Trimethoprim-resistant mutants of E. coli K12 have been isolated by-serial subculture in progressively higher concentrations of trimethoprim. High-level resistance depends on the accumulation of several mutational changes. Transduction with bacteriophage P1 has shown that all the mutations involved in resistance are associated with a locus, to be called tmr, between pyr A and pdxA and closely linked to pdxA. Resistance is accompanied by, and presumably due to, an increased activity of the target enzyme, dihydrofolate reductase. The tmr locus may include the structural gene for dihydrofolate reductase but the only mutations that have so far been observed are concerned with regulation.

The order of replication of a series of genes in Pseudomonas aeruginosa has been studied in synchronized cultures using a method based on the technique of sequential mutagenesis. This technique relies on the increased susceptibility of the replication point of the bacterial chromosome to mutagenesis by N-methyl-N′-nitro-N-nitrosoguanidine. The genes studied were those previously mapped by conjugation and whose order of replication had been studied by an investigation of gene frequencies in exponential populations. The results are consistent with the idea that there is two-way replication of the chromosome of P. aeruginosa starting at a point near trp-1 and arg-6. They also confirm that the two linkage groups which have been found by conjugation replicate at different times. If the assumption is made that there is only one chromosome in P. aeruginosa, the results can be used to show how the two linkage groups may possibly be joined together and the order is such that there would have to be two sites of attachment for the sex factor FP2.

Using the moments of gene frequencies, the drift variances of heterozygosity and genetic distance in transient states have been studied under the assumption that all mutations are selectively neutral. Interestingly, this approach provides a simple derivation of Stewart's formula for the variance of heterozygosity at steady state. The results obtained indicate that if all alleles in the initial population are equally frequent, the standard derivation of heterozygosity is very small and increases linearly with time in the early generations. On the other hand, if the initial allele frequencies deviate appreciably from equality, then the standard deviation in the early generations is much larger but increases linearly with the square root of time. Under certain conditions, the standard deviation of genetic distance also increases linearly with time. Numerical computations have shown that the standard deviations of heterozygosity and genetic distance relative to their means are so large that a large number of loci must be used in estimating the average heterozygosity and genetic distance per locus.

A considerable improvement in the selection of haploids in Aspergillus nidulans is described. Haploidization is used to assign genes to linkage groups (chromosomes) in fungi. The differential effect of a polyene antibiotic, N-glycosyl-polifungin, in respect to haploids and diploids, was utilized. In minimal medium only diploids (constructed from a ‘Master Strain’ and the investigated strain), which are fully heterozygous and thus have no growth requirements, germinate and are killed by the antibiotic. However, auxotrophic haploids, either induced or spontaneously formed, survive. These can then be used for linkage studies.

The first part of the paper provides strong supportive evidence for the previous findings (Cummins & Day, 1973; Day & Cummins, 1973) that the two alleles of the mating-type locus of the basidiomycete Ustilago violacea have different periods of inducibility during a cell cycle, and that the cell cycle characteristics of each allele are maintained in freshly isolated diploids. This difference in temporal properties of the alleles appears to be the basis of the dominance of allele a2 as it is inducible during a phase of the cell cycle when allele a1 is non-inducible. During G1 both alleles appear to be inducible and apparently ‘neutralize’ each other so that the cell cannot mate.

The second part of the paper provides evidence for a unique genetic control mechanism. The evidence suggests that the period of cell cycle inducibility of a locus governing a morphogenetic pathway may be regulated by a separate control gene the cc locus, with two known alleles ccstr(a stringent or restricted period of inducibility) and ccrel (a relaxed or non-restricted period of inducibility). This hypothesis stems from analysis of a diploid that was a1· ccstr/a2· ccrel and showed dominance of allele a2 during the S and G2 phases when freshly isolated, but which became incapable of mating after a period of subculturing. Analysis of haploids derived from this diploid strain showed that both mating-type alleles were functional but that it was now homozygous for ccstr, i.e. of genotype a1· ccstr/a2·ccstr· Thus the temporal and functional aspects of the mating type alleles are determined by different loci. It is postulated that cell cycle control loci may be widespread and serve to regulate the action of genes concerned with morphogenesis in relation to other cell cycle events.

This paper describes the results of an experiment to measure the effect on mean population fitness of recombination in the second chromosome of Drosophila melanogaster. There was a small and non-significant effect of recombination in lowering egg-to-adult viability of heterozygotes for wild-type chromosomes. A large (7%) and significant effect of recombinant chromosomes on the fecundity of Cy female carriers was detected.

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.

The genetic relationship between germspores and parental mycelia has been investigated in crosses involving multiply-marked heterokaryons and in simple difactor crosses.

It is concluded that for each zygospore the number of different diploid nuclei undergoing meiosis is often more than one, but generally not more than two. The meiotic products divide to produce the nuclei in the usually uninucleate germspore primordia and then continue dividing to produce germspores. Unmated nuclei do not pass directly from parents to progeny.

Each germsporangium contains differing numbers of viable germspores, with an average of about 7000; different genotypes are represented by varied numbers of germspores and often expected genotypes are missing. When the results of several germsporangia are added together or germspore stocks from many germsporangia are analysed, reciprocal genotypes appear in equal numbers and reliable recombination frequencies may be calculated.

Eleven mutants of R-factor R57 have been isolated which show constitutive expression of resistance to tetracycline (Tc). These derepressed (Tdr) mutants all gave a much greater resistance to Tc and to its analogue, minocycline, than could be obtained by optimal induction of cells carrying the wild-type (T+) determinant. Cells carrying each of the Tdr mutants together with T+ of either R6-S or of a plasmid found in Escherichia coli mi19 showed inducible Tc resistance, indicating that the Tdr mutants were all recessive, i.e. of repressor-negative type. Tdr1 was not recessive to the T-determinant of RP1, suggesting that the repressor gene products of the T-determinants in R57 and RP1 have different specificities.

A diffusion model was developed to investigate the effect of a mutant substitution by natural selection on heterozygosity at a linked neutral locus. Using this theory, we made extensive numerical analyses to compute the expected total heterozygosity (i.e. the sum of the fraction of heterozygotes over all generations until fixation or loss) at the neutral locus. It was shown that the hitch-hiking effect is generally unimportant as a mechanism for reducing heterozygosity. The effect becomes significant only when the recombination fraction between the selected and the neutral marker loci is smaller than the selection coefficient. In order to check the validity of the mathematical theory, Monte Carlo experiments were performed, and the results were in agreement. It has been suggested that linkage is important only in transient small populations such as at the time of speciation.

The R factor, R144drd3, mobilized chromosomal genes in Klebsiella pneumoniae strain M5a1 at similar frequencies to those observed for this R factor in Escherichia coli K12. In a derivative K. pneumoniae donor, strain HF3, R144 persisted in the autonomous state but now gave rise to polarized transfer of the chromosome in the order O his…arg 2…leu…trp. R144drd3 was unstable in K. pneumoniae M5a1; after successive subculture drug resistance, colicinogeny and transmissibility were lost. In strain HF3, transmissibility was preferentially lost, but determinants for drug resistance, colicinogeny and superinfection immunity were retained; 10–11 copies per chromosome of R144 were present in log phase cultures of both strains.