We analyse the progression of linkage disequilibrium produced by random genetic drift in populations subject to cyclical fluctuations in size. Our model is applied to natural populations of Drosophila which show an annual demographic cycle of bottleneck (finite size) and demographic burst (size supposed to be infinite). In these populations, linkage disequilibrium stabilizes in such a way that, at equilibrium, the expected square of the correlation of gene frequencies E(r2) shows a stable cycle from year to year. If two loci are tightly linked, E(r2) barely varies during the annual cycle. Its values remain close to the value expected in a population of the same but constant effective size. If two loci are loosely linked, fluctuations in E(r2) are large. The maximum value, reached at the end of the bottleneck, is 10 to 100 times greater than the value obtained at the end of the burst. Our results show that the interpretation of observed linkage disequilibrium, by means of statistical tests, requires an accurate knowledge of population demography.

Amoebae of the Colonia isolate of Physarum polycephalum produce plasmodia within individual amoebal clones.

This paper reports the derivation from the Colonia strain C50 of a strain CL (Colonia Leicester) which produces plasmodia in clones with high efficiency and which completes the life cycle reliably and repeatedly in single clones. The derivation of a line CLd (CL delayed plasmodium formation) is described and, using CLd, the construction of the isogenic heterothallic strains mt1; CL and mt2; CL.

The above strains provide a system for the isolation and genetic analysis of mutants of P. polycephalum within a uniform genetic background, in particular mutants expressed in the plasmodium.

Using this system two auxotrophic mutants have been isolated. Preliminary genetic analysis has shown that they are due to single gene mutations.

A high frequency of proviral acquisition has previously been reported in the offspring of SWR/J–RF/J hybrid mice. In the present study, it was investigated whether this proviral acquisition would be useful for large-scale insertional mutagenesis studies. A population of SWR/J–RF/J hybrid mice with a predominantly SWR/J background was created. Lines of mice with such a background and partially congenic for two active proviruses from the RF/J strain were generated (the insert lines). Control lines were derived from mice which had no proviral loci but had an otherwise similar genetic background. DNA samples of mice in the insert lines were screened for the appearance of new proviral loci by Southern hybridization. The rate of proviral acquisition, calculated from the observed number of new proviral loci was 0·023 new proviruses per mouse. This rate is lower than found in previous studies and too low for large-scale insertional mutagenesis studies. A sensitivity experiment indicated that there was adequate detection of new proviral loci.The number of segregating proviruses was consistent with the number of newly acquired proviruses actually detected. Two additional crosses between mice in the insert lines and SWR/J mice were performed. The rate of proviral acquisition was greatly increased when SWR/J females were initially mated to insert mice, but remained unchanged when SWR/J males were used. This suggested that mice in the insert lines had acquired a maternally transmitted factor, which was suppressing viral expression and thus reducing the rate of proviral acquisition.

Kimura used the heterozygosity and the number of low-frequency alleles to estimate that about 14% of mutations are selectively neutral. The method is shown to be subject to biases and to disruption due to bottleneck effects. Let deleterious alleles have selective disadvantage, s, compared with neutral alleles and let Ne denote the effective diploid population size. The estimator, , of the proportion of neutral alleles is positively biased if (roughly) 4NeS < 25 or if 4Nes > 200. In the former case, one cannot adequately detect the different influences of deleterious and neutral alleles, whereas in the latter case, deleterious alleles will rarely appear in the sample. These difficulties cause the biases in , and are likely to cause similar biases for any estimation method based solely on allele frequencies. There is substantial sampling variability in in cases of practical interest, when data from 11 loci, or even as many as 31 loci, are pooled. If there has been a recent contraction in population size, will be positively biased, often yielding values greater than 1 or even being infinite. But after a recent expansion in population size, the heterozygosity will not have made as quick an increase and will be negatively biased. Population expansion alone can produce values close to those observed by Kimura, even if all alleles are neutral. In an appendix, a new method for simulating samples of neutral and deleterious genes is described.

Equal variances within quantitative trait locus (QTL) groups in the segregating population are a usual simplifying assumption in QTL mapping. The objective of this paper is to demonstrate the advantages of taking into account potential variance effect of QTLs within the framework of standard interval mapping approach. Using backcross case as an example, we show that the resolution power of the analysis may be increased in the presence of variance effect, if the latter is allowed for in the model. For a putative QTL (say, A/a) one can compare two situations, (i) and (ii) . It was found that, if the variance effect of A/a is large enough, then in spite of the necessity to evaluate an increased number of parameters, the more correctly specified model provides an increase in the resolution power, as compared to the situation (i). This is not unexpected, if either in (ii) is lower than from (i). But our conclusion holds even if . These advantages are illustrated on sweet corn data data (F3 families of F2 genotypes). In particular, the log-likelihood test statistics and the parameter estimates obtained for a QT locus in the distal region of chromosome 2 show that the allele enhancing the trait is recessive over the opposite allele simultaneously for the mean value and variance.

