Showing papers in "Genetics Research in 1969"

Linkage disequilibrium due to random genetic drift

Abstract: The behaviour of linkage disequilibrium between two segregating loci in finite populations has been studied as a continuous stochastic process for different intensity of linkage, assuming no selection. By the method of the Kolmogorov backward equation, the expected values of the square of linkage disequilibrium z2, and other two quantities, xy(1 − x) (1 − y) and z(1 − 2x) (1 − 2y), were obtained in terms of T, the time measured in Ne as unit, and R, the product of recombination fraction (c) and effective population number (Ne). The rate of decrease of the simultaneous heterozygosity at two loci and also the asymptotic rate of decrease of the probability for the coexistence of four gamete types within a population were determined. The eigenvalues λ1, λ2 and λ3 related to the stochastic process are tabulated for various values of R = Nec.

The inheritance of the killer character in yeast.

Abstract: 1. First- and second-generation crosses between killer, neutral and sensitive strains of yeast have been carried out in all combinations. 2. The results of this analysis indicated that the killer character is under the control of two types of cytoplasmic genetic determinant. One type, (k), determines killing, and the other, (n), neutrality. The absence, (o), of both types of determinants confers the sensitive phenotype. 3. That both types of cytoplasmic determinant require the same dominant nuclear allele, M , for their maintenance has been indicated in two ways. First, both types are lost when the nuclear genotype is changed from M to m . Secondly, cells of genotype m (k) or m (n), which have been shown to occur among the first formed cells arising from spores of Mm (k) and Mm (n) diploids respectively, are unable to maintain their cytoplasmic determinants. On the other hand, spore cultures of M (k) and M (n) genotype derived from these same diploide continue to maintain the determinants. 4. Thus genotype of killer cells is M (k), of neutrals M (n), and of sensitivities either M (o) or m (o). 5. Cells maintaining both types of cytoplasmic determinant (i.e. of genotype M (k)(n) or M −(k)(n)) have been obtained by appropriate crosses, and shown to be of killer phenotype. 6. Alternative hypotheses to account for the results of this genetic analysis have been discussed.

A comparison of some biological characteristics of the mouse-passaged scrapie agents, 22A and ME7.

Abstract: Two mouse-adapted scrapie agents of different sheep origin were compared. The titre, reached in the brains of mice in the terminal stage of scrapie, is of the same order for both agents. There is a threefold difference between the incubation periods of the two agents in some mouse strains, of which C57 is one, and in this strain incubation of the 22A agent, given as a large dose by a peripheral route, occupies almost the whole life-span.The most fundamental difference between the agents concerns the reversal of the ranking of incubation periods, typically in the VM and C57 mouse strams: incubation of ME7 in VM takes almost twice as long as in C57, whereas most sub-lines of 22A take half as long in VM as in C57. The implications of this type of host-genotype, agent-strain interaction are discussed in terms of the possible nature of agent differences, the possibility of latent infection and the consequences for scrapie eradication programmes.

Genetic studies of ovulation rate in the mouse.

Abstract: Genetic and physiological studies of ovulation rate, both natural and induced, in a random bred population of laboratory mice led to the following results.(1) The heritability of natural ovulation rate in nulliparous females was 22±19%, estimated from the correlation between paternal half-sibs.(2) Selection applied to natural ovulation in primiparous females led to a response in both directions, with a realized heritability of 31% (h2 within litters = 0·18 ± 0·013). The number of eggs shed by the High, Control and Low lines after 12 generations were respectively 21, 16 and 14.(3) Selection applied to ovulation induced by 4 i.u. of PMS led also to a response in both directions, with a realized heritability of 22% (h2 within litters = 0·11 ± 0·038). The induced ovulation rates of the High, Control and Low lines after 12 generations were respectively 29, 19 and 14 eggs.(4) PMS-equivalents were calculated from dose-response curves, and were found to be nearly the same in all lines except the line selected for high natural ovulation rate, which had a PMS-equivalent nearly double that of the Control. From this it was inferred that the increase of ovulation rate resulting from selection for high natural ovulation was due to an increased FSH activity, whereas the changes of ovulation rate resulting from selection for low natural ovulation and for both high and low induced ovulation were due to changes of ovarian sensitivity.(5) Genetic correlations, estimated from correlated responses to selection, were: (i) between natural and induced ovulation in primiparous females, 0·33; (ii) between primiparous and nulliparous females in natural ovulation, 0·46; (iii) between primiparous and nulliparous females in induced ovulation, 0·82.(6) The selection for induced ovulation produced changes in litter size following natural ovulation that were nearly equal to the changes in natural ovulation rate. The selection for natural ovulation, however, produced no clear changes of litter size.

Genetics of host-controlled restriction and modification in Escherichia coli.

