Evidence for nucleus independent steps in control of repair of mitochondrial damage

Summary The induction of the cytoplasmic petite mutation by ultraviolet light in 21 UV-sensitive (rad) nuclear mutants of Saccharomyces cerevisiae was compared to that in the wild type. Six rad mutants showed an increased sensitivity and two were less sensitive than the wild type. Modifications in the dose-response paralleled that of UV-induced reversion in one nuclear locus (hi ₁) studied. In these eight mutants the repair of UV-induced mitochondrial lesions seems to be under nuclear control.A block in the repair steps controlled by eleven of the other rad genes studied did not interfere with the repair of mitochondrial damage. In strains carrying a mutation in any one of these genes the dose-response curve for petite induction could be superimposed on that of the wild type even though they differed from the wild type in respect of nuclear gene reversion. These steps of the mitochondrial repair pathway(s) are therefore likely to be controlled by a nuclear and/or a cytoplasmic genetic determinant whose product acts specifically on mitochondrial lesions or it may be that the products of these genes are not required in the process of induction of petites.     

Precision of neural timing: effects of convergence and time-windowing

We study the improvement in timing accuracy in a neural system having n identical input neurons projecting to one target neuron. The n input neurons receive the same stimulus but fire at stochastic times selected from one of four specified probability densities, f, each with standard deviation 1.0 msec. The target cell fires if and when it receives m inputs within a time window of msec. Let _n,m, denote the standard deviation of the time of firing of the target neuron (i.e. the standard deviation of the target neuron's latency relative to the arrival time of the stimulus). Mathematical analysis shows that _n,m, is a very complicated function of n, m, and . Typically, _n,m, is a non-monotone function of m and and the improvement of timing accuracy is highly dependent of the shape of the probability density for the time of firing of the input neurons. For appropriate choices of m, , and f, the standard deviation _n,m, may be as low as . Thus, depending on these variables, remarkable improvements in timing accuracy of such a stochastic system may occur.     

Transfer of the flocculation property to the Baker's yeastSaccharomyces cerevisiae by conventional genetic manipulation

Summary An haploid MAT strain derived from a non flocculent commercial Baker's yeastSaccharomyces cerevisiae was mated with aSaccharomyces cerevisiae 1209 FLO₁ strain (flocculation degree F₅) and a diploid and extremely flocculent strain was selected. After sporulation and separation of single spore clones of the diploid hybrid, both MAT FLO₁ and MAT FLO₁ haploid strains were isolated and aSaccharomyces cerevisiae diploid strain FLO₁/FLO₁ carrying a great part of the genome of Baker's yeast was obtained by mating these.     

Effects of elevation of strain-ploidy on transmission and recombination of mitochondrial drug resistance genes in Saccharomyces cerevisiae

Summary In order to study the effects of strainploidy on the transmission and recombination of the mitochondrial genes C, E and O conferring the resistance to chloramphenicol, erythromycin and oligomycin, respectively, haploids were crossed to diploids and the results of genetic analysis were compared with those from haploidxhaploid crosses. All haploidx diploid crosses showed an increased transmission of diploid derived alleles, relative to haploid derived ones, but the pattern of increase differed between homosexual and heterosexual crosses. In ^– haploid x ^– diploid homosexual crosses, the increase was of roughly equal magnitude at the C, E and O loci: there was a polar co-transmission of the diploid derived alleles. In ⁺ haploid x ^– diploid heterosexual crosses, on the contrary, a differential increase was observed at the different loci, the magnitude being the smallest at the C locus and the largest at the O locus. As a result, there was a preferential transmission in favor of the haploid derived C alleles and of the diploid derived O alleles. A near equal transmission from both parents was observed for the E alleles. A decrease and an increase in the recombination frequency were noticed in the above haploidxdiploid homosexual and heterosexual crosses, respectively.The above phenomena were ascribed to different dosages of mitochondrial genomes from parents. Experimental data were well accorded with the theoretical expectations which were obtained on the assumptions that diploids contain twice as many mitochondrial genomes as haploids, and that random pairing and recombination would occur among mitochondrial genomes from parents. The elevation of strain-ploidy did not affect the recombination polarity which is under the control of the gene.It was theoretically predicted that a preferential transmission in favor of diploid derived alleles at all the C, E and O loci would be seen in ^– haploid x ⁺ diploid heterosexual crosses as well as in ^+; haploid x ^+; diploid homosexual crosses, but that the magnitude of the polar transmission would vary depending upon the loci in the former crosses, while it would be the same at all the loci in the latter ones. The recombination frequency was predicted to decrease in both of these crosses.     

Folded chromosomes of mating-factor arrested yeast cells: comparison with G0 arrest

Folded chromosomes of haploid a or diploid aa cells arrested by -factor are characterized by a heterogeneous sedimentation distribution, distinguishable from the corresponding g ₀and g ₁ structures. The extent of heterogeneity appears to depend in part on the extent of the characteristic schmoe morphology. Under conditions where a and mating-type cells are mixed together without conjugation, folded chromosomes characteristic of -factor arrested a and aa cells appear in both cell types in a synchronous manner. Attempts to order G₀ and -factor arrest suggest that these two arrest stages represent two different branch pathways out of the cell cycle. These two branch pathways may have a common branch point, or they may be characterized by two different branch points.     

Caffeine enhancement of radiation killing in different strains of Saccharomyces cerevisiae.