Measurement of ColEl transfer by ColV2, in E. coli K12, showed that ColV2 specifies a surface exclusion system. This system is different from those specified by Flac, and by ColVBtrp and Rl-19. Like these other two systems, the ColV2 surface exclusion system is subject to inhibition by the appropriate transfer inhibitor, and is not expressed in stationary phase cells.

Two mosaic birds are described, from a sex-linked F1 cross (Rhode Island Red ♂ × Light Sussex ♀). They are sexually normal males and full brothers. Each has the following abnormalities on one side of the body: absence of the maternal dominants S (silver, sex-linked), W (white skin) and Co (Columbian restriction of black), and reduction in size. In one case all the melanocytes appear to be derived from the smaller side, so that the plumage pattern is identical on the two sides; in the other there is a minor deviation (modified expression of Co) from normal F1 plumage pattern on the larger side. Mated to Brown Leghorn hens, both mosaics bred as normal F1 males, with no evidence of germinal mosaicism. Over thirty full sibs were all found to be normal.

A purely paternal origin of the smaller side appears to be excluded in both cases by the lack of striping in the down, and by minor deviations from normal Brown Leghorn phenotype in adult plumage pattern. Origin by some kind of maldistribution of chromosomes at first cleavage division would require the X-chromosome and possibly as many as three autosomes to be affected simultaneously.

Two further unrelated crossbred mosaic males are described. Each has yellow skin (w) on one side of the body. In one case this is accompanied by a slight difference in plumage pattern, reduction in size and abnormal proportions in the limb bones; in the other merely by a slight reduction in size. Loss or non-disjunction of a single chromosome at first cleavage division is a possible explanation of these.

A high frequency of chromosomal non-disjunction occurs spontaneously in mice heterozygous for some Robertsonian translocations. If animals heterozygous for the translocation and homozygous for different alleles of a marker gene are mated together a few young homozygous for the marker arise through non-disjunction, and their frequency can be used as a measure. This method has been used with the Robertsonian translocation Rb(9.19)163H and the marker ruby ru (chr. 19); Rb(4.6)-2Bnr with brown (b) and misty (m) (chr. 4); and Rb(9.14)6Bnr with hairless (hr) and piebald (s) (chr. 14) respectively. The frequencies of marked young were: Rb163 0/5260 ruru; Rb2 21/1997 mm bb; and Rb6 19/1702 hrhr ss, and the corresponding calculated non-disjunction frequencies in each arm of the translocation were Rb163, <5 %; Rb2, 15%; Rb6, 15%. These figures show reasonably good agreement with values obtained by other methods. A search for genetic or environmental factors affecting the frequency of marked young in Rb2 and Rb6 revealed that in Rb2 the frequency increased with maternal age, whereas in Rb6 the maternal age of the marked young was non-significantly below that of the total progeny. The reasons for this discrepancy are not clear.

To test whether large changes in the enzyme levels of segmentally aneuploid Drosophila melanogaster can be ascribed to changes in kinetic properties of the enzymes affected, comparisons have been made with regard to the heat stability, substrate concentration dependency, and the presence of heat-stable inhibitors or activators within the extracts of aneuploid and control flies. By these criteria, no differences were found between controls and the α-GPDH activity of flies trisomic for chromosome II segments 27D–31E, 35A–40, 41–45F, and 57B–60F and no differences were evident between the IDH properties of 70CD–71B aneuploids and their controls. The enzyme changes observed in these aneuploids are more likely associated with changes in the rates of accumulation of the enzyme molecules. The IDH of flies trisomic for the 27D–31E region was more heat-stable than that of controls while the α-GPDH of flies trisomic for the 21A–25CD region displayed an apparent Michaelis constant for α-glycerophosphate lower than that of controls. The possible bases for these latter qualitative distinctions are discussed.

The R factor, R144drd3, mobilized chromosomal genes in Klebsiella pneumoniae strain M5a1 at similar frequencies to those observed for this R factor in Escherichia coli K12. In a derivative K. pneumoniae donor, strain HF3, R144 persisted in the autonomous state but now gave rise to polarized transfer of the chromosome in the order O his…arg 2…leu…trp. R144drd3 was unstable in K. pneumoniae M5a1; after successive subculture drug resistance, colicinogeny and transmissibility were lost. In strain HF3, transmissibility was preferentially lost, but determinants for drug resistance, colicinogeny and superinfection immunity were retained; 10–11 copies per chromosome of R144 were present in log phase cultures of both strains.

A mutant of Tetrahymena with heat-sensitive phagocytosis was obtained using a tantalum-particle enrichment procedure. The mutant phenotype is most likely determined by a somatic (macronuclear) mutation(s). The inability of the mutant to sustain cell division and to phagocytize at 37 °C are most likely determined by the same mutation. The phenotype of the mutant is stably inherited under vegetative propagation at 30 °C. At 37 °C, the mutation affects the development of the oral apparatus, the phagocytotic organelle. This mutant has proven useful for the study of cellular functions related to phagocytosis.