In a two-dimensional stepping-stone model of finite size, if a pair of alleles happen to segregate in the whole population, marked local differentiation of gene frequencies can occur only if migration between colonies is sufficiently rare so that Nm < 1, where N is the effective size of each colony and m is the rate at which each colony exchanges individuals with four surrounding colonies each generation. On the other hand, if Nm ≥ 4, the whole population behaves as if it were panmictic and the allelic frequencies become uniform over the entire distribution range unless mutation is unusually high. Tendency toward local differentiation is much weaker in two-dimensional than in one-dimensional habitats.

Epistatic models are examined for conditions which result in a negative covariance between a sire's purebred and test-cross progenies. It is found that heterozygote superiority is not a necessary condition for its occurrence. The frequencies of the same allele in the selected population and in the tester should be divergent. The implications of using the covariance in prediction are discussed. The important result to note is that selection in the crossbred may not be optimum even with a negative covariance.

The genetic basis of variation in rate of seedling growth, and development has been examined in the Australian commercial population of Phalaris tuberosa. A model of additive genetic maternal effects has been used, with seed weight of the female parent as an index of maternal ability. Rate of leaf appearance, rate of tillering and growth per tiller are all genetically variable in the population, with estimated heritabilities of 0·36, 0·23 and 0·34 respectively on an individual seedling basis. Total seedling growth has a lower heritability (0·17), due to a negative genetic correlation between tiller production and growth per tiller ( − 0·46). These two components have also been shown to be subject to qualitatively different seed size maternal effects. Genetic differences in seed size in the female parent have been found to influence growth per tiller, while environmental differences in seed size affect primarily the rates of leaf appearance and tiller production.

Bacteriophage εγ is capable of transduction both by replacement of a genetic segment of the recipient by the homologous genetic material from the donor strain and by the formation of defective transducing particles capable of lysogenizing the recipient strain of S. anatum.

The isolation of strains carrying such prophages, which have incorporated the lactose or arabinose operons, is reported. Lysogenic strains, carrying both normal and defective transducing prophage, form high-frequency transducing lysates. Other strains, carrying only defective prophage, show evidence that the association of prophage genes and transduced materials is stable since the loss of one frequently entails loss of the other.

Recombination data from crosses made at a single constant temperature of incubation were compared with those from crosses transferred to a different temperature at either the time of conidiation of protoperithecia by the strain of opposite mating-type, or after fertilization when crozier stages were first visible. Results were also compared from reciprocal crosses, from crosses made in different ways and from crosses in which protoperithecia were conidiated at different stages of maturity.

Different temperature regimes during vegetative growth and proto-perithecial development had highly significant effects on subsequent meiotic recombination, while temperature differences during later premeiotic stages (between conidiation of protoperithecia and the crozier stage) had no or little effect. It was found that premeiotic controls could have as great, or greater, effects on meiotic recombination than those operating directly during meiosis. The possible adaptive significance of this is discussed.

Recombination frequencies were affected by the method of making a cross (joint-inoculation of strains of opposite mating-type, or conidiation of protoperithecia), and by protoperithecial age at the time of conidiation by the opposite mating-type. Differences in recombination between reciprocal crosses were obtained and were dependent on temperature of incubation and age of protoperithecia at the time of conidiation. Recombination was not affected by different lysine concentrations in the medium. Genetic differences in premeiotic effector-production between the strains used were inferred.

The process leading to gene recombination can be interrupted in the filamentous bacteria Streptomyces coelicolor by growing mixed cultures on cellophane disks lying on complete medium. The mycelium is harvested, broken, diluted and the broken hyphae plated at different time intervals. By this means some markers can be excluded from heteroclones or from recombinant progeny in early samples. The recombinant pattern clearly changes with time, with an increase of markers contributed to the recombinant progeny. In crosses between male (NF) and female (UF) strains, the maleness is the first donor trait to appear in the cells of the recipient parent. The fertility factor does not produce a transfer origin on the donor chromosomes; the donor contribution may extend on either side or on both sides of the factor which appears to be compulsory for zygote formation. The longer the time of contact between parental cells, the longer the segment of the donor chromosome contributing to the recombinant progeny. When spores are formed they contain almost exclusively recombinant nuclei derived from segregation processes.

A bacteriophage was isolated from a strain of Salmonella manilla. This phage, υ15, was found to have adsorption and conversion properties similar to phage ε15. The two phages show similar transduction efficiencies but phage υ15 lysogenizes more efficiently. Phage υ15 is not subject to repression by ε15 type prophage. The densities of the two phages are indistinguishable.

Phage υ15 can ‘cure’ ε15 type prophages and recombination is possible between genomes of the two phages. It seems probable that they are heteroimmune representatives of a single bacteriophage family.

