An R factor for ampicillin and streptomycin resistance (AS) was identified in Salmonella enteritidis. The AS factor transferred freely to Escherichia coli K12, but only two of 260 K12(AS) clones from this cross would transfer AS to S. typhimurium, although all lines tested transferred it to S. enteritidis and K12. A study of one of the two exceptional lines“ revealed that it also transferred AS to S. paratyphi B, S. thompson and S. anatum. The R factor maintained its transferability when cycled between these serotypes and K12. Transfer to S. enteritidis, however, resulted in loss of the ability of AS to transfer to the heterologous serotypes, that is, it apparently became host specific for S. enteritidis. S. paratyphi B and S. anatum also imposed host specificity on AS, but S. typhimurium and S. thompson did not. The R factor would always enter S. enteritidis, whatever its previous salmonella host but, once it had done so, it became specific for S. enteritidis. AS could always transfer to K12, which did not seem to modify its host range. These phenomena are most easily explained by analogy with host-controlled modification of phage. Their possible significance in relation to apparent host specificity of R factors is discussed.

A selection of fi+ resistance factors and transfer factors, when introduced into K 12F+, showed a range of inhibitory activity of lysis by the male-specific phage μ 2. This range can be used to subdivide the fi+ factors into fi+1, fi+2, fi+3 and fi+4 classes, according to the degree of inhibition of μ 2 lysis. To this subdivision can be added restriction of the ‘female-specific’ phage φ 2.

Introduction of all the fi+ factors tested into K 12 HfrH totally inhibited lysis by μ 2 in spot tests, but with two fi+1 and one fi+2 factors visible lysis was obtained in agar-layer tests. These three factors caused least inhibition of transfer of pro by HfrH. It can be assumed that both tests reflect lower inhibition of sex fimbrial formation by these fi+ factors than by the remainder.

The fi− factors, when introduced into K 12, can be subdivided on the basis of restrictive effects on phage φ 2.

These effects can be added to phage restriction in the salmonellae for the purposes of further classification of the transfer factors and R-factors.

A comparison was made of the amount of penicillinase produced by strains of Staphylococcus aureus carrying chromosomal penicillinase genes, plasmid penicillinase genes and strains carrying both chromosomal and plasmid genes. Plasmid genes were found to produce at least twice as much enzyme as homologous chromosomal genes. The results suggest that there are, on average, two plasmids per chromosome in exponentially growing cells.

Two inducers were used in these experiments—methicillin and 2-(2′-carboxyphenyl)-benzoyl-6-aminopenicillanicacid (CBAP). The latter proved a more efficient inducer than methicillin and this difference was most apparent when more than one i+ gene was present, which suggests that the efficiency of induction reflects the ability of the inducer to react with represser.

Chromosome 5R of Secale cereale var. Imperial has been substituted for chromosomes 5A, 5B and 5D of hexaploid wheat. There is evidence of compensation, in terms of vigour, in all substitution lines. Chromosome 5R rectifies the male sterility of nullisomics 5A and 5D; however, it is unable to circumvent the meiotic irregularity and male sterility of 5B-deficient stocks. Another instance of wheat-rye chromosome pairing, in the absence of chromosome 5B, is reported. The data reported here demonstrate that the long arm of 5R is the more essential arm and indicate that chromosome 5R is homoeologously related to the group 5 chromosomes of wheat.

Mutants of Hfr strains of Escherichia coli K12 defective in conjugation owing to failure to produce sex pili were isolated by resistance to F-specific phage. Tests of the ability of six de-repressed F-like R factors and four de-repressed I-like R factors to restore Hfr donor behaviour to these mutants indicated that there were several ways in which such restoration could occur. Of a total of 26 defective mutants, 12 were restored by F-like R factors. In these twelve, the function of the integrated F factor was evidently restored, because the sex pili contained F pilin subunits, distinguishable from the R pilin by serological tests. In contrast, among the four I-like R factors, only two were effective, butin all 26 defective mutants; the restored Hfr bacteria produced only I-like pili. The I-like sex factors, in restoring Hfr donor behaviour, did not therefore act by complementing the defective F.

Plants of two rye genotypes, one highly inbred, the other not, were grown with varying amounts of available mineral phosphate. In two experiments, one using culture solutions, the other a field experiment, the results show an effect of phosphate treatments on mean chiasma frequency at first metaphase in meiocytes. Plants given increased amounts of phosphate showed an increase in chiasma frequency. A similar effect of phosphate on chromosome size and mass at mitotic metaphase in meristems is known and there may be a direct link between chromosome size and chiasma frequency.

Following an experiment in which mice had been selected for the length of the mitochondrial section (midpiece) of their sperm tails, an attempt has been made to determine experimentally the adaptive significance of this character; this was by the artificial insemination of mixtures of selected and unselected (control) spermatozoa, and the subsequent disclosure of their competitive fertilizing ability in the paternity of the offspring. After the birth of nearly 500 offspring, there was no indication that the control cells were—by this criterion—functionally superior. From the discrepancy between the pairs of males sampled, however, it is possible that other biological or technical factors have been important.

