An extension of two-way selection (i.e. selection of mutant from wild-type and vice versa within the same locus and with the same efficiency) to four different mutational or segregational situations was made possible by using acetate, fluoro-acetate and other substances related to their metabolism.

Two types of mutants resistant to fluoro-acetate were selected, the first of which (designated fac) cannot grow on acetate as the sole carbon source, while the second (designated fan) can.

Commencing with either a fac or a fan strain a double fac fan strain may be isolated, which is much more resistant to fluoro-acetate than either single mutant strain. Such double mutant strains may also be obtained by crossing a fac to a fan strain. Various characteristics of growth response of these strains on various media were observed.

fac mutants are recessive and map in three meiotically unlinked loci, one in linkage group V and two in linkage group VIII.

fan mutants are recessive and map in five loci, one in each of the linkage groups V, VII and VIII, and two linked in linkage group VI.

Most fac mutants isolated did not revert and this failure is considered genuine. Of the revertants tested, most resulted from extra-cistron suppressors, while revertants of two fac mutants resulted from very closely linked or intra-cistron suppressors.

It is argued that the findings indicate the existence of two pathways for acetate utilization in Aspergillus nidulans, a major and a minor; fac mutants are blocked in the major pathway, fan mutants in the minor pathway.

(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.

An experimental evaluation of the effect of mating systems and selection upon an additive trait thought to be highly heritable was made. There were two similar replications. Each consisted of a mass selected and randomly selected group, with five mating systems within each group.

Realized heritabilities in the mass selected lines were considerably less than was expected prior to the initiation of the experiment, and averaged approximately fourteen percentage points less than heritability estimated from the zero generation. This in turn resulted in smaller correlations between the genotypes of mates than had been previously expected in the assortatively and disassortatively mated lines.

The average response of the mass selected, assortatively mated lines was slightly more than the mass selected, randomly mated lines, though not statistically significant. This result seems to conform to theoretical expectations.

In the mass selected lines, estimates of phenotypic and genetic variance declined regardless of mating systems. There was a tendency for phenotypic variances to decrease in the randomly selected lines, but this was not the case for estimates of genetic variance.

As an aid to selection, it seems that assortative mating would be of little value with traits of low or intermediate heritability but might be useful if the trait is highly heritable.

Four ‘master’ strains of Aspergillus nidulans, three of which are marked on each of the eight chromosomes, are used for assigning genes of unknown location to linkage groups. In this technique a heterozygous diploid is synthesized between one ‘master’ strain and a strain carrying an unlocated marker. This diploid is haploidized by growth on p-fluorophenylalanine, and the haploids thus obtained are classified for recombination between the unlocated marker and each of the markers of the ‘master’ strains. The marker or markers of the ‘master’ strain which show no recombination with the unlocated marker identify the chromosome pair on which the latter is located. One example is presented for each of the four ‘master’ strains.

The present series of papers is concerned with the variation shown by date of ear emergence, seed weight, and measures of seedling growth rate in the Australian Commercial population of Phalaris tuberosa L. In this first communication, the statistical theory necessary for the interpretation of the available experimental observations is developed. The treatment involves a consideration of the effects of partial self-fertilization under open-pollination, of phenotypic assortative mating, and of maternal effects, on the expectations of the observed covariances among relatives.

The variation shown by the Australian Commercial population of P. tuberosa in respect of date of ear emergence, seed weight and seedling weight has been analysed, and the interrelationships among the variables characterized. All three characters show appreciable additive genetic variation, in the sense that approximately seven generations of artificial selection would be sufficient to push the population mean for each trait beyond the range shown by introduced ecotypes.

Apart from the positive association between seed weight and seedling weight due to maternal influence, the three variables are to a large extent genetically independent in this locally adapted interbreeding population. It is therefore probable that the negative ecotypic correlations between seedling growth rate and date of ear emergence, and between seed weight and date of ear emergence, would rapidly be dispelled under random mating in a synthetic population.

Under open-pollination, date of ear emergence has been shown to be subject to phenotypic assortative mating of degree ρ = 0·78, approximately 53% of the variation being additive genetic. Variation in seed weight within the strain is extensive, with a heritability of 0·79. Neither character shows evidence of important genotype × years interaction.

Variation in seedling weight involves an appreciable genotype × environment interaction component, and has a heritability of only 0·17 which includes variation due to genetically determined maternal effects. The correlation between the seed weight of an ovule parent and the ‘ true’ mean seedling weight of the derived maternal half-sib group is of the order of 0·57. It has been estimated that the correlated response per generation in seedling weight, due to selection for seed weight, is 0·54 times that expected from direct selection.

