A major difference in developmental stability has been demonstrated between two populations produced by artificial selection for supernumerary scutellar bristles. The test system involves the substitution of an X-ray induced partial revertant of sc1 for the wild-type allele at the scute locus, enabling direct comparisons to be made of the degree of canalization at the wild-type level of expression of the character. One population is comparable with the unselected Canberra stock in stability, though it differs appreciably in mean bristle number: the other population shows a marked reduction in the level of regulation of bristle number variability. The alleles responsible for the reduced level of canalization are rare in the base population, and are of particular importance in the determination of limits to directional selection. Their effects on developmental stability have been shown to depend on the activity of the allele at the scute locus.

The genetical and developmental aspects of six dumpy mutants of Drosophila melanogaster have been investigated. The mutants obm, lm and olv (possibly alleles), lv, lv1 and lvI (possibly alleles) were known to be lethal when homozygous. Previously the lethal effect has been treated as a uniform effect. However, the lethal stage is not the same for all homozygotes, being egg/larval (E/L) for the three lv alleles, egg (E) for olv, larval/larval ecdysis (L/L) for lm and E/L and larval (L) for obm. Not all heterozygous combinations are lethal, i.e. obm/lm and obm/lvI are not lethal. Phenotypically the lethal heterozygotes fall into two patterns: (i) combinations not involving the allele obm and (ii) combinations involving obm. In the former, the mutant with the developmentally later expression is ‘dominant’ to the mutant with the developmentally earlier expression. In the latter, the genotypes manifest different proportions of individuals at the lethal stages E, E/L and L. Previous observations suggested that the lethality of the homozygote obm/obm was associated with the presence of an independent lethal in the stock. Observations presented here suggest that the lethality is a function of the obm allele itself. Complementation between some of the lethal mutants is not in accordance with the general rule for dumpy that compounds manifest the traits they have in common.

Studies on the growth kinetics of R+ and R− cultures of Escherichia coli in the presence of nalidixic acid (NA), acriflavine (AF) and kanamycin (Kan) showed that each drug caused a decline in viability of both R+ and R− cells for several hours. During further incubation the viability rose rapidly for the R+ cultures, but either rose less rapidly (AF and Kan) or continued to decline (NA) for R− cultures. Distribution curves of the resistances of individual clones of R+ and R− bacteria to atabrine, NA, AF and Kan suggested that the presence of an R factor in the host bacterium increased its mutation rate to resistance to these drugs: this would account for the more rapid growth rate of R+ cells during the latter stages of incubation in their presence. The mutations causing increased resistance to NA and to Kan were located in the bacterial chromosome and not in the R factor.

By the use of appropriate strains of Escherichia coli, Shigella flexneri and Salmonella typhimurium with and without an R factor, R100, the mechanism of ‘curing’ of R factor by acridine dyes was examined. This R factor was shown to confer increased sensitivity to acriflavine upon the host cells. E. coli strain W-3630, once infected with R100, has never been observed to segregate R− cells. When mixtures of R+ and R− cells of this strain were grown in acriflavine broth, the proportion of R− cells increased and was also correlated with the proportion in the initial inoculum. Other bacterial strains carrying R100 segregate R~ cells spontaneously. Growth tests starting with varying proportion of R+ and R− cells of these strains in acriflavine broth also gave a marked correlation between the initial and final proportions of R− cells, and indicated that the main cause of ‘curing’ the R factor was the selective enrichment of R− segregants present in the initial inocula or arising spontaneously during growth of the R+ culture. These results suggest that the mechanisms underlying the ‘curing’ of F and R factors are different. Tests with several acridine dyes gave results similar to those with acriflavine.

The sul gene of B/rw (Witkin) and 26 independently isolated sul mutants were examined for their suppression of radiation sensitivity. The suppressed lon gene was demonstrated by transducing it into a K12 proC recipient. The sul gene of B/rw and all but one of the B/r mutants was found to be linked to a gene controlling azide resistance. Transduction data established the order of markers as leu azi mutT sul. One B/r mutant was found not transducible with azide resistance, sul mutations were found to suppress the capsular polysaccharide production of lon strains.

