1. A series of experiments have been carried out with Drosophila melanogaster in order to improve further a population, highly selected for a quantitative character, by introducing genetic material from an inferior population. The effects of selecting the latter before crossing to the selected line, of waiting after crossing before restarting selection and of varying the intensity of selection after crossing have been studied. The inferior population used was the large random breeding population from which the selected line had been produced by selecting downwards for sternopleural bristles.

2. There proved to be some incompatibility between the conditions necessary to achieve the two criteria of success—the time to surpass the selected line and the extent by which it was eventually exceeded. The lines which surpassed it soonest had not been selected before crossing and had been selected intensely immediately afterwards. Those which surpassed it most had had the greatest amount of selection before crossing followed by the most intense selection afterwards.

3. Crossing-over did not appear to be a limiting factor after crossing. Neither a period of relaxation after crossing nor a lower intensity of selection after crossing increased the chance of success.

4. Genetic analyses were made of several lines which had exceeded the original selected line. In separate lines, second, third and fourth chromosomes were found which were superior to the chromosomes of the selected line. This and other evidence shows that the limits in such lines with a small number of parents are not in any way absolute but artefacts of the selection programme. The extreme third and fourth chromosomes were both recessive to that of the selected line in the effect on bristle score.

5. A theoretical discussion of such selection programmes is given, assuming that the character is controlled by independently segregating loci. It is shown that, if two alleles at different loci have an equal but small chance of fixation in the initial selection, one because it is rare and the other because, though frequent, it has little effect on the character, such a programme will tend to pick up the former rather than the latter. Of two such rare alleles with an equal initial chance of fixation, the one additive and the other recessive, the chance of fixation of the latter will be greater in a crossing programme.

1. The response to selection for increased number of bristles on one abdominal segment was studied over 12 generations using a factorial design of three population sizes (10, 20 and 40 pairs of parents) and five selection intensities (10, 20, 40, 80% and controls).

2. The responses on the average agreed well with those expected from the estimated base population heritability, but individual replicates diverged considerably.

3. Larger populations tended to give greater response to selection, due mainly to larger realized heritabilities.

4. There was no consistent effect of selection intensity on realized heritability.

5. For populations with the same number of individuals scored, less intense selection gave greater realized heritabilities.

1. An experimental evaluation of Robertson's (1960) theory of limits in artificial selection was attempted. A number of lines from the Canberra base population were selected for abdominal bristle number over 50 generations with population sizes of 10, 20, and 40 pairs of parents and selection intensities of 10, 20 and 40% as well as unselected controls.

2. In general, the total response obtained increased with an increase in Nī (product of population size and standardized selection differential).

3. Thus, total response increased with increase in the number of individuals scored, or, for a fixed number of parents, increase in selection intensity increased both rates of response per generation and total response.

4. But for the same total number scored, the response increased as selection intensity decreased. However, the proportion selected had only a small effect as compared with that of the total number scored.

5. Sublines in which the population size was reduced after 16 generations of selection but with the selection intensity kept constant, immediately fell behind their parent lines and gave much less response.

6. Agreement between replicate lines was generally poor, particularly for the 10- and 20-pair lines.

7. Patterns of response in individual lines were frequently irregular and ‘waves of response’ were not uncommon.

8. The results are discussed in terms of several theoretical models of selection limits. In general, agreement with these models was poor, as much of the response appeared to be due to a few genes (or gene combinations) with large effects on bristle number.

1. In order to determine the nature of the genetic variation causing the response to selection in our lines (Jones et al. 1968), various analyses were performed.

2. There was no consistent change in heritability, estimated from half-sib correlation or from the phenotypic correlation between the bristle numbers of two abdominal segments, after 10 to 20 generations of selection.

3. Realized heritabilities over the 10 generations subsequent to the heritability estimations were less than in the early generations but bore little relationship to the estimated values.

4. Six lines contained recessive lethals with appreciable effects on bristle number as indicated by high variances, large regression on relaxation and large response to reverse selection.

5. Reverse selection lines taken from the main lines at generation 40 indicated that genetic variation was still present in almost all of the lines. Only one line failed to respond to further forward or to reverse selection.

6. The three highest lines were crossed in pairs and reselected. Two of the three possible crosses gave further response, exceeding the higher parent after one and three generations, but the other cross failed to pass the highest parent line.

7. A combination of large gene effects, linkage, and gene interaction effects have been suggested as the cause of irregularities in the response of the lines. It has not been possible to determine the relative importance of these effects.

Xasta flies appear to segregate five types of gametes in unequal numbers, namely two which contain both or neither of the chromosomes affected by the translocation and inversions, two further classes which contain one affected and one unaffected chromosome, and finally the remainder which have an unbalanced chromosomal content. These conditions are necessary to fit the results observed in crosses involving the Xasta stock. Xasta exhibits balanced polymorphism under crowded conditions and this may be due to the production of toxic substances by Xa larvae which delay the development of wild type-larvae.

A monosome was identified as chromosome 7 of the A genome of the tetraploid cottons by its linkage with the Lc1yg2R2 markers. This makes eight chromosomes of the tetraploids now identified by monosomes. For over a decade the N locus has been considered to be linked with the Lc1yg2R2 loci. Monosomic analysis, however, reveals that N is on a separate chromosome. Studies with a telocentric of chromosome 7 show that the Lc1yg2R2 loci are located in the long arm. It is suggested that the R2 locus rather than the Lc1 locus is in the proximal position. These findings are discussed in relation to data, particularly that involving the linkage group, which supposedly illustrate the compensatory recombination phenomenon in cotton.

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.

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.

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.