Nuclear DNA syntheses have been measured microspectrophotometrically in course of meiotic nuclear development in Neurospora crassa. The purpose of the study has been to establish the timing of these DNA syntheses in relation to meiosis and in the formation of its final products in an ideal genetic material, where the products of the meiotic division can be recovered and identified. Spectrophotometric analysis has indicated that the last premeiotic DNA replication is completed before caryogamy; a postmeiotic DNA replication takes place at the tetrad of the nuclei level; and finally a second postmeiotic DNA replication occurs in the ascospores to result in eight binucleated ascospores. The timing of postmeiotic replication is compatible with the models explaining gene conversion through the postulated mechanisms involving heteroduplex formation.

Inositol independent (inl+) strains were obtained either as transformants following treatment of the inositol requiring (inl) strains of Neurospora crassa with the wild-type DNA or as revertants without any DNA treatment. A significant number of the inositol-independent transformants were also found to have acquired additional mutations called osmotics (os) which made them unable to grow on 1 m-NaCl medium. None of the inositol-independent revertants were found to possess such osmotic mutations and their growth remained unaffected by the presence of NaCl. Many of the osmotic mutants described here were found to be new alleles of the previously known os–1 mutation on the linkage group I of Neurospora crassa. The remainder were found to map at two new genetic loci designated as os-6 and os-7; these loci were found to be closely linked to os–1. Among the new osmotic mutants only os-1 and os-6 mutants showed intragenic complementation. The mechanism of DNA-induced mutation during transformation is discussed.

Plasmodium formation in the Myxomycete Physarum polycephalum normally involves fusion of haploid amoebae, carrying different alleles at the mating type (mt) locus, to give diploid plasmodia. Strains carrying the mth allele are capable of undergoing the amoebal–plasmodial transition with high efficiency within amoebal clones, resulting in the formation of haploid plasmodia. NMG mutagenesis of mth amoebae, followed by an enrichment procedure, was used to isolate mutants in which such clonal plasmodium formation was either delayed or absent. Thirteen mutants of the second type were analysed. Three of these were temperature-sensitive for plasmodium formation. All thirteen mutants were able to form diploid crossed plasmodia when mixed with a mt1 strain. Three new genes were identified and designated npfA, npfB and npfC. A mutant allele of npfA rendered clonal plasmodium formation temperature-sensitive, but did not prevent crossing at the non-permissive temperature with derived strains carrying the same mutant allele. No recombination was detected between npfB or npfC and mt, but npfA was unlinked to mt and a locus (apt-1) shown in a previous study to be involved in plasmodium formation. The genes npfB and npfC were distinguished by complementation analysis. Strains of the genotype npfB−; npfC+ behaved in the same way as strains carrying the mt2 allele. The nature of the mutants and the role of the mating-type locus in the initiation of plasmodium formation are discussed.

R388s is a mutant of the R factor, R388. Unlike its parent R388s does not confer trimethoprim resistance, though it does confer sulphonamide resistance and the ability to transfer. It is shown that when bacteria harbour R388s mutations to trimethoprim resistance occurred at a high frequency (3·5 × 10−4), whereas such mutations never occurred when the host lacked the R factor. Bacteria harbouring R388s did not mutate to resistance to any other antibiotics at a detectable frequency.

The expression of trimethoprim resistance in bacteria possessing R388s appears to require two genes, one located on the R factor and the other situated presumably on the chromosome. The mechanism of trimethoprim resistance in these bacteria is probably due to reduced permeability and is thus quite unlike the insusceptible target site mechanism of the parent R factor, R388.

