1. The comparison of the dry weight of thin layer haploid and diploid colonies of A. niger on complete medium and complete medium supplemented with p-fluoro-phenylalanine led to the conclusion that there is a difference in growth rates of hyphae under these different conditions.

2. The growth curves of the same strains on both media were established. On complete medium, haploids and diploid show a growth rate increasing linearly for about 20 h after germination and reaching a maximum which is then maintained. On p-fluorophenylalanine, the haploids show a similar curve, although the maximum growth rate reached and maintained is about half that on complete medium; for the diploid, however, the maximum is less than the corresponding one in the haploid and, once this maximum has been reached, the growth rate goes down linearly to a very low value which is then maintained.

3. The cytological study of the hyphal tip cell showed, in the presence of the amino acid analogue, a reduction of the mean size of the diploid nuclei together with an increase of the number of nuclear fragments. This explains the growth rates observed and is accepted as a confirmation that p-fluorophenylalanine, by its action on the mitosis, favours chromosome losses which lead finally to the production of haploid nuclei.

Body size in Drosophila pseudoobscura is a continuously varying character with a high heritability; it is almost certainly related to fitness. Natural populations of D. pseudoobscura from Canada to Mexico have been sampled and found to vary geographically in body size. The geographic variation for the genes determining size is to some extent correlated with the physiographic division of the West. The populations from the Pacific coast have genetically smaller flies than do those from the interior provinces. Experimental populations derived from the samples of seven widely separated natural populations were crossed to yield F1 and F2 hybrid generations. Body size in the F1's varied irregularly, while the F2's showed a consistent ‘breakdown’, the F2's being significantly smaller than their F1 parents. The F1's were significantly less variable than their parents, while the F2's were significantly more variable than their parents of the F1 generation. The natural populations possess coadapted genetic systems, with genes mutually adjusted by selection for favorable interactions. Recombination disrupted the balanced genic complexes to give the F2 breakdown and the increased F2 variability. D. pseudoobscura differs from D. subobscura in showing the effects expected in crosses between coadapted systems. This species difference lends additional support to the hypothesis that the gene pools of these two successful species respond in different ways to environmental variation. The gene pool of D. pseudoobscura is flexible and changes readily, while that of D. subobscura is relatively rigid.

An additional Y-chromosome in either male or female did not have an influence on the mutagenic action of formaldehyde–food. It is concluded that the lack of response of female germs cells to this treatment is not due to lack of a Y-chromosome, and that the specific response of spermatocytes among germ cells is probably not due to the striking activity of the Y in these cells.

Kershner's (1949) observation that the sex-linked lethal frequency in X-rayed males was significantly reduced by an additional Y-chromosome could not be confirmed.

The first two steps towards increasing ampicillin resistance in Escherichia coli concern the genes ampA and ampB which are located at least 20 min from each other (Eriksson-Grennberg et al. 1965 and the two preceding papers). This paper describes a third class of ampA-containing mutants (designated ampAB) which are resistant to D, L-ampicillin concentrations of 100 μg/ml. When such as F-strain (D31) was crossed to different Hfr strains analysis of the trp+ and proB+ recombinants indicated that resistance genes were located between trp and proB. Altogether five classes of recombinants were produced but only the two genes ampA and ampB were recovered. One interpretation suggested is that the resistance of D31 is due to the presence of ampA and ampB and a chromosomal aberration by which ampA was moved to a position near ampB. It was possible to transduce both intermediate levels of resistance as well as the full resistance of an ampAB donor strain, but the strains produced were unstable. In crosses the presence of ampAB in the recipient reduced the number of recombinants by reducing the number of stable pairs. In an Hfr strain ampAB was shown to give rise to additional difficulties in establishing the cell contact during mating. Some ampicillin resistant mutants also showed decreased ability to propagate the RNA phage MS2.

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.

Virus-like particles can be seen in sections of kappa of stocks 7 and 562. The virus-like particles are specifically associated with R bodies and are rare or absent in kappa particles lacking R bodies. This observation clearly links the virus-like particles with kappa's toxic activity; it also supports the suggestion that kappa is infected with a lysogenic virus whose induction results in the production of the R body and viruses. Many ‘unfilled’ virus-like particles are present in whole kappa particles. Wound R bodies of stock 7 are surrounded by a sheath, whose breakage may provide the mechanism which causes the unrolling of the R body. One edge of the R body ribbon stains more intensely than the other.

UV induces lesions in DNA which lead to the death of cells, mutation and, in yeast, intragenic and intergenic mitotic recombination. We have investigated the interaction of two post-treatments, dark holding and photoreactivation, on the frequencies of these events. It was found that dark holding reduces cell death and intergenic recombinants, but causes an increase in intragenic recombination frequency. Photoreactivation reduces cell death and intragenic recombination, but has no effect on intergenic recombination. After dark holding, photoreactivation has no further effect on cell survival or intergenic recombination, but may reduce the frequency of intragenic recombinants. After photoreactivation, dark holding still causes an increase in cell survival and the frequency of intragenic recombination, and reduces the frequency of intergenic recombination.