In a population of Drosophila melanogaster started from an inbred wild-type strain the recessive second chromosome lethal studied had shown overdominance which after many generations was lost. In the present study the persistence of this lethal was tested in three series each of five populations. The genetic backgrounds of the different series of populations were obtained from (a) the inbred strain, (b) the above original population after the overdominance had been lost, and (c) a population started from the same inbred strain and where another lethal had shown overdominance which subsequently had been lost. The lethal was overdominant in the (a) background but detrimental to the heterozygous carriers on the other backgrounds. The detrimental effect of the lethal was stronger in the (b) background than in the (c) background. The varying behaviour of the lethal is possibly due to different adapted background genotypes and/or different degrees of heterozygosity of the gene pools.

The effect of dietary supplements of individual l-amino acids on the expression of the eyegone and eyelessK mutants of Drosophila melanogaster are compared. In both mutants, eye size is reduced by excess levels of tryptophan, phenylalanine and methionine, and in each case the effects are independent of metabolic competition for pyridoxal phosphate. A dietary interaction involving methionine and UNA can be demonstrated in the eyK strain, but the mechanism of action of this amino acid is obscure. Tryptophan metabolism is examined in detail. Although both tryptamine and serotonin have significant effects, the action of tryptophan on eye development is largely independent of its metabolic products. Conversely, the effect of dietary supplements of certain catecholamines is consistent with the action of phenylalanine. The action of certain metabolic inhibitors provides additional support for the suggestion that the catecholamines have an important effect on morphogenesis in the eye imaginai disks. Eye development is also affected by increasing concentrations of γ-amino-butyric acid, and this, taken together with the effect of the catecholamines and indolalkylamines, suggests that physiological control of the action of the mutant genes on eye development involves a group of compounds characteristically associated with nervous tissue. Eye development in the eyK strain may be influenced by the availability of acetyl CoA, which would be expected to affect acetylcholine biosynthesis. Possible mechanisms of action of the effective dietary treatments are discussed, together with a tentative hypothesis regarding the mode of action of the mutant genes on eye development.

Hybrids between four diploid species of Aegilops and species of Secale were obtained by using embryo culture. There was a marked incompatibility in the crosses between Secale species and each of the four species in Section Sitopsis of Aegilops and Ae. mutica. It is suggested that this genetic incompatibility with Secale species is an additional similarity between these species of Aegilops and the diploid species of Triticum.

Most chromosomes of Aegilops (A) and Secale (S) are univalent during meta-anaphase of meiosis in these hybrids, but some appeared to associate and others to pair as apparently normal chiasmate bivalents. Analysis of non-chiasmate and chiasmate associations showed that the frequencies of autosyndetic (AA and SS) and allosyndetic (AS) associations fitted the 3AA: 7AS: 3SS ratio expected if association and pairing is at random. Any deviations from random involved a deficiency rather than an excess of Aegilops-Secale pairing. There is no evidence that the chromosomes of Secale are homologous with those of Ae. caudata, Ae. comosa and Ae. umbel-lulata, and it is suggested that the genome of Secale species does not show any homology with the genomes of the genera Aegilops. This does not preclude the presence of homologous segments. It is suggested that the possibility of random association of chromosomes should be considered when occasional pairing in interspecific hybrids is analysed, and that identification of chromosomes and recognition of chiasmata are required. The possibilities of chiasmata between non-homologous chromosomes, of a genetic mechanism in rye which suppresses the pairing of homoeologous chromosomes, and of other factors causing asynapsis and pseudo-synapsis between genetically similar chromosomes are discussed.

The sequence of development of salivary gland chromosome puffs in Drosophila melanogaster cultured on live yeast and RNA-deficient medium has been studied in late larval and prepupal stages. Differences in puffing pattern between individuals cultured in the two environments are almost entirely due to shifts in the relative timing of puff development at specific chromosome sites. Detailed studies on the right arm of the second chromosome indicate that, at certain sites, puffing activity spreads to adjacent lettered subdivisions of the chromosome. There are differences in the extent of lateral spreading of activity into adjacent regions in the two environments which may be due to clustering of functionally related genes.

