The effect of nalidixic acid on conjugal recombination was studied in matings with recipient strains carrying recA200, a mutation which confers a thermosensitive Rec− phenotype. Addition of nalidixic acid to Hfr Nals × F−NalRrecA200 matings at low temperature (35 °C) caused a sharp 10- to 20-fold decline in the yield of recombinants if plating on selective agar was delayed. Two separate processes were identified as being responsible for this decline. Those merozygotes in which the transferred DNA was free of the donor cell lost the ability to form recombinants through inactivation of this DNA, an effect which could be prevented by using exonuclease deficient (recB sbcB) recipients or by prior growth of exonuclease proficient recipients in medium containing 0·25 M sodium chloride. No more than 50% of the observed loss of recombinants could be attributed to this effect. The remaining merozygotes lost their ability for recombinant formation provided mating pairs, and presumably the displaced donor DNA strand, remained intact. This process was thought to involve withdrawal of transferred DNA (DeHaan & Gross, 1962) and was studied in isolation in matings with recA+, or recA200 recB sbcB recipients. A mechanism involving re-annealing of the displaced Hfr DNA to the donor molecule as a result of nalidixic inhibition of gyrase activity in the donor causing relaxation of DNA supercoils is proposed to account for this withdrawal event.

Crossing-over between the α and the β loci constituting the A incompatibility factor gives rise to two new specificities which are compatible with both parental As and with each other. The frequency of monokaryotic mycelia carrying re-combinant A factors is shown to be under genotypic control in a multiple crosses programme (6 × 6), selection for high and low recombination frequencies (two generations), and crosses between the selection lines. The recombination values based on samples of 100 monokaryons, range from 0 to 21%; however, the more accurately estimated values of the ‘High’ and ‘Low’ selections are 14% and 4%, each being based on approximately 2,000 mycelia.

The data are compatible with a gene-system consisting of the postulated locus rec which has a major effect on recombination and which is linked to the A factor, and several minor effects by other loci. Alternative interpretations are presented and discussed. The apparent dominance of low frequencies of recombination on high frequencies can be related to the breeding behaviour of S. commune. Thus close linkage between α and β allows a high number of A specificities to be maintained in a population as well as high out-breeding potential, while the inbreeding potential (i.e. dikaryotic combinations between monokaryons originating from a single fruit-body) is kept low and near its minimum.

The significance of the two-locus structure of the incompatibility factors is examined theoretically in an Appendix at the end of the Discussion section.

Mice were selected for high and low body weight at 5 and at 10 weeks of age. Selection was performed (1) separately for each trait, and (2) for various combinations of the two traits, using (a) independent culling levels and (b) restricted indices. Two-way selection for each trait separately gave large responses and correlated responses. Selection by independent culling levels intended to increase 5-week weight while restricting change in 10-week weight gave no demonstrable response; selection by culling levels intended to decrease 5-week weight while restricting change in 10-week weight resulted in decreases in body weights at both ages. Index selection, intended to change weight at one age while holding that at the other age constant, was generally successful. Observed responses did not conform very well with predicted responses for either index or culling levels selection. The significance of these observations in regard to the problem of selection involving restriction of traits is discussed.

The variety Chinese Spring of Triticum aestivum is susceptible to cold treatment applied at the juvenile plant stage, while the variety Cappelle-Desprez shows resistance to such treatment. By cytological and backcross procedures single homologous pairs of chromosomes from Cappelle-Desprez were substituted for their homologues in Chinese Spring. Assay experiments carried out on each of the 21 possible substitution lines indicated that three chromosomes, 4D, 5D and 7A of Cappelle-Desprez were involved in the determination of cold resistance. The resistance expressed by the three substitution lines carrying these chromosomes was less than the reaction of Cappelle-Desprez to cold treatment. Also the magnitude of this resistance supported a hypothesis that the action of the three chromosomes was additive on the scale of measurement used. The possible relationships of the genes for cold resistance to the established genes controlling other developmental characters, also carried by these three chromosomes, are discussed.

Plasmid-mediated sensitivity to filamentous phage IKe is shown to be a property exclusive to plasmids of the N incompatibility group. As with other sex factor-specific phages, IKe sensitivity results from the provision of a plasmid-encoded receptor. However, direct evidence for IKe adsorption to a sex pilus-like structure is so far lacking.

Mutations in an N plasmid were obtained which affected IKe infect-ability and N transfer frequency simultaneously, though to different extents. IKe receptors could be removed to a limited extent by high speed blending, but only under more extreme conditions (higher speed and in low ionic strength medium) than F pili. As with F-specific filamentous phages, IKe adsorption was partially blocked by Zn2+.

We tentatively suggest that the results accord with the IKe receptor being a sex pilus rather different from F and I pili (possibly in being much shorter in liquid culture), but other interpretations of these data are possible.

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.

Material sampled along 11 rivers of the western part of Salix viminalis L. natural range (in Poland, Germany and Austria), as well as in stands in Sweden and Belgium, was assayed for 15 isozyme loci and cuttings were installed in two field experiments located in a nursery south of Uppsala, where growth traits were measured. These data were used to test hypotheses on the origin of Swedish populations, on the part played by rivers in the genetic differentiation and on the relative differentiation at isozyme and quantitative trait loci. Although significant, the overall population differentiation was low, the FST value being around 4%. Much higher FST values were observed between subpopulations from southern (Skåne) and central Sweden. This strong population differentiation, accompanied by significant linkage disequilibria, suggests the recent and diverse origin of Swedish populations. Degrees of differentiation between and within Polish river systems were of the same magnitude, indicating the presence of gene flow between river systems. Flow-regulated waterways, associated with higher human disturbance, may well explain why populations along rivers of the western part of the study area exhibited significant differentiation patterns while no differentiation could be detected along the less disturbed riparian habitats of eastern Poland. Finally, higher FST values were obtained for quantitative trait loci than for isozyme loci but, with two notable exceptions, their 95 % confidence intervals overlapped.