The rates of mitotic recombination at the loci ade3, ade5–7, ade6 and ade8 were approximated by the method of ‘increasing proportion of variants with growth’. The values (per cell generation) were 3·5 × 10−4 for ade8, 1·4 × 10−4 for ade5–7, 4·0 × 10−5 for ade3 and < 2 × 10−6 for ade6. The relative frequencies of mitotic recombination in regions between ade 5–7 and the centromere were different from those obtained with radiation-induced recombinants.

Genetic systems involving developmental inactivation of entire chromosomes occur in two widely different groups of organisms: mammals and coccids (Homoptera: Insecta). The two groups show several similarities and some interesting contrasts with respect to this unusual cytogenetic phenomenon. Although mammalian X chromosomes and coccid paternal sets are components of different genetic systems, comparisons between them nevertheless suggest approaches that might prove to be of value. Further, the occurrence of facultative heterochromatization in these two wholly unrelated taxa must mean that this type of heterochromatization represents a fundamental capacity of chromosomes.

Associative overdominance due to linked detrimental mutations was investigated using the method of moment equations based on diffusion models. The expectation of the apparent selective value at the marker (neutral) locus has been evaluated. Assume two linked loci, at one of which the steady flux equilibrium is reached under constant mutational input of deleterious mutations (with rate v) having disadvantages hs in heterozygote and s in homozygotes. At another locus, the neutral alleles are segregating with frequencies near 0·5. Let Ne be the effective size of the population and c be the recombination fraction between the two loci. Then the coefficient of associative overdominance at the neutral locus can be obtained by taking the expectation with respect to chromosome frequencies at steady flux equilibrium. It becomes approximately

where (LI−L0) is the inbreeding depression caused by deleterious mutations under complete inbreeding, and Nehs ≫ l and hs ≫ v are assumed. More generally, if the inbreeding depression of a chromosome segment with a length of recombination fraction C is (LI−L0) then s′ at the neutral marker at the edge of the segment is

where hs is the average heterozygote disadvantage of detrimentals.

The significance of the associative overdominance is discussed in relation to actual observations. It is proposed that the most of the observed heterozygote superiority including inversion chromosomes of Drosophila, isozyme alleles in Avena and ABO blood group genes in man could be explained by the associated detrimentals.

1. Coprinus lagopus produces two non-specific phosphatases: a constitutive acid phosphatase, and an alkaline phosphatase which is repressed during growth on media with a high inorganic phosphate concentration.

2. The alkaline phosphatase is also repressed when Coprinus is grown on an organic phosphate source; but if the acid phosphatase is selectively inhibited by fluoride the alkaline phosphatase is de-repressed and growth is comparable to that observed on an inorganic phosphate source.

3. Alkaline phosphatase is not repressed in aerial mycelium or sporophores even when grown on high phosphate medium.

4. Mutants altered in their capacity to synthesize alkaline phosphatase were selected from two compatible wild-type strains, H2 and H5.

5. Mutants producing a higher level of alkaline phosphatase than wild-type (‘regulator’ mutants) fall into four (or possibly five) complementation groups. Assuming five separate genes, two pairs are linked; the remaining one is independent and on another chromosome.

6. Mutants deficient in alkaline phosphatase synthesis fall into at least three groups. They were tested for linkage to ‘regulator’ loci but so far there is no evidence of this.

Salmonella anatum lysogenized by a doubly mutant strain of bacteriophage εγ shows a markedly rough colonial phenotype on tetrazolium agar. Mutants that abolish the roughness are reported.

Recombination between chloramphenicol-sensitive (Cms) mutants of Rl, and R100, has been demonstrated in Escherichia coli K12 rec+; it occurs at reduced frequency in recB and recC, and is not detectable in reeA, indicating that R factor recombination depends on host functions. Some mutants of R1 also recombine with an R100 mutant in a similar way. recA cells carrying an R1 and an R100 Cms mutant (hetero-R state) have a low level of chloramphenicol-resistance, and form a chloramphenicol acetyl transferase that has lower specific activity than enzyme from hosts carrying wild-type or recombinant factors. These results suggest the occurrence of interallelic complementation between mutant R factors.

Hyperhaploid and hyperdiploid colonies of Aspergillus nidulans, disomic or trisomic for one or more chromosomes respectively, are abnormal in phenotype and somatically unstable. It has been shown for Glasgow strains that a different specific phenotype arises, depending upon which chromosome is additional (Pollard, Käfer & Johnston, 1968). This report extends this system and shows that irrespective of the residual genetic background of the isolate, the system of phenotypic specificity applies.

In the fission yeast Schizosaccharomyces pombe, growth responses of mutants strains in the de novo purine synthesis pathway, in the purine interconversion system, and of various double mutants, have been studied upon different purine-supplemented media. The results show that exogenous purine utilization as nucleotide source is based exclusively upon pyrophosphorylation of purine bases, and they make it possible to identify most of the enzymic steps acting, in this organism, upon a purine ring to give another purine ring.