Fifteen mutant strains of Chlamydomonas reinhardi were isolated which showed defects in some aspect of the process of cell-wall formation. Genetic analyses indicated that most of the mutations were due to single gene changes; two were anomalous in that non-Mendelian segregations were obtained on crossing with other genotypes, and on selfing they frequently gave rise to wild-type phenotypes.

Occasional somatic revertants were also obtained from these two strains. On the basis of these analyses it is suggested that there are two levels of control operating in the process of cell wall biogenesis - one concerned with subunit production at the nuclear level and another, possibly concerned with three-dimensional organization, at another level. Electron-microscope analyses of the different mutants showed the mutants to be divided into three main categories: those in which the wall was formed but was not attached to the plasma membrane, those in which the wall was attached to the membrane, and those in which very little wall was formed. In the last class in particular, vesicles containing wall precursors were clearly visible, and were shed through the plasma membrane into the medium.

The recombination between the methA alleles of Aspergillus nidulans is very strongly polarized. The mutants can be mapped in respect to each other and in respect to the flanking markers, both on the basis of the relative frequency of one of the P classes and one of the R classes. In all crosses it is the distal mutant which converts predominantly.

In one combination of the mutants crossed the low temperature at which meiosis proceeded increased recombination frequency and modified the recombination pattern.

The duration of meiosis and its stages has been estimated in Hordeum vulgare variety Sultan grown at 20·0 °C with continuous illumination. The results are in good agreement with a previous estimate of the duration of meiosis and some of its stages in barley. The duration in barley does not differ greatly from that predicted from the previously calculated regression of DNA amount on meiotic duration in diploid plant species. As in other cereal species, meiosis is comparatively short.

Some quantitative implications of the gene conversion models of Holliday and Whitehouse & Hastings are discussed. These models predict two kinds of mispairing when a heterozygous mutant is in hybrid DNA. It is pointed out that the latter model, in contrast to statements in the literature, does not require identical repair rates in both kinds of mispairing.

Furthermore, some peculiarities are discussed which result if, among the corrected fractions of hybrid sites, the frequency of repair to wild-type is the same in both kinds of mispairing (r = s). In the Holliday model a minimum results under this condition for the frequency of asci which show a normal 4:4 segregation, but which have originated by repair from meioses with hybrid DNA.

Bacterial hybrids were produced to contain genetic material of Salmonella typhimurium, Escherichia coli, S. montevideo and S. abony origins. Analyses by transduction provide evidence that 6% of the original typhimurium genome has been replaced in the production of these hybrids. Although a number of biosynthetic pathways are affected by this gene substitution, the growth rate of these hybrids in minimal medium is unchanged. Supporting evidence for the close relatedness between S. typhimurium and the other three species is not observed in recombination studies. Available results favour the concept that differences in base sequences are responsible for the low frequency of recombination obtained in heterologous crosses.

A strain of mice that had ceased to respond to selection for high litter size was inbred with continued selection. Depression of the mean proved the existence of residual genetic variance. Four lines survived the inbreeding, and one reached 20 generations with a mean equal to the original strain, thus disproving overdominance as a major cause of the residual variation. The four selected inbred lines were crossed and a new strain derived from the cross was maintained in parallel with the original strain. The new strain showed an improvement of 1·5 mice per litter over the original strain. Thus selection with inbreeding was able to achieve an advance beyond the limit attained by the original selection.

