Chemostat cultures in which the limiting nutrient was lactose have been used to study the relative growth rate of Escherichia coli in relation to the enzyme activity of β-galactosidase. A novel genetic procedure was employed in order to obtain amino acid substitutions within the lacZ-encoded β-galactosidase that result in differences in enzyme activity too small to be detected by ordinary mutant screens. The cryptic substitutions were obtained as spontaneous revertants of nonsense mutations within the lacZ gene, and the enzymes differing from wild type were identified by means of polyacrylamide gel electrophoresis or thermal denaturation studies. The relation between enzyme activity and growth rate of these and other mutants supports a model of intermediary metabolism in which the flux of substrate through a metabolic pathway is represented by a concave function of the activity of any enzyme in the pathway. The consequence is that small differences in enzyme activity from wild type result in even smaller changes in fitness.

We describe the use of DNA reassociation kinetics to determine the total genome size and complexity together with the individual complexity and copy number of the single copy, middle repetitive and highly repeated DNA fractions of cell line and larval DNA from the mosquito, Aedes aegypti. The genome of Ae. aegypti is both large and complex, being one third the size of the human genome, and exhibits a short period interspersed repeat pattern. The implications of patterns of sequence arrangement and genome complexities for experiments aimed at isolating specific classes of DNA sequences, such as mobile genetic elements, are discussed.

In this paper we describe the use of a free duplication, sDp2 (I;f), for the recovery, maintenance, and analysis of mutations defining essential genes in the left third of Linkage Group I of Caenorhabditis elegans. The lethals were induced in a strain of genotype (sDp2) + /dpy-5 + unc-13/ dpy-5 unc-15 +, using either 12 mM ethylmethane sulphonate or 1500 r of gamma radiation. Lethal mutations linked to the dpy-5 unc-13 chromosome were recognized by the absence of Dpy-5 Unc-13 individuals amongst the self progeny and were maintained by isolating Unc-13 hermaphrodites. These strains – which have two mutant alleles of the essential gene and a wild-type allele on the duplication – are balanced, since crossing-over does not occur between sDp2 and the normal homologues. Using this sytem we have recovered 58 EMS-induced mutations. These have been characterized with regard to map position and complementation. Twenty-nine of the EMS-induced mutations lie to the left of dpy-5 and define 20 complementation groups; 3 were inseparable from dpy-5 and define 3 complementation groups; 21 were to the right and define 17 complementation groups. Among a set of 29 gamma radiation-induced lethal mutations, 17 appear to be single gene mutations or are very small deletions. We estimate that we have identified from one-sixth to one-half of the essential genes in the sDp2 region.

For large population sizes, gene frequencies p and q at two linked over-dominant loci and the linkage disequilibrium parameter D will remain close to their equilibrium values. We can treat selection and recombination as approximately linear forces on p, q and D, and we can treat genetic drift as a multivariate normal perturbation with constant variance-covariance matrix. For the additive-multiplicative family of two-locus models, p, q and D are shown to be (approximately) uncorrelated. Expressions for their variances are obtained. When selection coefficients are small the variances of p and q are those previously given by Robertson for a single locus. For small recombination fractions the variance of D is that obtained for neutral loci by Ohta & Kimura. For larger recombination fractions the result differs from theirs, so that for unlinked loci r2 ≃ 2/(3N) instead of 1/(2N). For the Lewontin-Kojima and Bodmer symmetric viability models, and for a model symmetric at only one of the loci, a more exact argument is possible. In the asymptotic conditional distribution in these cases, various of p, q and D are uncorrelated, depending on the type of symmetiy in the model.

The ribosomal RNAs produced by the multigene families on the X and Y chromosomes of Drosophila melanogaster are very similar despite the apparent evolutionary isolation of the X and Y chromosomal rDNA. X–Y exchange through the rDNA is one mechanism that may promote co-evolution of the two gene clusters by transferring Y rDNA copies to the X chromosome. This hypothesis predicts that the proximal rDNA of X chromosomes will be Y-like. Consequently, rDNA variants found only on the X chromosome (such as those interrupted by type 1 insertions) should be significantly clustered in the distal X nucleolus organizer. Proximal and distal portions of the X chromosome nucleolus organizer were separated by recombination between the inverted chromosomes In(1)scv2 (breakpoint in the centre of the rDNA) and In(1)sc4Lsc8R (no rDNA). Molecular analyses of the resulting stocks demonstrated that rRNA genes containing type 1 insertions were predominantly located on the chromosome carrying the distal portion of the X rDNA, thus confirming a prediction of the X–Y exchange hypothesis for the co-evolution of X and Y chromosomal rDNA. Distal clustering is not predicted by the alternative hypotheses of selection or gene conversion.

