1. Rates of gain and loss of acclimatization to temperature of males from two inbred lines and the hybrids between them were measured by recording their survival times in dry air at a lethal temperature (34°C).

2. All hybrid males lost acclimatization to temperature more quickly than did inbred males. B/K males gained acclimatization to temperature more quickly than any other group, but the K inbred males gained acclimatization more quickly than did either the K/B males or the B males. In all cases acclimatization is gained more quickly than it is lost.

3. The extent of acclimatization to temperature, as measured by the difference in survival times of 15°C. and 25°C. acclimatized flies in a range of lethal temperatures, was not found to be different in inbreds and hybrids.

4. The results suggest that hybrids can produce the enzymes necessary for acclimatization to temperature more rapidly than inbreds and confirms the hypothesis that hybrids are biochemically more versatile than inbreds.

5. The difference between the rates of gain and loss of acclimatization to temperature suggests that the processes involved in the enzyme changes are temperature dependent.

6. The absence of a difference in the extent of acclimatization to temperature indicates that both inbred and hybrid D. subobscura are capable of producing those enzymes necessary for temperature acclimatization.

7. The high values of the temperature coefficients for heat death indicate that this process involves protein (enzyme) denaturation.

8. An ageing effect was observed in inbred flies.

1. A histochemical method has been applied to the detection of alkaline and acid phosphatase mutants in single colonies of Aspergillus nidulans.

2. With the above method it has been possible to isolate mutants in which the alkaline and acid phosphatase activities are affected either separately or simultaneously.

3. Crude extracts of wild-type A. nidulans contain four electrophoretically distinct phosphatase components, two with activity at alkaline pH and two with activity at acid pH. Genes affecting three of the four components have been identified.

4. Two suppressor mutants of an alkaline phosphataseless mutant (palB7) have been isolated. In a strain carrying palB7 and one of these suppressors, the restoration of an alkaline phosphatase component is accompanied by loss of the faster acid phosphatase component. In a similar strain carrying the other suppressor, the partial restoration of the alkaline phosphatase component goes with an electrophoretic alteration of the slower acid phosphatase component.

5. Genetic analysis of twenty-seven mutants has resulted in the identification of fifteen loci affecting the phosphatases. All these loci have been assigned to linkage groups, and twelve of them were also mapped meiotically in relation to other loci.

6. One possible model (based on heteropolymeric proteins) has been proposed to account for the electrophoretic and genetic data on the various phosphatase and suppressor mutations.

According to the polaron hybrid DNA model, the initial nucleotide-chain breakage leading to genetic recombination takes place only at the linkage points which define the ends of each polaron. The term dissociation cycle is proposed for the postulated series of events from primary breakage at a linkage point to the final breakdown of unpaired chains. At mutant sites, the mispairing in hybrid DNA may persist and give rise to post-meiotic segregation, or a correction process may operate by which molecular homozygosity is restored. This causes conversion, which may be evident as reciprocal or as non-reciprocal recombination.

The implications of this model are that a crossover occupies a segment of the chromosome, and that conversion is a process which takes place when a mutant site happens to lie within such a segment. Although crossovers appear to be initiated at fixed points outside or at the ends of the genes, they extend into the gene on one side or the other. The negative interference between recombination events over short intervals of the linkage map is attributed to the association between crossing-over and the conversion which is likely to occur at any mutant sites which happen to lie within the crossover. In the same way, non-crossover hybrid DNA can also lead to conversion, and hence to negative interference.

The relevant data on genetic recombination have been found to fit this model, and have led to the following main conclusions:

(1) The polaron may coincide with the cistron, or in some instances may possibly include more than one cistron. As a corollary to this, there appear to be two kinds of cistrons: unipolar, where all the recombination is initiated from the same end, and bipolar, where it is sometimes initiated from one end and sometimes from the other.

(2) In bipolar cistrons there is usually a preponderance of recombination initiated from one end over that from the other. In five genes where the orientation with respect to the centromere is known, two show a preponderance in favour of the proximal end and the other three in favour of the distal end. It seems possible that this asymmetry within the gene may reflect intrinsic differences in the frequencies with which the dissociation of the DNA molecules is initiated at different linkage points.

(3) The hypothesis of a fairly constant amount of newly-synthesized DNA per dissociation cycle, irrespective of how it is distributed along the four templates from a linkage point, leads to a number of predictions for which there is evidence in support. These concern the detailed pattern of crossover and non-crossover hybrid DNA within the gene.

(4) Specific differences in intragenic recombination are attributed to differences in the pattern of DNA synthesis. The predominantly non-reciprocal recombination found within cistrons of Neurospora crassa would be explained if synthesis extends unequally from the linkage point in the two chromatids. The higher frequency of reciprocal recombination found in Aspergillus nidulans is attributed to more equal extension.

(5) Differences between meiotic and mitotic intragenic recombination both in A. nidulans and in Saccharomyces cerevisiae are explained on the supposition that more DNA synthesis per dissociation cycle occurs at mitosis than at meiosis.