Abstract: The sites of mutations affecting host-controlled modification (HCM) have been mapped in E. coli K and E. coli B by conjugation and transduction experiments between mutants. These mutations all map close to the serB locus on the opposite side to the marker thr. Non-parental HCM has been observed among colonies obtained from Pl transduction experiments between HCM mutants. Control experiments have shown that these non-parental recombinants can not be accounted for by reversion of either parent and must result from recombination between mutants. Several genetic models are suggested which could account for these recombinants and an attempt is made to distinguish between various models by testing for complementation between mutants in a zygotic induction complementation test.

Controlling elements in the mouse X-chromosome. I. Interaction with the X-linked genes.

Abstract: The mouse X-chromosome controlling elements, detected by their influence on the position effect variegation caused by the X-autosome translocation T (1; X) Ct, have been found to modify the heterozygous phenotypes of two X-linked genes. It is proposed that X-inactivation can be incomplete, the level of inactivation or the frequency of cells in which inactivation is incomplete being dependent upon the ‘state’ of the controlling element located in the X. The data suggest that this is a consequence of a reversal, or partial reversal, of inactivation of the X as a whole in some cells rather than a vairable spread of inactivation along the length of the X.

Somatic segregation of the killer (k) and neutral (n) cytoplasmic genetic determinants in yeast.

Abstract: When killer and neutral strains of Saccharomyces cerevisiae are crossed the resulting diploid clones possess a killer phenotype and when spored yield a complete range of tetrad ratios.The combined results of analysing tetrads and vegetative cells of diploid clones derived from two different neutral × killer crosses (K1 × N1 and K2 × N1) demonstrate that the range of tetrad ratios can be accounted for by the occurrence of somatic segregation of killer (k) cytoplasmic determinants prior to sporulation. Such results support the genetic model for the inheritance of the killer character in yeast already proposed (Somers & Bevan, 1969).During the course of these studies a correlation was found between the strengths of the killer phenotypes of diploid colonies and the proportions of killer spore cultures obtained after sporulation of their cells.

Chromosomal basis of dosage compensation in Drosophila. I. Cellular autonomy of hyperactivity of the male X-chromosome in salivary glands and sex differentiation.

Abstract: Morphology and the rate of RNA synthesis of the X -chromosome in XX/XO mosaic larval salivary glands of Drosophila melanogaster have been examined. For this purpose the unstable ring- X was utilized to produce XX and XO nuclei in the same pair of glands. The width of the X -chromosome and the left arm of the 3rd chromosome (3 L ) of larval salivary glands was measured and the rate of RNA synthesis by them was studied upon the use of [ 3 H]uridine autoradiography in such XX (female) and XO (male) nuclei developing in a female background (i.e. otherwise genotypically XX ). In such mosaic glands the width of the single X -chromosome of male nuclei is nearly as great as that of the paired two X 's of female nuclei, as is also the case in normal male ( X Y ) and female ( XX ). The single X of male nuclei synthesizes RNA at a rate equal to that of the paired two X 's of female nuclei and nearly twice that of an unpaired X of XX nuclei. Neither the developmental physiology of the sex nor the proportion of XO nuclei in a pair of mosaic salivary glands of an XX larva has any influence on these two characteristics of the male X -chromosome. It is suggested that dosage compensation in Drosophila is achieved chiefly, if not fully, by a hyperactivity of the male X , in contrast to the single X inactivation in female mammals, that this hyperactivity of the male X is expressed visibly in the morphology and metabolic activity of the X -chromosome in the larval salivary glands of the male, and that this hyperactivity and therefore dosage compensation in Drosophila in general is not dependent on sex-differentiation, but is a function of the doses of the X -chromosome itself.

Sexual and parasexual analysis of Ustilago violacea.

Abstract: Forty-two mutants of the anther smut fungus Ustilago violacea were mapped by means of complementation tests, mitotic haploidization, and meiotic segregation. Spontaneous mitotic haploidization was very rare, but haploids were induced at a high frequency using p–fluorophenylalanine (PFP). Haploid segregants appeared as fast-growing, spherical colonies (papillae) which grew away from the diploid growth on PFP medium. Thirty-three markers, classified by complementation tests into 21 genes, were mapped by mitotic haploidization in 10–12 linkage groups. There were no discrepancies in the linkage data, and all the markers could be assigned unequivocally to linkage groups. Although about 250 diploids were analysed, there were no segregants in which mitotic crossing-over and mitotic haploidization appeared to have occurred simultaneously.Thirteen of the 33 markers, in six or seven genes, were expressed infrequently (0–5%) in the papillae produced on PFP medium. These markers, which behaved unusually and were designated missing-markers, were found to be on two chromosomes which tended to remain disomic on PFP medium. Thus 8–10 chromosomes haploidize readily on PFP medium, whereas two other chromosomes are resistant to the effects of PFP and remain disomic. Meiotic segregation was investigated in crosses of genetically marked haploid stocks and also hi diploids, using the host plant. Some of the results enabled preliminary maps to be made of three linkage groups. The results from meiotic segregation were fully compatible with those from mitotic haploidization and the complementation tests.The genetical evidence for a haploid chromosome number of at least 10–12 is in conflict with the observations of several cytologists that n = 2 in this species.