Three-point transduction analysis was employed to determine the sequence of four structural and two regulatory methionine genes (metA, B, F, H and metI, J, respectively) on two apparently adjacent transduction fragments in Salmonella typhimurium. These fragments were subsequently orientated with respect to each other and the rest of the linkage map by the analysis of recombinants obtained in conjugation experiments. The following clockwise sequence of genes is proposed: -metJ-metB-metF-argF-thi-purD-metA-metI-metH-. The bearing of these results on the regulation of the metB–F and metA–H clusters is discussed.

1. Triticum aestivum ssp. vulgare variety Chinese Spring (2n = 6x = 42) is susceptible to yellow rust caused by Puccinia striiformis while the wild annual grass Aegilops comosa (2n = 14) is resistant to all the physiologic races for which it has been tested.

2. By a backcrossing programme initiated from Chinese Spring × Ae. comosa hybrids, using Chinese Spring as the recurrent parent, a line was isolated with a single chromosome of Ae. comosa, determining rust resistance, added to the full complement of Chinese Spring.

3. The alien chromosome substituted with good genetic compensation only for the chromosomes of homoeologous group 2 of Chinese Spring. This demonstrated that the chromosome determining rust resistance is in homoeologous group 2. It was designated 2M since Ae. comosa has the M genome.

4. In order to induce recombination between 2M and its wheat homoeologues, hybrids were made using Ae. speltoides which has the capacity to suppress the activity of chromosome 5B that normally prevents homoeologous synapsis. A backcrossing programme, using Chinese Spring as the recurrent parent, was reinitiated from the 29-chromosome hybrids carrying chromosome 2M and the haploid complements of Chinese Spring and Ae. speltoides.

5. Selection was practised for rust resistance and ultimately a resistant plant with 42 chromosomes, that formed 21 bivalents at meiosis, was isolated. This plant was heterozygous for a dominant rust resistance allele (Yr8) derived from Ae. comosa. Homozygotes were isolated in its progeny and in this way the rust resistant breeder's variety, Compair, was established.

6. Compair differs from Chinese Spring in its yellow rust resistance which was shown to be determined by a chromosome corresponding to 2D of Chinese Spring. This chromosome of Compair has the short arm, the centromere and a proximal segment of the long arm of chromosome 2M and a distal segment of the right arm of chromosome 2D. The modified chromosome, which is designated 2M/D, arose by homoeologous recombination in the Ae. speltoides hybrid or in the immediately succeeding backcross generations. Chromosome 2M/D carries the Yr8 gene in the proximal segment of the long arm derived from chromosome 2M.

7. In hybrids between Compair and standard wheat varieties, chromosome 2M/D pairs regularly with chromosome 2D so that regular segregation of Yr8 can be expected and Compair treated like any other parental variety in wheat hybridization programmes.

8. This work illustrates the way that homoeologous recombination can be induced and exploited both in cytogenetic analysis in wheat and in practical breeding work. The nature of the meiotic synapsis of chromosome 2M/D with its partial homologues raises questions concerning the means by which chromosome 5B influences the specificity of meiotic synapsis.

Plaque-forming transducing (pt) particles of bacteriophage ø 80 have been used to transduce genes governing tryptophan synthesis. Main interest was devoted to strains which carry deletions extending for varying distances into the tryptophan (trp) operon, covering trp A and ending in trp B, trp C or further. If the deletion does not over-reach trp C, infection by pt particles carrying the trp (CBA)+ segment gives rise to stable partial diploids which express the complete tryptophan independence. If the deletion is further extended, transductants are not entirely complemented and appear only on medium containing indole. If the recipients do not carry any prophage in the attachment site of ø 80 (att80) the indole-requiring transductants are very unstable; if the recipients do carry such prophage, even defective, all transductants are very stable. Stable indole-requiring diploids can be superinfected by other pt phages carrying the complementary genes of the tryptophan operon, giving rise to supertransduced indole-independent bacteria. Transduction to complete independence of tryptophan or any precursor has also been achieved in three steps. Supertransductants yield after u.v. induction pt phages carrying the initial trp segments and segregate rare cells with incomplete ability to synthesize tryptophan. They are therefore polylysogenic polyploid heterogenotes carrying overlapping segments of the trp operon linked to prophage genomes.

When recipients are carrying a prophage in att80, transductants appear at a low frequency. The reduction is particularly pronounced with ø 80 pt transducing segment trp (CBA)+ to immune bacteria.

Different types of chimerism result from double fertilization. Differences result from fertilization of first and second polar bodies and from early fusion of two zygotes. These three types of chimerism have different phenotypes or different proportions of possible phenotypes, and variation is greater in respect of gene loci close to the centromere. The phenotype frequencies in the different types of chimerism are tabulated and the phenotype frequencies expected in different mating types are shown. Studies of such cases offer the only method of autosomal gene localization relative to the centromere at present available in man.

A strain of Ustilago maydis has been isolated, after treatment with nitrosoguanidine, which lacks an extracellular endonuclease capable of causing two strand breaks in DNA. Genetic analysis suggests that the strain carries two mutations and is probably also deficient in a second nuclease.

Genotypic and phenotypic changes of HCM in P. aeruginosa are due to a variety of causes. In addition to interstrain genetic differences and the semi-permanent effects of growth at 43°C, mutations affecting both restriction and modification have been isolated by direct selection and through the pleiotropic effects of p–fluorophenylalanine resistance mutations. It is suggested that some of these changes affecting HCM may be taking place through alterations of the ribosomes.