The level of liver catalase activity in substrain C57BL/6 mice is only approximately half that in substrain C57BL/He. That this difference was due to a single major gene with low level of activity dominant to high was indicated by analysis of the F1, F2, and backcross hybrids and confirmed by progeny tests of first back-cross male segregants. The gene symbol Ce is suggested.

The absence of any difference between reciprocal hybrids indicated no extra-chromosomal maternal influence.

There was no evidence that this gene controlling liver catalase had any influence on the level of kidney catalase activity for this did not vary between the two parent substrains or the hybrid groups.

Males consistently had higher liver and kidney catalase activity than females. The difference was more pronounced in respect to kidney catalase, males having about twice the activity found in females.

1. The maintenance of mu particles and metagons from Paramecium aurelia (stock 540, syngen 1) in another ciliate protozoan Didinium nasutum has been shown to occur.

2. It has been shown that mu particles could not be supported in Didinium without metagons. One particular strain of Didinium has, however, never been able to support mu even when metagons were present.

3. The continued production of metagons in Didinium was shown to take place even when the Didinium was fed on killer Paramecium of genotype m1m1m2m2.

4. After destruction of the metagons in Didinium by ribonuclease there was no subsequent reappearance of metagon activity.

5. Metagons and mu particles were eliminated from Didinium during encystment.

6. It is considered that the increase of metagons in Didinium is due to self-replication. In Paramecium the metagons replicate only slowly or not at all.

7. It is concluded that the metagon is a gene product in Paramecium which takes on at least one other property, replication, when introduced into Didinium.

1. Heterozygous (K/k) killer paramecia containing kappa particles from stock 51, syngen 4 of Paramecium aurelia were passed through autogamy, and the kk offspring tested for killing ability and for presence of stainable kappa particles at various fission stages.

2. After eight fissions, a small proportion of the paramecia were non-killers, and a few lacked stainable particles. After the 12th fission nearly all cells lacked kappa.

3. Mild starvation of the kk kappa-bearing paramecia resulted in complete loss of kappa and conversion to non-killers, even when applied to animals only one fission after the loss of gene K at autogamy.

4. The results obtained here show some discrepancy with data obtained earlier, both with kappa, and with mu particles in stock 540, syngen 1. The reasons for this discrepancy are at present not clear.

A total of eighty-seven revertants were induced by ultra-violet light in an am3 strain. All of these revertants appear to be the result of mutation at sites in or close to the am locus. Fourteen of the eighty-seven revertants were partial revertants in that under some conditions of assay they possessed low glutamate dehydrogenase activity compared with the wild-type although their growth rate was similar to that of the wild-type. Enzyme extracts of thirteen of the partial revertants were assayed for glutamate dehydrogenase in various ways in order to establish qualitative distinctions between different kinds of mutant enzyme. On the basis of these tests six different groups were established, of which one contained six revertants, one three and the others one. All except one of the mutant enzyme types showed a marked activation when incubated with α-oxoglutarate plus NADPH2, and all of these had Michaelis constants for ammonium ion much higher than is found for the wild-type enzyme. The remaining group of three revertants gave, at first, no enzyme activity in any of the assay systems. Two of these (the third was not tested) were shown to produce an enzyme variety which becomes quite inactive in phosphate buffer at pH 8·0 but can be fully activated by the addition of ethylenediamine tetra-acetic acid. Forced heterocaryons between each of six partial revertants and eleven am mutants were made and the resultant sixty-six heterocaryons assayed for glutamate dehydrogenase activity. The partial revertants differed among themselves in their complementation characteristics. Some complemented with none of the am mutants, some with am1 only, and some with am1 or with am7. The complementation tests confirmed the differences established by the enzyme studies. The data presented here, together with previous work, demonstrate that ultra-violet light induced mutation in an am strain can result in at least eight types of revertant differing from each other in respect of the glutamate dehydrogenase variety which each can produce.

Ten U.V.-induced mutants at the adenine-1 locus of Schizosaccharomyces pombe differ not only in their intragenic location but also in their spontaneous, HNO2- and U.V.-induced frequencies of reversion to adenine-independence. Seven mutants reverted with HNO2 and of these only two reverted also with U.V.; the other three did not revert with either HNO2 or U.V.