A scheme of the interconversion system in Schizosaccharomyces pombe is given.

R-factor 1818, which is refractory to curing by acridines or ethidium bromide from Escherichia coli K 12 and Klebsiella aerogenes 418 hosts, was eliminated from thymineless mutants of these organisms under conditions of thymine starvation. Neither R-7268 nor R-TEM could be eliminated by this method. The concurrence of R-TEM with R-1818 inhibited the curing of R-1818, whereas R-7268 had no effect on R-1818 elimination

Haploid segregante from vegetative diploids of Ustilago violacea appear as spherical colonies (papillae) growing above a background of dead diploid cells on complete medium containing DL-p–fluorophenylalanine (PFP). Most mutants segregate normally but one-third of the mutants were expressed infrequently in 0–5% papillae only. These mutants, designated ‘missing-markers’, were found to be on either of two chromosomes that remained disomic after treatment with PFP. When cells from a disomic papillum were streaked on complete medium, monosomics in which ‘missing-markers’ were expressed segregated spontaneously at a low frequency. Thus, of 10–12 linkage groups identified in U. violacea, two remain disomic after PFP treatment. Possible reasons for these differences between chromosomes in the same genome are discussed.

Haploid and diploid stock cultures did not differ either in resistance to PFP, or in the production of papillae on PFP medium. Haploid segregante from a diploid were slightly more resistant to PFP than the wild-type haploid cultures under some conditions, but were very different in that they no longer produced papillae on PFP medium. These haploid segre-gants resembled one of three PFP-resistant mutants (pfp-A) isolated from a wild-type haploid stock grown on PFP medium. The significance of these results to the mechanism of haploidization in U. violacea is discussed.

A class of abnormal sex-ratio (SR) condition in Drosophila characterized by the elimination of male zygotes is caused by infection with maternally transmitted SR-spirochaetes. A given spirochaete strain, WSR, was shown to have an associated DNA virus, spv-2, in a latent condition. This virus is induced to multiply when another DNA virus, previously designated spv-1, infects WSR spirochaetes; spv-1 is normally associated with another strain of SR-spirochaetes, NSR, and is in a manifest state. Spv-2 infects and lyses NSR, thus eliminating spirochaetes from fly hosts which, in these experiments, were D. melanogaster.

The elimination of WSR through lysis by spv-1 results in the ‘curing’ of the SR-condition in the host flies. However, the elimination of NSR by spv-2 does not lead to the immediate elimination of the SR-condition in the host. A hypothesis is presented in favour of the view that a substance, androcidin, is produced by SR-spirochaetes and that this substance is actually responsible for the death of male zygotes. NSR may produce a more potent androcidin than WSR.

Plants of Triticum aestivum (2n = 6x = 42) ditelocentric 5BL were treated with EMS in order to produce mutations in the 5B system by which meiotic pairing between homoeologous chromosomes is normally prevented. To check for the occurrence of mutation T. aestivum ditelo-5BL plants were pollinated with rye (Secale cereale 2n = 14) and meiosis was examined in the resulting hybrids.

Wheat-rye hybrids were scored for the presence of mutants when the wheat parents were either the EMS-treated wheat plants, or their selfed derivatives, or their progenies obtained after pollination with untreated euploid individuals.

Mutants were detected by each of these procedures and mutant gametes were produced by the treated ditelocentric plants with frequencies between 1·5 and 2·5%, but there were differences between the mutants in the extent to which homoeologous pairing occurred in the derived wheat-rye hybrids. The differences may have resulted from the occurrence of mutation at different loci or to different extents at the same locus.

Two mutants, Mutant 10/13 and Mutant 61, were fixed in the homozygous condition. Mutant 10/13 was made homozygous both in the 5BL ditelocentric and in the euploid conditions but these genotypes regularly formed 21 bivalents at meiosis, and there was no indication of homoeologous pairing although the mutant 10/13 gave rise to homoeologous pairing in wheat-rye hybrids.

Genetic analysis of spontaneous half-sectored colonies of a diploid strain homozygous for ade2 and heterozygous for linked markers aro1A, hom2, ade8 and trp4 was undertaken. Some of these were due to an unexpectedly high rate of ade2 reversion and others to a low incidence of haploid sectors. About 50% resulted from mitotic recombination. Only a few were due to mitotic crossing over alone, the majority resulted from non-reciprocal ‘conversion’, including recombination extending over two loosely-linked loci, and some colonies featured both mitotic crossing over and mitotic conversion. The latter indicate that negative interference operates for mitotic recombination. The rate of mitotic recombination for ade8, calculated from the frequency of half-sectored colonies was 3·1 × 10−4 per cell division.