Factor responsible for spermatozoan abnormality located on the y chromosome in mice.

Abstract: Two inbred strains of mice differing in the mean percentage of spermatozoa with abnormal heads were used: KE (16·1%) and CBA (5·9%). The F1 resulting from the crosses exhibited a heterosis effect, while in the backcrosses an obvious segregation of genotypes was observed; both generations showed a reciprocal difference, depending on the source of the Y chromosome. The character of sperm head abnormality seems to be polygenically determined, one of the genes being located on chromosome Y.Seven generations of backcrosses were performed in which the Y chromosome from CBA was introduced to the genetical background of the KE strain. In the seventh generation 10·2% of abnormal spermatozoa were found, which is significantly lower than in the KE strain. The difference shows the net effect of the Y-linked locus. A correlated difference was found in the fertilization rate, indicating that a factor influencing male fertility is located on chromosome Y. It does not seem to influence the shape of normal spermatozoan heads.Karyotype analysis did not reveal gross abnormalities in the KE strain.

Interactions between de-repressed F-like R factors and wild type Colicin B factors: superinfection immunity and represser susceptibility

Abstract: F-like R factors are related to ColB-K77, but not to ColB-K98 or ColB-K166, by the criterion of superinfection immunity. Tests of de-repressed mutant R factors for susceptibility to repression by ColB-K98, known to produce represser for F, showed that these could be either repressor-sensitive or insensitive. The former were independently shown not to produce repressor, which was thus, presumably the reason for their de-repressed state.

Effect of a recA gene on cell division and capsular polysaccharide production in a lon strain of Escherichia coli.

Abstract: In Escherichia coli the u.v. sensitivity gene recA suppressed u.v.-induced filamentation in a lon u.v. sensitive strain without affecting capsular polysaccharide production. recA appears to prevent the stimulus that leads to filamentation in a lon strain.

Natural selection and gene substitution.

Abstract: Using models which describe the change, by natural selection, of the actual numbers of genes rather than their relative frequencies, it is demonstrated that the equation familiar to geneticists, i.e. dp/dt = sp(1 − p), is appropriate under a wide range of circumstances. It was pointed out that, for realistic treatment of the evolutionary process through which gene substitutions are repeated, the models must have the property such that the total population number remains constant or nearly constant throughout the process, and is not appreciably influenced by the genes being substituted.The load or cost for a gene substitution was studied assuming a haploid population and the effects on the load of such factors as epistatic gene interaction in fitness, finite population number and slow change of environment were investigated. The load may become very large under a strong ‘reinforcing’ type epistasis between advantageous genes. In a finite population, the load for one gene substitution may be inflated by about unity if the product of the effective population number (Ne) and the selection coefficient (s) is large but Nesp0 is much smaller than unity, where p0 is the initial gene frequency. On the other hand, slow change of environment may decrease the load somewhat. It was concluded that despite these and other complicating factors, Haldane's original formula, –logep0, for a haploid population (−2 logep0 for the case of a diploid without dominance) is still useful for assessing the approximate amount of selective elimination that accompanies the process of gene substitution in evolution.

On the theory of artificial selection in finite populations.

Abstract: The effect of selection on individual performance for a quantitative trait is studied theoretically for populations of finite size. The trait is assumed to be affected by environmental error and by segregation at a single locus. Exact formulae are derived to predict the change in gene frequency at this locus, initially by finding the probability distribution of the numbers of each genotype selected from a finite population of specified genotypic composition. Assuming that there is random mating and no natural selection the results are extended to describe repeated cycles of artificial selection for a monecious population. The formulae are evaluated numerically for the case of normally distributed environmental errors. Using numerical examples comparisons are made between the exact values for the predicted change in gene frequency with values obtained using approximate, but simpler, methods. Unless the gene has a large effect (α) on the quantitative trait, relative to the standard deviation of the environmental errors, the agreement between exact and approximate methods is satisfactory for most predictive purposes. The chance of fixation after repeated generations of selection is also evaluated using the exact method, and by means of a diffusion approximation and simple transition probability matrix methods. Except for very small values of population size ( N ) and large α the results from the diffusion equation agree closely with those from the exact method. Similar results are found from tests made of the prediction from the diffusion equation that the limit is only a function of N α for a given intensity of selection and initial frequency, and that the rate of advance in gene frequency is proportional to 1/ N for the same set of parameters.