The prophage of the phage B3 of P. aeruginosa exhibits superinfection inhibition towards certain R plasmids such as R18-1 or R18-3 as shown by a severe depression (c. 10−6) in the conjugal transfer of these plasmids into B3 lysogens. The transfer of other R plasmids into such lysogens and the transfer of R18-1 or R18-3 into bacteria lysogenic for unrelated phages was unimpaired. R18-1 or R18-3 was non-transducible into B3 lysogens although transducible into non-lysogens. The interaction appeared to be reciprocally expressed since B3 lysogenization of bacteria R+ for R18-1 or R18-3 was also reduced. Consistent with the failure of co-existence of the prophage and the plasmids was the observation that when the transconjugants from the cross R18-1 donor ×B3 lysogenic recipients were tested, transfer-deficient plasmid derivatives were encountered (about 40%). Other transconjugants were cured of prophage, or carried mutant B3 prophage with lytic, lysogenization or superinfection inhibition functions impaired.

The homothallic amoebal clones of Physarum polycephalum are of potential use in understanding the developmental genetics of this organism. Such an application requires that complementation and recombination analysis be possible between pairs of homothallic clones. This paper is a report of the formation of mixed plasmodia by pairs of homothallic amoebal clones. In order to detect such mixed plasmodia use was made of two marker genes involved in plasmodial fusion, fusA and fusB. Sporulation of a mixed plasmodium formed from two homothallic (delayed) amoebal clones yielded progeny amoebae which were genetically recombinant. It is deduced from the ratios of various genotypes in these progeny clones that the mixed plasmodium was diploid and that meiosis was associated with sporulation. There is therefore no impediment to the use of the homothallic strains for genetical analysis. The progeny amoebal clones were observed to be showing segregation for the characters homothallic (rapid) and homothallic (delayed). This observation, taken together with other related observations, suggests that the homothallic (delayed) character is produced by mutation of the homothallic (rapid) character. The rare plasmodia formed by a homothallic (delayed) amoebal clone are the result of reversion of this mutation. Amoebal clones of the homothallic (delayed) type are therefore developmental mutants unable to perform the differentiation from amoeba to plasmodium.

A total of 39 mutants at the grey-3, grey-4 and grey-5 spore colour loci in Sordaria brevicollis have been investigated for conversion pattern by crossing them with wild type and counting aberrant asci. Twenty-one of the mutants were obtained with ICR170 and all showed postmeiotic segregation only rarely (0–8% of the aberrant asci); two showed conversion predominantly to wild type (class A) and the other 19 predominantly to mutant (class B). Six mutants were obtained with ethylmethane sulphonate and one with nitrosoguanidine, and they all showed postmeiotic segregation frequently (14–54% of the aberrant asci) and conversion usually about equally frequently in each direction, though with considerable diversity between mutants (class C). Eleven UV-induced mutants comprised one of class B and ten of class C. There was considerable variation in aberrant ascus frequency between alleles, but conversion pattern seemed to be independent of this frequency.

The frequency of adjacent-2 disjunction in mice carrying the reciprocal translocation T(9; 17)138Ca was studied by mating together animals heterozygous for the translocation and carrying different recessive marker genes, using Tt for chromosome 17 and cwcw for chromosome 9. The proportion of marked young arising from adjacent-2 disjunction varied according to the markers carried in the two parents. When the female carried Tt the frequencies of marked young were always higher than when non-T females were used, and when Tt and cwcw were carried in the same parent there was a shortage of marked young obtaining both copies of the proximal region of chromosome 17 from the father. Both these effects were regarded as probably another example of the phenomenon discovered by Johnson, of inviability of young lacking a maternal homologue of a certain region of chromosome 17. There were other variations in frequency of marked young, among crosses using non-T females, which may have been due to differences in transmission ratio of male gametes carrying various t-haplotypes or to true variations in frequency of adjacent-2 disjunction.

This paper describes a method for selecting for conditional lethal mutations in the filamentous fungus Schizophyllum commune. The method is, to our knowledge, the only successful application of filtration enrichment to Basidiomycetes. In addition, it is far more powerful than filtration enrichment methods reported previously for any fungus; it permits recovery of up to 80% auxotrophs starting with material untreated with any mutagen.