‘Wider ratio’ octads (8:0, 0:8, 7:1 and 1:7) regularly occurred in wild-type(+) × white ascospore(w) crosses of the Pasadena strains of Ascobolus. Control crosses showed that phenocopies and false octad clusters were absent or rare; no reversion from w to + occurred, but mutation from + to w was found at a number of loci, with nearly all 0+:8w and many 2+:6w octads in + × w crosses arising from mutation, not conversion. Nearly all 8+:0w, 7+:1w and 6+:2w octads appeared to arise by conversion.

The finding of genuine wider ratio octads implies hybrid-DNA formation at corresponding sites in both pairs of non-sister chromatids in the same bivalent, which conflicts with models of the synaptinemal complex requiring that only two of the four chromatids pair intimately at any point. Octad types arising from hybrid-DNA formation at corresponding sites in both pairs of non-sister chromatids were described and formulae were derived for their frequencies. The lack of genuine wider ratio octads in several other Ascobolus studies was shown to be explicable quantitatively in terms of their conversion frequencies.

‘Corresponding-site interference’ is defined as interference between the two pairs of non-sister chromatids of a bivalent in hybrid-DNA formation at exactly corresponding sites. Formulae based on observed octad frequencies were derived for calculating coincidence values for this kind of interference. Corresponding-site interference was found to be weak, with coincidence values differing between crosses with high and with low conversion frequencies.

In Great Britain by far the greater part of the genetically effective dose of man-made radiation to the human population is due to high-intensity irradiation of the gonads in medical radiology; about a third of it is received by post-natal males, a third by post-natal females, and a third by foetuses, mainly in the later stages of gestation (Osborn & Smith, 1956).

Chromosome transfer has been studied in mating pairs formed during a 5-minute interval, under conditions that prevent the formation of new pairs. It has been found that, despite restriction of the period of effective contact formation, there is a considerable spread in the time at which transfer is initiated in the different mating pairs. In some pairs there is a delay of as much as 15 minutes before transfer commences.

The presence of broth in the medium in which transfer takes place leads to a marked reduction in the stability of pairs involving some Hfr strains, but has little or no effect on pairs involving others. The action of broth can be ascribed to the effect of a complete mixture of amino-acids on the metabolic activity of the donor cells.

A segment of chromosome which has penetrated from the Hfr into the F− cell may be subsequently withdrawn. Withdrawal may take place in more than half of the mating pairs, and probably occurs at the time when the donor and recipient cells separate. It is prevented if the chromosome breaks when conjugation is interrupted.

The sterility which is associated with male recombination induced by 31.1 MRF was studied genetically and cytologically. In all crosses it was found that female sterility mainly involves failure of the heterozygous females to lay eggs because their ovaries are atrophic. Under the optical microscope, the atrophic ovaries were seen to contain only germaria in their ovarioles. It was also found that in some cases 31.1 MRF affects only one of the two ovaries of the same female. This observation suggests that defective development of atrophic ovaries is not due to influences from the rest of the body but should be attributed to the inability of the germ cells to differentiate. Moreover, various stocks as well as homologous chromosomes were found to react differently to 31.1 MRF with respect to female sterility. In their effect on male sterility it was observed that some strains behave as neutral and others as reactive when mated with 31.1/Cy L4 males.

The singedvery weak mutation was created by the sequential addition of two P transposable elements to the singed gene. The mutation can be somatically unstable through the action of a dominant maternal effect mutation on the second chromosome. It is also unstable in the germ line in these conditions. Sequencing of the region of the P insertions in the mutation reveals that the two inserted elements have single internal deletions, and the larger of the two is a copy of the KP element. The mutation will generate, at high frequencies, strongly singed and pseudo-wild type products by reversions occurred in the germline. These are the result of the precise excision of the smaller and the larger elements respectively. By PCR amplification of dissected thoraces we show that the somatic instability of the mutation, from a weak to a strong singed phenotype, is also caused by the excision of the smaller of the two elements.