(6) Clustering of sites in maps of alleles based on recombination frequencies is attributed to a rather limited range of variation in the lengths of the newly synthesized nucleotide chains.

By comparing genetic correlations between an X-rayed and a control population of rice, parameters for the strength of pleiotropic effects of quantitative genes have been estimated. It is evident that the gene-set which is responsible for main rachis formation and is therefore of primary importance for panicle formation, is pleiotropically controlling the development of rachillas and basal internode. It is found that the pleiotropic control over the development of higher rachillas is very powerful but less strong over the lower ones and basal internode.

Developmental relationships among these characters have also been investigated by measuring the genetic correlations between their developmental instabilities. The basis of our inference is the hypothesis that if two characters which are found to be pleiotropically controlled by the same set of genes take their own developmental paths from a very early stage, they will show only slight correlation between their developmental instabilities. If, on the contrary, they are taking the same developmental path and their differentiation sets in late, the correlation coefficient will be high.

A method is now available for synchronizing the division of populations of cells of the smut fungus Ustilago maydis. In two experiments carried out with a diploid strain heterozygous for several biochemical markers, samples of cells were removed at intervals through the synchronized division cycle and treated with a constant dose of ultra-violet light. Cell survival and the frequency of the various recombinants resulting from induced mitotic crossing-over were recorded. In addition the period of DNA synthesis in the unirradiated population has been measured. During this period the cells are very sensitive to ultra-violet light and those that survive contain the highest proportion of induced recombinants. In so far as the markers make it possible to locate the position of cross-overs, cells which are irradiated early in the period of genetic replication show most crossing-over towards the ends of the chromosome arms, whilst cells which are treated late in this period show crossing-over near the centromeres. The data are most easily interpreted by supposing that chromosome replication begins at the ends of the arms and proceeds to the centromere, and that the temporary interruption of this process by ultra-violet light can result in pairing and crossing-over in the vicinity of the points of interruption.

Six new am mutants of Neurospora, am14–am19, with defective formation of NADP-linked glutamate dehydrogenase, were isolated following treatment of conidia with nitrous acid. This brings the number of genetically distinct and viable am mutants to sixteen. Heterocaryon-compatible isolates of each of the new mutants were tested for complementation with each of the mutants previously known. A number of new complementary combinations were found which add to the complexity of the complementation map which, however, remains linear.

The mutants which show complementation are am1, am2, am3, am7, am14 and am19. It has proved possible to isolate an altered form of glutamate dehydrogenase from each of these except am14, which appears to produce no protein fractionating like the normal enzyme. Among the non-complementing mutants, am4 produces an easily-isolated inactive form of glutamate dehydrogenase. These results agree with the immunochemical findings of Roberts & Pateman (1964).

Of the mutationally altered forms of the enzyme, the am1, am2, am3 and am7varieties are electrophoretically identical or closely similar to the normal protein, while the am4 and am19 varieties move faster towards the anode at pH 8·5, am19being the faster of the two. The am19 protein, like the am2 and am3 proteins previously reported on, can be activated to give some enzyme activity, but the activity obtained is much lower for this mutant than for am2 or am3.

All the complementary pairs of mutants, including those involving am19, form active enzyme varieties which have very close to normal electrophoretic properties. In several cases the complementation enzyme is less stable to heat than the wild-type enzyme.

1. Individual genes of a multigenic system of resistance to actidione in yeast were analysed in a series of crosses involving resistant strains and sensitive strains.

2. Resistance conferred varied in the range 0·5 p.p.m. to 20 p.p.m.

3. Positive interaction was seen between resistance genes in recombinant strains.

4. Recessive modifier genes were found which do not themselves confer resistance but which modify the activity of specific resistance genes in a positive way, that is towards a greater degree of resistance. Both a two-fold and a five-fold increase in resistance is seen depending on the gene combination.

5. In selection experiments, strains resistant to relatively high concentrations of actidione (1000 p.p.m.) were obtained but only by selection on media containing successively higher concentrations of the drug, that is, by the step wise build up of multiply mutant forms. Mutations of interacting resistance genes and specific modifier genes were obtained by selection methods including pretreatment of cells with ultra-violet light.

Six experimental populations of Drosophila pseudoobscura were maintained in three different environments for almost four and a half years. All the populations have originally descended from the same founders. The populations were examined for evidences of genetic divergence. The longevity of the flies from four of these populations, and their F1 and F2 hybrids, was studied at 16°C. and at 27°C. The results indicate that the gene pools of some of the populations have diverged from each other.

In zygotes from matings between Hfr, F′ or F+ donors of E. coli K and F− recipients of E. coli B the restriction of λ. K and λ.C is 100 times less than in unmated B cells. This decrease in the capacity of zygotes to restrict phage λ is transient and is not related to the transfer of genes controlling host-induced modification during the cross.