Summary Haploid and diploid wild type strains, and three classes of radiation-sensitive mutants of Saccharomyces cerevisiae were tested for enhancement of UV-inactivation by caffeine in growth medium. In addition, the sensitizing effect of caffeine was studied in a haploid and a diploid wild type strain after gamma-irradiation. The drug sensitized the UV-irradiated cells of all strains except those reported to be only slightly UV-sensitive but highly sensitive to ionizing radiation. After gamma-irradiation, no caffeine-enhancement of killing was observed in stationary phase cells of either the haploid or the diploid strain. However, log-phase cells of both strains were partially sensitized.The results of both sets of experiments suggested that caffeine interferes with a recombinational repair occurring in cells in S or G2 phase.     

Chromosomal loci of genes controlling site-specific restriction endonucleases of Bacillus subtilis

Summary In Saccharomyces cerevisiae, diploid strains which are respiratory deficient (e.g., rho^–) or are homozygous for the mating-type locus (i.e., either a/a or /) are unable to sporulate. In order to induce sporulation in these nonsporulating strains, the technique of protoplast fusion mediated by polyethylene glycol was adopted. In this study, the products of protoplast fusion were induced to sporulate without reversion to normal cells.Protoplasts from a respiratory-deficient diploid strain were mixed with those from a respiratory-competent haploid one carrying mitochondrial drug resistance markers, treated with 30% polyethylene glycol-4000 and 25 mM CaCl₂, and incubated in 0.1 M potassium acetate containing 0.8 M sorbitol as an osmotic stabilizer. After two days' incubation, asci with three to eight spores were formed at a frequency of 1×10^–3 to 2×10^–4. Sporulation was also observed in products of fusion between an a/a diploid and haploid strains and between an / diploid and a haploid strains. The analysis of the genotypes of spores revealed that when fusion products were cultured under conditions for sporulation, karyogamy did not take place, diploid nuclei underwent meiosis, and both diploid and haploid nuclei were able to develop into spores.     

Circadian rhythms of corneal mitotic rate,retinal melatonin and immunoreactive visual pigments,and the effects of melatonin on the rhythms in the Japanese quail

We investigated circadian ocular rhythms in the Japanese quail, Coturnix coturnix japonica. The birds were placed under light-dark cycles (LD 1212), constant light (LL) and constant darkness (DD), and the retinas were dissected out at four-hour intervals throughout 24 h. Following measurements were performed. (1) Melatonin content in the retina was measured by radioimmunoassay. It was low in light and several folds higher in darkness under LD 1212. The rhythm continued in DD, but disappeared in LL. (2) Mitotic figures in the corneal epithelium were counted. Similar rhythms to the melatonin content were observed in the corneal mitotic rate with a slight phase delay. (3) The retinas were fixed at 4-h intervals and immunostained with anti-bovine rhodopsin serum and anti-chicken iodopsin monoclonal antibodies. The outer segments of photoreceptor cells were stained intensively throughout 24 h in LD 1212, LL and DD. In contrast, the stainability of the locus close to the outer limiting membrane where the Golgi apparatus exists changed diurnally. Scores showing the ratio of cells with positive staining indicated high values from 4 h after the onset of light to the beginning of dark phase under LD 1212. The values were high throughout 24 h in LL and intermediate or low in DD. (4) To investigate the effect of melatonin on the corneal mitotic rate and visual pigments at the Golgi region, melatonin was injected into one eye and saline into the contralateral eye. Melatonin induced a phase advance in the corneal mitotic rate under LD 1212, but did not induce a rhythm under LL. The ratio of photoreceptor cells with positive staining to anti-visual pigment antibodies at the Golgi region was not affected by melatonin injection.Abbreviations ANOVA analysis of variance - BSA bovine serum albumin - DD constant darkness - Io-mAb monoclonal antibodies against chicken iodospin - LD light-dark - LL constant light - mRNA messenger ribonucleic acid - PBS phosphate buffer solution - Rh-As antiserum against bovine rhodopsin - SCN suprachiasmatic nucleus - T transducin - T transducin      

Cytogenetics of the parthenogenetic grasshopper Warramaba (formerly Moraba) virgo and its bisexual relatives

Triploid hybrids have been obtained by crossing individuals of the diploid Warramaba virgo with males of two undescribed related species of Warramaba, P169 (neo-XY) and P196 (X₁X₂Y). In both cases, offspring which receive a Y-chromosome from the father are males, while those that receive a neo-X from P169 or an X₁ and an X₂ from P196 are females. The triploids can be distinguished from diploids, even in the earliest nymphal instars, by the larger size of their eye facets. Their gonads are undeveloped and abnormal so that they are mostly sterile (the males absolutely so). Nevertheless, in the case of female hybrids (both the ones between virgo and P169 and those between virgo and P196) a few oocytes do develop and it was possible to obtain a further generation of hybrids by parthenogenesis. These, which are all female, and have karyotypes identical to those of their mothers, are derived from eggs which have undergone the virgo type of meiosis, with a premeiotic doubling of the chromosome number, followed by synapsis restricted to sister chromosomes. — Some diploid hybrids have also been obtained between the bisexual species P169 () and P196 (). In this case the male offspring died in the embryonic stage or immediately after hatching. Female hybrids, on the other hand, were viable but had under-developed ovaries, so that they only laid very few eggs. Some of the latter developed into embryos with a karyotype identical to that of the mother, but the meiosis of these eggs has not yet been studied, so that it is not known whether it is of the virgo type. These hybridization experiments support the hypothesis that virgo originated as a hybrid between P196 and P169. — A single male hybrid between Warramaba picta () and P169 () was obtained; it had active spermatogenesis, but many meiotic abnormalities.