We report the results of an ontogenetic analysis of quantitative genetic variance components with two replicates drawn from the randombred ICR strain of mice. A total of 432 mice from 108 full-sib families raised in a cross-fostering design were used to estimate direct effects heritability, maternal effects, and environmental effects for weight, head length, trunk length, trunk circumference, and tail length at 17, 24, 31, 38, 45, 52, 59, and 66 days of age. There was no significant difference in heritability between the replicates. Heritabilities either stayed more or less constant with age at about 0·30 (weight, trunk length, trunk circumference) or increased slightly with age (head length, tail length). Maternal effects decreased with age from a maximum of about 0·50 at weaning to about 0·15 at age 66 when growth was nearly complete. Environmental effects increased in relative importance during ontogeny.

The amino acid analogues p-fluorophenylalanine (PFP) and ethionine (ETH) are strongly mutagenic in Coprinus lagopus. The most pronounced effect was found with suppressor mutations of the met-1 locus. PFP, at a concentration of 2·4 × 10−4 M, increased the mutation frequency 500 fold and ETH, at a concentration of 2·4 × 10−3 M, 30 fold over the spontaneous mutation frequency. From the spectrum of suppressors of the met-1 locus and the dominant revertants of the ad-82 locus, induced by analogue treatments, it was concluded that both analogues induce single base-change mutations. The dose response curves follow a sigmoid plot, revealing that within a certain range of analogue concentrations, muta-genesis is strongly dose dependent.

Using analogue resistant mutants, it has been shown that PFP mutagenesis is a function of its incorporation into protein. However, ETH mutagenesis is independent of protein incorporation but can be correlated with the degree of ethylation of nucleic acids. The synergistic effect PFP and ETH supports the evidence of the different mutagenic actions of the two analogues.

Twenty-four reciprocal crosses between two green and six chimera cultivars, containing respectively normal green and mutant white plastids in their germ layers, were analysed for their segregation patterns of green, variegated and white embryos, and for their fertility data.

Analysis of variance of fertility data showed that W × G crosses were generally less fertile than the reciprocal G × W crosses, but this was not correlated with selection against any particular class of embryo.

Analysis of segregation patterns showed that they were essentially the same with a constant female and varying male parent, but change the female and the pattern could be radically altered. The six different mutant females with both constant green males could be arranged in a sequence in which white plastids were increasingly successful, and white and variegated embryos increasingly frequent. A similar sequence was demonstrated with three green females and two constant white males. It was concluded that the major control of plastid inheritance was determined by the female nuclear and plastid genotypes, with the male having only a minor, modifying influence even when male plastids were more successfully transmitted than female ones.

The very low frequencies of variegated embryos in some crosses led to a rejection of the classical hypothesis of sorting-out from mixed cells, and its replacement by the hypothesis that pure green or pure white embryos arise by the replication of only one plastid at a time, whereas variegated embryos arise by replication of both plastids.

Initial Fertility (IF) strains of Streptomyces coelicolor are able to convert recipient strains (UF) to the IF condition by contact, without concomitant transfer of chromosomal markers. The conversion is prevented by the presence of acridine orange in the medium of the mixed culture. Acridine orange is also moderately effective in inducing the formation of UF variants from IF-treated strains. No effect of the drug is observed on UF variant formation from Normal Fertility (NF) strains nor on the behaviour of the fertility factor in NF × UF mixed cultures. The hypothesis is put forward that the fertility factor works as an episome in S. coelicolor, fixed to the chromosome in the NF strains, free in the IF strains and missing in the UF strains.

1. The growth of a number of inbred lines from the Pacific cage population have been compared under different conditions of temperature and nutrition. Body size and duration of the larval period were taken as measures of performance. Sub-optimal diets were provided by growing larvae on chemically defined synthetic media.