A note on the inheritance of erythromycin-resistance in Paramecium aurelia

Abstract: In yeast it has been reported that some types of erythromycin-resistance are due to cytoplasmic genetic factors, localized in mitochondria (Thomas & Wilkie, 1968; Slonimski, 1969). In view of the current interest in genetical aspects of mitochondria, and the ease with which cytoplasmic heredity can be demonstrated in Paramedum aurelia, some tests have been made to determine whether erythromycin-resistant variants could be obtained in this ciliate and, if so, to determine their genetic basis. Paramecia were placed in bacterized lettuce medium at pH 7-0, to which erythromycin had been added to a concentration of 0-25 mg/ml. It was found that the organisms passed through two or three fissions, and then stopped dividing. Usually, however, they remained alive for days or weeks in presence of the drug, getting gradually smaller. To kill the paramecia outright, much higher concentrations of erythromycin were necessary. In the present study, however, higher concentrations of the drug than 0-25 mg/ml were not used. Paramecia having the ability to grow in the presence of erythromycin were obtained in the following way. Samples of 10 ml, containing approximately 10000 organisms of P. aurelia (stocks 513 and 168, syngen 1), previously grown in bacterized lettuce medium, were put in centrifuge tubes, and gently spun down until most of the paramecia were at the bottom but alive. The supernatant was poured off and bacterized medium containing 0-25 mg/ml erythromycin added. The tubes were kept at 25 °C, and at twice-weekly intervals the contents again centrifuged and resuspended in freshly prepared bacterized medium with added erythromycin. After varying periods of time—between 2 and 4 weeks —some of the tubes were found to contain actively growing paramecia, and on isolating samples of these on to depression slides, the animals were found to grow at the normal rate (3 fissions per day at 25 °C) in erythromycin-containing medium. Once obtained, these clones were stable in regard to their resistance to erythromycin, for after being placed in normal bacterized lettuce medium and allowed to pass through several hundred fissions and several autogamies in the absence of the drug, the progeny were still found to be able to grow normally in erythromycin-containing medium. To study the genetic basis of erythromycin-resistance in P. aurelia, conjugation was brought about between resistant animals of stock 513, and sensitives of stock 168. Pairs were isolated, and the cytoplasmic parentage of each ex-conjugant clone determined by testing one animal of each with specific antisera, after one post-conjugational fission (as described elsewhere, Beale, 1954). The sister animals were then tested for ability to grow in the presence of 0-25 mg/ml erythromycin. The Fx progeny of 28 pairs were tested in this way, and in every case the exconjugant clones descended cytoplasmically from the erythromycin-resistant parent were found to consist of resistant animals, and the ex-conjugant clones descended cytoplasmically from the sensitive parent were all sensitive. Backcrosses were made between erythromycin-resistant Fx animals and stock 168 sensitives, and again one member of each pair was found to yield a clone of resistant

Brachyphalangy, an allele of extra-toes in the mouse

Abstract: Brachyphalangy ( Xt / bph ) is an allele of extra-toes ( Xt ) in linkage group XIV of the mouse. The phenotypes of both heterozygote and homozygote are distinguishable from, but similar to, those of extra-toes.

Nucleic acid content and Chromosome morphology in Chrysanthemum

Abstract: 1. The DNA contents of twenty-eight different species and forms of Chrysanthemum have been measured by photometry. It is shown that there are large differences in DNA content between some species with identical chromosome numbers.2. The DNA contents of natural polyploids are frequently not those expected when comparison is made with diploid forms of the same species. The DNA contents of induced polyploids are those expected.3. Chromosome length and volume are positively correlated with DNA content.4. The relationship between chromosome number, chromosome size, DNA content and gene number is considered, and it is suggested that the differences in DNA content may result from the presence of differing amounts of genetically inactive DNA in the chromosomes.

Genetic polymorphism and interspecific competitive ability in Drosophila.

Abstract: A simple method is given to compare the relative fitnesses of various populations of one species when they compete with another species. Populations of Drosophila psevdoobscura polymorphic for the chromosomal inversions CH and AR are superior to monomorphic populations CH or AR when they compete for the available resources with D. serrata.

Investigation of the mating system of Pseudomonas aeruginosa strain 1. IV. Mapping of distal markers.

Abstract: Thirteen selected distal markers have been mapped by time of entry studies and linkage analysis. They appear to fall on two distinct linkage groups and with the ten previously described there are nineteen markers on one group and four on the other. There is as yet no clear evidence that the two groups are linked, but this possibility has not been excluded.Of the thirteen markers described, six can be mapped by measuring their time of entry into suitable recipients. It should now be possible to establish the order of any further marker and to provide more accurate information for any marker transferred within 35 min from the time of mixing the cells.