Genetic analysis of revertants showed that they could be due to suppressor mutations at a locus or loci far from the adenine-1 locus, or to events extremely close to, if not identical with, the original site of mutation. Many distant suppressors were found among spontaneous revertants of some adenine-1 and adenine-7 allles, but such suppressors appear to be less frequent among the five adenine-8 mutants tested. There are large differences between different sites at the same locus in the proportion of their spontaneous revertants due to distant suppressor mutations. Nitrous acid-induced revertants of adenine1-, adenine-7, and adenine-8 mutants were almost all of a type containing no detectable suppressor mutation. In the three cases studied, U.V.-induced revertants could be either predominantly of a suppressor or non-suppressor type, depending on the particular mutant examined.

Striking examples of mutagen specificity were found in which a particular mutant gave mostly suppressor-type revertants with one mutagen, while with another mutagen revertants were mainly of a type containing no detectable suppressor mutation.

A number of mutants of E. coli K 12, deficient in purine biosynthesis, have been isolated and the biochemical blocks have been determined. The mutations were mapped in conjugation experiments. In some cases the differences in penetration times were too small to determine the exact order of the loci by interrupted mating experiments. In these cases the recombination frequencies were determined in four factor crosses. In this way the location of eleven different purine markers has been determined. The loci are scattered over the chromosome. Only two groups of linked genes were found.

The effects of P32 disintegration in the Hfr donor cell on the genetic recombination of Escherichia coli K 12 have been studied. The dose-response relationship for the transfer of markers at a distance x from the chromosomal origin, N/N0 = e−αxD, which summarizes the findings of Fuerst et al. (1956) for early markers of Hfr H, has been found to hold true to a good approximation for more distal selected markers in this strain, as well as for selected markers of Hfr CS, which transfers its chromosome in the opposite direction. The inheritance by recombinants of non-selected male markers located between the origin and the selected marker is not affected at all by prior P32 dose to the Hfr. The sensitivity of each Hfr strain studied to the lethal effects of P32 decay corresponds to the sensitivity to transfer effects of a male marker located about one-third of the total chromosomal length away from the origin. Genetic and lethal effects in Hfr H showed the same dependence on storage temperature during P32 decay. Our data, therefore, supports the concept of a common origin for the lethal and genetic effects of P32 decay.

The quantitative predictions of current models for chromosome transfer with regard to radiation mating experiments have been examined. A simple model, involving no DNA synthesis, as well as the replicon model of Jacob et al. (1963) predict exponential marker inactivation curves of the kind found experimentally. The model of Bouck & Adelberg (1963) predicts a more complex dose dependence, but one not nearly so different from that of the other two models as was anticipated by Jacob et al. It appears unjustified to attempt to exclude any of the three models on the basis of presently available data from radiation mating experiments.

By performing matings using various Hfr strains having different origins of transfer, it has been observed that male chromosomal markers located very near the origin of transfer in conjugation in E. coli appear in recombinants with a lower frequency than expected from an extrapolation of the transfer gradient. The effect is greatest for markers closest to the origin, and becomes negligible for markers transferred more than 4–5 min. after the origin. The results suggest that in the zygote a random cross-over between male and female genomes is necessary somewhere between the origin and any male marker in order for that marker to appear in a viable recombinant. It was also deduced that the entry time for the origin itself occurs after a 3–5 min. delay after mixing male and female cells together.

In summary, rII mutations induced by exposure of T4 phage particles to either visible light or proflavine are acridine revertible. In both cases mutation occurs without inactivation. The rII mutations induced by the photodynamic action of proflavine on T4 phage particles are equally divided between the acridine revertible and base analogue revertible classes. It is proposed that photodynamic action induces base analogue mutations and that the acridine revertible category arises from the separate effects of light and proflavine.

From studies involving inhibition of DNA synthesis in Hfr strains of Escherichia coli K12, either by thymine starvation (Pritchard, 1963) or amino-acid starvation (Suit, Matney, Doudney & Billen, 1964), during mating with F− strains, it has been concluded that transfer of DNA from males to females can occur in the absence of DNA synthesis. This conclusion is at variance with the hypothesis (Jacob, Brenner & Cuzin, 1963) that the energy required for transfer is derived from the process of DNA replication. On the other hand, a second prediction from this hypothesis, that one polynucleotide chain of the DNA transferred during mating will have been synthesized during transfer, is strongly supported by recent experiments (Ptashne, 1965; Blinkova, Bresler & Lanzov, 1965; Gross & Caro, 1965).