval instar to late pupation. Shorter heat pulses defined discrete larval, larval/pupal, and pupal TSPs for lethality. In addition, homozygous Q-III females exhibit ts sterility and maternal effects, indicating that the Q-III gene product is essential throughout development. Heat-pulse experiments revealed a number of adult developmental abnormalities, involving derivatives of eye-antennal, leg, wing and genital imaginal discs. Many defects, for example, those involving the eye or antenna (eye-antennal disc), male genitalia (genital disc), and scutellum (wing disc), have larval TSPs; whereas others, such as bristle or sex comb traits, have pupal TSPs. It is suggested that the former defects may be related to cell death in the larval anlagen; while the latter are more likely due to blockages in differentiation during pupation. Q-III also interacts in ts fashion with several non-allelic mutations . Thus, at 29°C, Q-III is lethal when combined with DI, Ly and Dfd; suppresses the sex comb phenes of Msc and Pc; and produces wing nicking effects when combined with vg or Sex. TSPs were defined for the vg, Dl and Sex interactions. It is suggested that many of these interactions arec:metabolic rather than specific. The fact that Q-III phenotypically resembles bobbed and ts suppressor of forked[sup ts], strengthens the notion that Minute gene products are active in translation. It is concluded that translational defects can fully account for the pleiotropy of Q-III. / Science, Faculty of / Zoology, Department of / Graduate

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No description available.