2. Gene-environment interaction is widespread and often very great. The phenotypic effects of inbreeding on body size, even on a live yeast medium, may be greatly influenced by temperature. In one set of comparisons, inbred lines averaged 20% smaller at 25° C. but only 3% smaller at 18° C.

3. Sub-optimal diets of different chemical composition, which lead to about the same average decline in body size, may differ greatly in the level of heterogeneity of response among the same set of inbred lines. Thus much greater heterogeneity was found on diets deficient in RNA than on diets with low protein levels. Such information is a useful guide to further study of gene-environment interaction in the outbred population.

4. Diets which lead to a decline in body size of flies of the foundation population do not necessarily cause greater proportional decline on the part of inbred lines. Individual lines have been encountered in which body size is quite unaffected by changes in diet which reduce the size of the outbred flies by 25% or more.

5. A series of crosses between lines from the same foundation population showed a striking level of homeostasis. The average body size and development time of the F1's was close to that of the population of flies on the favourable and two alternative sub-optimal diets. Also, compared with the parent lines, there was little evidence of gene-environment interaction among the crosses.

(1) F1-hybrid mice derived from a cross of the highly inbred strains: C57BL/6 and BALB/c, were tested for inherited changes of histocompatibility by an orthotopic inter-exchange of tail-skin grafts. The fathers of tested mice received either 522 rads of gonadal X-irradiation, or received no irradiation 2 months prior to mating.

(2) Thirty-two mice with altered histocompatibilities were found in a total of 2572 complete tests. All of those mutant mice (twenty-one) that produced an adequate number of offspring were shown to pass the incompatibility on to their progeny.

(3) Mutants were classified as to whether they effected a gain, a loss or both a gain and a loss in antigen specificity as determined by whether they rejected skin of donor mice or their skin was rejected by host mice. Twenty-six were clearly of the gain type, five were most likely gain type and only one showed both a loss and a gain effect. There was no clearcut evidence that loss types had occurred. The preponderance of gain types was tentatively explained as an artifact of the system used for the assay.

(4) Several of the detected mutants were probably from parents carrying mutations that originated in past generations, for some mutant mice occurred in clusters.

(5) There was no apparent effect of paternal irradiation (522 rads) on mutation frequency. The induced mutation rate was estimated to be less than 2·6 × 10−5/ gamete/rad.

(6) Independent data on isografts from F1 hybrids of proven non-carrier pedigreed parents provided an estimate of spontaneous mutation rate of 6·75 × 10−3/ gamete.

(7) The estimate of doubling dose (greater than 260 rads) was consistent with the estimates for recessive lethals and visibles in mice.

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.

A general representation of multilocus selection is extended to allow recombination to depend on genotype. The equations simplify if modifier alleles have small effects on recombination. The evolution of such modifiers only depends on how they alter recombination between the selected loci, and does not involve dominance in modifier effects. The net selection on modifiers can be found explicitly if epistasis is weak relative to recombination. This analysis shows that recombination can be favoured in two ways: because it impedes the response to epistasis which fluctuates in sign, or because it facilitates the response to directional selection. The first mechanism is implausible, because epistasis must change sign over periods of a few generations: faster or slower fluctuations favour reduced recombination. The second mechanism requires weak negative epistasis between favourable alleles, which may either be increasing, or held in check by mutation. The selection (si) on recombination modifiers depends on the reduction in additive variance of log (fitness) due to linkage disequilibria (υ1 < 0), and on non-additive variance in log (fitness) (V′2, V′3,.. epistasis between 2, 3.. loci). For unlinked loci and pairwise epistasis, si = − (υ1 + 4V2/3)δr, where δr is the average increase in recombination caused by the modifier. The approximations are checked against exact calculations for three loci, and against Charlesworth's analyses of mutation/selection balance (1990), and directional selection (1993). The analysis demonstrates a general relation between selection on recombination and observable components of fitness variation, which is open to experimental test.