Seven strains of guinea pig (Cavia porcellus) with different degrees of inbreeding were investigated for genetic variability by starch gel electrophoresis. Variants for six out of 31 loci (Gpd-1, Aat-1, Ak-2, Pgm-1, Ada and Gpi) are reported. The data suggest that the strains all come from a small founding population and therefore are genetically more homogenous than the visible characteristics imply. Differences between isoenzyme data and other genetic characters are described and possible underlying genetic mechanisms are discussed.

1. A number of stable nystatin-resistant mutants of the yeast Saccharomyces cerevisiae have been isolated from platings of a sensitive wild-type strain on low concentrations of the antibiotic.

2. These mutants were found to be resistant to 10, 15 or 60 units of drug/ml.

3. Analysis of meiotic segregants from crosses of these mutants to wild-type indicate that resistance is determined by two types of genes; resistance genes and modifiers.

4. Functional analysis of the mutants demonstrated the existence of three recessive resistance genes, nys-l, nys-2 and nys-3 and that nys-1 and nys-2 were linked.

5. Genetic analysis showed that nys-1 was affected by two modifiers, Mnys-1 and Mnys-2, but that only Mnys-2 affected nys-2 and nys-3.

6. The modifiers Mnys-1 and Mnys-2 are dominant.

7. An investigation of the effects of temperature and medium on resistance demonstrated marked interactions between genotype and environment for both the resistance genes and the modifiers.

8. Second-step mutants have been isolated by plating first-step mutants on higher concentrations of the drug. Some of these are resistant to 800 units/ml.

9. Some possible mechanisms of nystatin resistance are discussed.

1. The occurrence of fusion between plasmodia produced from amoebal clones of P. polycephalum was studied.

2. The occurrence of fusion was found to be strain-dependent and the factors responsible segregated in the progeny of a cross.

3. The segregations found in crosses between several strains led to the conclusion that four alleles (f1–f4) of one gene f were controlling fusion in these strains.

4. Fusion occurs only between plasmodia carrying identical f alleles, except in one class of results.

5. A model accommodating all the results, including the ‘exceptional’ class, is proposed. It requires that the action of the f factors is to inhibit fusion between dissimilar strains rather than to promote fusion between identical strains. Certain physiological deductions from this model are discussed.

6. The locus (mt) determining mating type of the amoebae is not concerned in plasmodial fusion and is unlinked to f.

7. The rate of fusion between some pairs of strains is apparently influenced by modifying genes.

8. It is suggested that, as a result of the operation of the f gene and of the previously described killing reaction, heterokaryons will occur rarely in natural populations of P. polycephalum.

Current models predict that large increases over wild-type in the activity of one enzyme will not alter an organism's fitness. This prediction is tested in Saccharomyces cerevisiae through the use of a high copy plasmid that bears one of the following: hexokinase B (HEXB), phosphoglucose isomerase (PGI), phosphofructokinase (PFKA and PFKB), or pyruvate kinase (PYK). Transformants containing these plasmids demonstrate a four to ten-fold increase in enzyme specific activity over either the parent strain or transformants containing the plasmid alone. Haploid and diploid transformants derived from independent backgrounds were grown on both fermentable and non-fermentable carbon sources and evaluated for several components of fitness. These include growth rate under non-limiting conditions, maximum stationary phase density, and viability in extended batch culture. Cell viability is not affected by overproduction of these enzymes. Growth rate and stationary phase density do not differ significantly among strains that overexpress HEXB, PGI or contain the vector alone. PFKA, B transformants show reduced growth rate on glucose in one background only. For these loci the current model is confirmed. By contrast, when grown on glucose, yeast overexpressing PYK demonstrate reduced growth rate and increased stationary phase density in both backgrounds. These effects are abolished in cells containing plasmids with a Tn5 disrupted copy of the PYK gene. Our results are consistent with reports that the PYK locus may exert control over the yeast cell cycle and suggest that it will be challenging to model relations between fitness and activity for multifunctional proteins.

Inositol independent (inl+) strains were obtained either as transformants following treatment of the inositol requiring (inl) strains of Neurospora crassa with the wild-type DNA or as revertants without any DNA treatment. A significant number of the inositol-independent transformants were also found to have acquired additional mutations called osmotics (os) which made them unable to grow on 1 m-NaCl medium. None of the inositol-independent revertants were found to possess such osmotic mutations and their growth remained unaffected by the presence of NaCl. Many of the osmotic mutants described here were found to be new alleles of the previously known os–1 mutation on the linkage group I of Neurospora crassa. The remainder were found to map at two new genetic loci designated as os-6 and os-7; these loci were found to be closely linked to os–1. Among the new osmotic mutants only os-1 and os-6 mutants showed intragenic complementation. The mechanism of DNA-induced mutation during transformation is discussed.

Five biometrical traits (thorax length, wing length and width, sternopleural and abdominal chaetae numbers) were measured on 13 equatorial African strains and 30 French strains. In all cases highly significant differences were observed between the two geographic groups. These results are added to previously known variations concerning adult weight and ovariole number. In each place, the genetic particularities of the wild populations seem to be maintained by the selective pressure from environmental conditions, resulting in a homeostatic focusing of the best fitted average genotype. Analysis within each group showed that variations between strains were in most cases poorly or not correlated, so that partial or total genetic independence between the various traits measured seems likely.