A technique for obtaining bryozoan chromosome spreads is described. Karyotypes were prepared for the following phylactolaemate species: Plumatella emarginata, 2n = 14; Hyalinella punctata, 2n = 14; Fredericella sultana, 2n = 14; and Pectinatella magnifica, 2n = 18. Approximate numbers were obtained for Lophopodella carteri, 2n 18–22, and three ctenostomes, Pottsiella erecta, 2n = 22 or 24; Amathia semiconvoluta, 2n about 30; and Paludicella articulata, 2n about 20. Differences between the bryozoan karyotypes seen in this study correlate with taxonomy.

A metric character determined by a large number of loci without epistasis is normally distributed. In the absence of linkage the joint distribution in two or more relatives is multivariate normal, so that all regressions are linear and have constant residual variance. In the presence of linkage this is no longer true except in the case of parent and child; for all other types of relatives the regression line is unaffected by linkage but the residual variance about this line is no longer constant but increases away from the mean.

A computer model is developed that simulates Marker Assisted Selection (MAS) in a population produced by a cross between two inbred lines. Selection is based on an index that incorporates both phenotypic and molecular information. Molecular markers contributing to the index and their relative weights are determined by multiple regression of individual phenotype on the markers. The model is applied to investigate the efficiency of MAS as affected by several factors including total number of markers in the genome, number of markers contributing to the index, population size and heritability of the character. It is demonstrated that selection based on genetic markers can effectively utilize the linkage disequilibrium between genetic markers and QTLs created by crossing inbred lines. Selection is more efficient if markers contributing to the index are re-evaluated each generation than if they are evaluated only once. Increasing the total number of markers in the genome as well as the number of markers contributing to the index does not necessarily result in a higher efficiency of selection. Moreover, too many markers may result in a weaker response to selection. Population size is shown to be the most important factor affecting the efficiency of MAS.

Cellular functions of a double mutant of Escherichia coli K 12 deficient in recombination (recA) and defective in excision of pyrimidine dimers (uvrB) have been compared to those of isogenic recA or uvrB single mutants and ‘wild type’ bacteria. A combined effect of the two mutations on cell survival both under normal conditions of growth and after exposure to ultraviolet light or mitomycin C was demonstrated. The ratio of optical density to the number of colony formers in growing cultures of the double mutant is three times greater than in similar cultures of the recA single mutant and 9 times greater than in either uvrB or in ‘wild type’ cultures. The doubling time in growing recA uvrB cultures is 90 min, compared to 60 min, for the recA single mutant and 40 min for the uvrB single mutant and ‘wild type’ bacteria. Growing cultures of recA uvrB (λcI857) bacteria contain a substantial fraction of cells which are unable to form colonies at 32 °C, but produce phage when heated to 42 °C. No such cells were found in cultures of the single mutants or the ‘wild type’ bacteria lysogenic for λc1857. The double mutant is 10 times more sensitive to ultraviolet light and twice more sensitive to mitomycin C than the recA single mutant. In contrast to recA bacteria, exposure of the double mutant to mitomycin C induces little additional breakdown of cellular DNA. Induction of the prophage by mitomycin C is, however, prevented in both recA uvrB (λ) and recA (λ) bacteria. Exposure to mitomycin C creates conditions which render the prophage inducible by a newly transduced ree A gene. This effect of mitomycin C persists and can be revealed in complete medium at 37 °C after 100 min of incubation. The decay of the prophage, in cells exposed to mitomycin C, proceeds at a similar rate in both the double mutant and the recA single mutant. The inability of recA lysogens to be induced to phage production is discussed in the light of the present findings.