The hypothesis that the residual variation was due to genes with simple dominance was tested by seeing if it could account for the observations with reasonable values of the relevant parameters. The improvement made by the inbreeding and crossing required the elimination of about 30 recessive genes with effects (homozygote difference) of 0·5 phenotypic standard deviations and gene frequencies of 0·2. Consideration of the mean levels of the selected inbred lines in conjunction with the rate of depression found on inbreeding without selection showed that the selection with inbreeding had eliminated about 75% of the segregating reces-sives. The number of genes contributing to the residual variance was therefore about 40. The additive variance generated by these genes was just consistent with the estimate of zero from the realized heritability. Consideration of the original selection showed that about half the genes could have been still segregating when the response ceased. The hypothesis therefore requires the number of genes in the base population to have been about 80. The number of genes required, though large, does not seem impossible, and the hypothesis of genes with simple dominance can account for all the observations.

Using polarography the uptake of oxygen by intact and homogenized mycelium of a wild-type strain and strains of slow-growing non-perithecial (sgp) mutants was compared. It was found that whilst the oxygen uptake of intact, wild-type mycelium increased on the addition of glucose or succinate as substrate, uptake of oxygen by mutant mycelium increased only when glucose was used as substrate and was unaffected by succinate. When, however, homogenates of mutant mycelium were used oxidation of the succinate occurred. It was concluded that the inability of the mutants to utilize succinate was due to their impaired ability to take up the compound across the hyphal wall and this was confirmed using radioactively labelled substrates. It is tentatively suggested that the abnormal growth of the sgp mutants on glucose medium and their impaired permeability to tricarboxylic acid cycle intermediates may be due to reduced availability of high energy compounds caused by a lesion in their oxidative phos-phorylation system.

Analysis of a series of exceptional ry+ half-tetrads, produced in mass matings involving rosy mutant heterozygous half-tetrads, provides rigorous demonstration of the occurrence of non-reciprocal as well as reciprocal recombination events within the rosy cistron of Drosophila melanogaster. Inferences about allele recombination drawn from this and other studies in Drosophila provide a strong argument that gene conversion occurs as a regular event in higher eukaryotes.

A study of the structural and functional organization of the rudimentary (r: 1–54·5) locus of Drosophila melanogaster has demonstrated that alleles of this gene reside in a number of recombinationally separable sites, and display a complex pattern of interallelic interaction. Data relating to interallelic interaction have been utilized to construct a linear complementation map consisting of 7 complementation units and 16 complementation groups. Comparison of the genetic fine structure map and the complementation map shows that the two maps are approximately co-linear. Totally non-complementing alleles reside at both ends of the fine structure map. The r locus is best interpreted by the model of a single cistron whose product affects several distinct developmental processes and whose alleles display a complex pattern of interallelic complementation. Intragenic recombination within the r locus is accompanied by the appearance of parental and recombinant flanking marker classes not expected on the basis of reciprocal recombination. Studies with half-tetrads demonstrate that intragenic recombination can occur either by gene conversion or by a reciprocal exchange mechanism. The pattern of organization seen at the r locus is similar to patterns of organization found in work with fungal genes.

We found that la is located in linkage group XVIII. It is highly probable that la and tg are alleles, and closely, linked to Es-1. Mice of the genotype la/tg are abnormal, with clinical signs similar to tg, although more severe. They develop earliest signs at about 15 days of age, similar to la, are runted but fertile and can live for months. Clinical signs are ataixa, stiffness, retarded motor activity and intermittent focal seizures. The pathological basis for these symptoms is still elusive. The three types of mice, la/la, la/tg and tg/tg are thus distinct clinically, la/tg resembling in some respect either of the other two.

The usual systems of assay using techniques of chromosome substitution demonstrated in Drosophila and wheat are not applicable to barley (Hordeum vulgare). Chromosomal material for assay may, however, be substituted from one variety into another by using translocations to mark and restrict crossing over in the chromosomes to be transferred. This paper describes the isolation and assay of lines derived in this way.

Seven substitution lines derived from the donor variety Maris Badger and the recipient variety Mars have been scored for quantitative characters in two field trials.

The results indicate that variation in flowering time and other associated characters is largely determined by genes on chromosome 2 and that chromosome 4 is involved in the control of plant height.

The feasibility of the technique as a method of assaying the contributions of chromosomes to qualitative characters by substitution in a diploid is discussed.