The interrelationship between an F prime and an F-like or I-like R factor was studied in Escherichia coli K 12 strains carrying both. The donorability of such strains suggested that these plasmids were transferred independently by their own transfer systems. Plaque formation by several male- and female-specific phage was tested; the inhibition by Flac of several female-specific phage was not affected by an F-like R factor. Neither did an R factor affect the incompatibility between Flac and Fhis. However, the surface exclusion characteristic of Flac was lost in the presence of an F-like R factor. This is probably closely related to the simultaneous loss of donorability, the F-pilus, and the f+ antigen: all may be the result of the inhibition by an R product of the formation or activity of a single F product.

The b locus of Ustilago maydis is both an incompatibility locus and a pathogenicity locus. There are several different b factors, and they are randomly distributed and equally frequent in the natural population. A sample of 62 lines contained 18 b groups and permitted an estimate of no more than 25 b groups in the natural population. All lines within the same b group are indistinguishable in their mating behaviour. No linkage was detected between b and any of the four biochemical markers tested. No intrafactor recombinants at b were found among more than 6700 progeny. Differences in degree of pathogenicity between solopathogenic diploids and comparable haploid by haploid crosses were detected.

Single ascospore cultures from triploids were screened as a potential source of disomic tester stocks for the purpose of genetical mapping of chromosomes in yeast. A high tolerance of aneuploidy was found, and strains disomic for only one chromosome were rare. There was a high occurrence of strains disomic at specific chromosomes and in certain multiple combinations of disomic chromosomes.

The colour difference (crimson and yellow) in Kerria lacca (Kerr) was used to test the ‘Lecanoid’ system of chromosome behaviour proposed on cytological evidence. The colour strains bred true for colour. Reciprocal matings between the colour strains produced only crimson sons and daughters, confirming that the colour difference is not sex-linked and that the male is somatically a diploid. The phenotypes of the F2 and the backcross generations differed according to the heterozygosity and the direction of cross of the F1 parents, since heterozygous females produced two kinds of gamete but heterozygous males produced only the maternal kind. Thus, while elimination of the paternal gene and hence chromosome was confirmed during the formation of male sex-cells, demonstration of somatic diploidy of the male is not compatible with a ‘Lecanoid’ system in K. lacca. Instead, the available cytogenetic evidence is suggestive of a chromosome system with heterochromatization and elimination of the paternal chromosome-set confined to the male germ line.

By analogy with the situation in coccids it is suggested that in mammalian XO embryos the single X turns heterochromatic in some cells, but that such a change does not result in cell death because the X then reverts back to an euchromatic and active state. This testable alternative to the Gartler–Sparkes hypothesis would imply that the anomalies of the XO Turner syndrome are largely due to imbalance of sex-linked genes rather than developmental damage resulting from cell death and that mammalian X inactivation might become reversed in response to special developmental needs.

Klebsiella aerogenes strain V9A carries determinants AK and TK, giving resistance to ampicillin and tetracycline, and a plasmid FKlac, but no active sex factor. F− and I-type sex factors were able to transfer TK from V9A to Escherichia coli K12 and between strains of K12, and TK behaved as a separate plasmid with its own replicon. AK could not be transferred, except possibly by a sex factor carrying its own A determinant, but the evidence for such transfer was inconclusive. It is suggested that AK is either a chromosomal gene or is in a plasmid with a cell attachment in Klebsiella not represented in E. coli K12. AK produces a β-lactamase.

In the present study, skin fibroblasts and leukocyte cultures from patients with acute infectious mononucleosis were examined to determine if any in vitro marker chromosomes could be detected. No alterations in chromosomes could be found in fibroblasts either from the region of the typical rash or from other areas of skin. This would tend to suggest but does not necessarily prove that this tissue was not extensively involved in the disease process.

When a determinant for neomycin-kanamycin resistance (K) was transferred by an F-lac factor into Salmonella typhimurium, the resulting KF-lac strain was sensitive to the male-specific phage μ2 and F-lac was derepressed. However, F-lac alone is repressed in S. typhimurium. When kanamycin resistance is spontaneously lost from S. typhimurium KF-lac an element persists which derepresses F-lac in S. typhimurium. The results are consistent with the hypothesis that a locus der, for derepression of F-lac in S. typhimurium, lies on the K plasmid. The R factor Rldrd19 is derepressed in K12 but is repressed in S. typhimurium. It also is derepressed by der. In contrast to F-lac and R1, another R factor, R136drdH8, is derepressed in both K12 and S. typhimurium, so that the intervention of der is unnecessary for its derepression in the salmonella host.

Anomalous frequencies of transferrin phenotypes were found among the offspring of matings between deer mice heterozygous for the two forms of transferrin most common in natural populations. Matings involving other phenotypes produced offspring in the expected phenotypic frequencies. Selective penetration by facility, occurring only in the reproductive tract of the heterozygous females, appears to provide the most satisfactory explanation for the observations.