In the housefly, mosaics appear spontaneously but rarely. Sexual mosaics or gynandromorphs also appear in strains in which sex determination is based on autosomal sex factors. Rare cases of recombination in the male have been reported by some authors. In field and laboratory populations, mitotic plates with figures indicating exchange of chromatid segments are regularly observed in tissues of individuals of both sexes and at all stages of development. All these anomalies are interpreted as outward manifestation of the same phenomenon: mitotic recombination. The cytological basis of mitotic recombination, its relative frequency, its influence on linkage and genetic variability are discussed.

Ten A factor mutants and an unlinked A suppressor mutant were isolated from fruit bodies formed by common A heterokaryons of Coprinus lagopus. All A factor mutants formed false clamps and lacked A mating-type specificity. Four A5 and two A6 mutants had functional α sub-units which were recovered in tests for intra-factor recombination in matings between mutant and wild-type factors. No functional β sub-units were recovered in tests of two of the A5 mutants and the two A6 mutants which possessed functional α sub-units. From three other mutants (two A5 and one A6) neither α nor β sub-units were recovered.

The suppressor mutant also formed false clamps but was recessive since stocks carrying it retained their A and B specificities when mated with wild-type testers. Fruit bodies formed by monokaryons carrying the suppressor mutant had pale gills bearing varying proportions of abnormal tetrads with less than four spores.

1. An auxotroph of Salmonella typhimurium, pro-401, was isolated in a strain that was unstable at the su-leuA locus. The auxotrophy of pro-401 is probably due to the attachment of a controlling episome to the proline region of the genome where it suppresses gene expression.

2. The controlling episome frequently transposes over short distances so that all clones consist of cells, mixed for the site at which the controlling episome is attached; homologous transductions yield prototrophs.

3. The controlling episome can transpose to a different complementation group; homologous transductions yield abortive transductants; syntrophism occurs between cells that are ‘mutant’ in different complementation groups to give reversions consisting entirely of auxotrophic cells which are called auxotrophic reversions.

4. The controlling episome transposes over very short distances and never to beyond the limits of this proline region of the genome; no wild-type reversions were found.

5. The controlling episome can be located at relatively distant proline sites in different clones; prototrophs from transductions between clones that are separated by many subculturings can be 100 times more frequent than from homologous transductions.

6. The controlling episome has its frequency of transposition to different complementation groups increased by UV; irradiation increases the frequency of auxotrophic reversions.

7. The controlling episome continues to transpose in stored cells.

8. The pattern of reversions of pro-401 is different in these studies from its pattern two years previously. This is discussed.

Previous studies have shown that En-1, a homeobox-containing gene, maps close to or at the Dh locus in the mouse. Since homeobox-containing genes are key genes in the control of development the close proximity of En-1 to the developmentally significant gene Dh raised the possibility that the Dh mutation represented a mutant allele of En-1. A genetic analysis involving En-1, Dh, and other chromosome 1 markers (Emv-17, In and Pep-3) shows that although Dh and En-1 are closely linked they are separable by recombination (4/563). The likely gene order and recombination frequencies of these loci are: In (5.2±0.9) Emv-17 (1.1±0.4) Dh (0.7±0.4) En-1 (3.0±0.7) Pep-3. This shows that Dh is not a mutant allele of En-1.

The size of nuclear DNA from wild-type Ustilago maydis was determined to be approximately 6·09 ± 0·3 × 108 daltons from neutral sucrose gradient sedimentation analysis. Following exposure to ionizing radiation the nuclear DNA size was reduced due to the production of double-strand breaks in the DNA. These breaks were repaired when the irradiated cells were incubated in medium for at least one hour after irradiation. The repair was seen as a shift in the DNA profile from a low molecular weight region where the control DNA sedimented. Inhibition of protein synthesis by cycloheximide prevented this type of repair. Blocking protein synthesis also decreased the survival of irradiated wild-type cells but not radiation-sensitive mutants. Protein synthesis was necessary within the first one and a half hours after irradiation for the survival of wild-type cells to be unaffected. The results provide additional evidence for an inducible repair process in U. maydis.

The sex factor F is itself exceptional among conjugation factors in being freely expressed in all the bacteria of a culture, while related factors which are largely unexpressed are widespread. These can be recognized by the presence of a small proportion of cells sensitive to phage specific for F+ bacteria, and may sometimes constitute the conjugation factors associated with colicinogeny. Moreover, F itself can mutate to the repressed state so that derivatives of E. coli K12 which appear F− include some in which F is still present but rarely expressed.