1. A number of mutants affecting changes in the pigment of larval and adult An. gambiae are reported.

2. Two other pigment characters are also described. One of these, diamond, is probably inherited as a single factor.

3. Two of the mutants are sex-linked and the experimental results show An. gambiae to have a Drosophila form of sex-linkage.

4. The sex-linked white-eye mutant is shown to be epistatic to the gene for collar and the characters diamond and red stripe.

5. Attention is drawn to the difference in sex-linkage between An. gambiae, and C. pipiens and Ae. aegypti which may indicate different methods of sex-determination.

White-spored mutants of A. immersus were used in experiments carried out to study the relationship between conversions occurring simultaneously in linked genes, and to check whether conversion occurs at random in the two chromatids of a chromosome.

Conversions in two loosely linked loci were shown to be independent of each other and randomly distributed between sister chromatids. On the other hand, conversions in two closely linked loci showed a high correlation of occurrence.

Four ascospore-pigmentation mutants were crossed with their wild-types by methods giving a considerable degree of synchrony in perithecial development; segregation patterns were scored from perithecia at different stages of maturity. The observed second-division segregation frequencies for asco in N. crassa were at a maximum when little or no ascal dehiscence had occurred but decreased markedly as dehiscence proceeded. No significant change in this frequency with maturity occurred for tan spore in Neurospora but in S. fimicola the observed second-division segregation frequencies for gray and hyaline rose with increasing perithecial maturity. As similar changes in this frequency were observed when asci were mounted in water instead of the 2 m sucrose solution normally used, it was concluded that changes in this frequency with maturity generally resulted from a differential maturation and bursting of asci with different spore arrangements, rather than from changes in crossover frequency in successive meioses. Some evidence of the latter phenomenon was found in two of the Sordaria crosses.

In a cross of asco × rib-1 in Neurospora, the observed frequency of recombination between the two loci was significantly higher in dispersed spores collected from young perithecia than in those from more mature perithecia: this effect was probably an artifact resulting from the differential maturation and bursting of asci with different divisions of segregation for asco. In a cross of crisp × + in Neurospora, an excess of crisp progeny was obtained from dispersed spores and an excess of +-progeny from spores from harvested perithecia. These deviations from the expected 1:1 ratio for cr: + were thought to result from a differential maturation of asci with different segregation patterns for crisp and the failure of many dehiscing asci to expel all eight spores.

Hypotheses to explain the various phenomena observed were discussed. Suggestions were made concerning the avoidance of bias from the differential maturation and bursting of asci in experimental procedures used in the study of recombination and gene conversion. This bias may be responsible for some phenomena previously attributed to events at meiosis.

Four crosses were made between bacteriophages T4 and partially non-excludable T2 exr+4. Each cross was designed as a four-factor cross: in total seven pairs of homologous genes were used as markers, extending over half the circular map from the locus for adsorption specificity (h+) along the map segment for the early functions to osmotic-shock sensitivity (o).

The frequencies of T2 markers recovered in the progeny show polarized exclusion: the frequencies are low for h+2 at the beginning of the map segment for the early functions and increasing to o+2. Recombination of T2 exr+4 is restricted for all loci concerned and the recombinational events are probably non-reciprocal. Restriction of sequential replication from a fixed starting point is proposed to explain polarized exclusion of T2 genes. Non-homology is dicussed in relation to restricted recombination.

1. Auxotrophs were sought in a slow-growing reversion of the tryA47 strain of S. typhimurium. This reversion differs from tryA47 by a genetic change that is inseparable from the 47 site and has been designated 47S. Out of thirty-nine auxotrophs that derived from independent mutations, twenty-two grew on minimal medium supplemented with anthranilic acid. Eight of these auxotrophs were examined and each was shown to have the 47S site unchanged and to carry a further mutation in the tryA gene. These further mutations were shown to be at different sites in different auxotrophs.

2. Auxotrophs were sought in the wild-type LT-2 strain. None out of thirty-six were mutant in the tryA gene.

3. The 47S site in tryA47S was replaced by 47+ transduced from wild-type LT-2. Auxotrophs were sought in this strain and only one out of nineteen was mutant in the tryA gene.

4. Auxotrophs were sought in a wild-type reversion of tryA47. Out of twenty-six none were mutant in the tryA gene.

5. The 47+ site in wild-type LT-2 was replaced by 47S transduced from tryA47S. Auxotrophs were sought in this strain and twenty-two out of fifty-five were mutant in the tryA gene.

6. We conclude that tryA auxotrophs are only recovered at a high frequency when the 47S site is present in the tryA gene.

7. In strains with the 47S site in the tryA gene the frequency of auxotrophs that will grow on minimal medium supplemented with indole but not on minimal medium supplemented with anthranilic acid is appreciably higher than in those strains without the 47S site in the tryA gene. These auxotrophs are mutant in genes that are in the same operon as tryA.

Inheritance of DDT resistance has been studied in crosses between the highly resistant ‘T’ strain of A. aegypti (constituted by inbreeding from the TRINIDAD DDT-resistant stock) and the ‘64’ susceptible strain.

Larval DDT resistance derives from a major gene RDDT1 on linkage group II, the order being RDDT1–s–y. Linkage group III may also contribute to larval resistance. Linkage group I makes no contribution.

Adult DDT resistance derives from a major gene RDDT2, 18·2 ± 2·1 units from the market blt on linkage group III. Linkage group II has no influence on adult resistance.

Selection with DDT to retain only RDDT1/+ segregants in larvae of backcrosses RDDT1/+×+/+ did not increase resistance in resulting adults, confirming the difference in genetic mechanism at the two stages.

The F1 progenies from reciprocal crosses between ‘T’ and ‘64’ differed slightly but significantly in larval resistance, modifying the influence of the major gene RDDT1 in the heterozygote.

The early developmental stages of the RDDT1/+ phenotype (up to the fourth larval stage) were more viable than the +/+ phenotype in backcross segregation. The difference in mortality probably exceeded 30%.

Selection for and against the canalized phenotype in scutellar bristles was attempted in two selection lines and a randomly selected line was used as control. The selection lines were the Decanalization line (D) and the Canalization line (N). The D line was maintained by matings of scute males (scwbl) with three scutellars with wild-type females (scwbl/yw) with five bristles, in the N line scute males with four bristles were mated with wild-type females also with four bristles, while in the C line males and females of the above genotypes were selected at random. The lines were established from a sample of flies taken from a line selected for high scutellar numbers.

After eighteen generations of selection the C line was characterized by a regression of mean bristle number without appreciable change in variance. Relative to the N line, the D population showed a lower proportion of flies having four scutellars, a higher variance in bristle numbers, and a higher proportion of four-bristle scute flies having abnormal patterns.

Two alternative hypotheses were advanced to account for the results of this experiment. The first postulated a relative change in the widths of the four-bristle canalization zones in the selection lines, while the second suggested a relative change in frequencies of specific modifier genes for scutellars in scute and in wild-type genotypes of the lines. The evidence favours the latter hypothesis.

1. An investigation of the serum esterases of the field-vole, M. agrestis, has been carried out by the method of starch-gel electrophoresis.

2. A total of 254 animals, mainly from Carron Valley, near Stirling, have been examined.

3. Evidence is presented for a system of four enzymes controlled by two genetic loci, which it is proposed to call Es-3C and Es-3R.

4. Each locus specifies one polypeptide sub-unit of the E3 enzyme, which is a dimer.

5. The two loci each have two alleles present at high frequencies in the population examined.

Contrary to the conclusion of earlier workers, penetrance of a very rare genetic defect, congenital tremor in White Leghorn chicks, was found to be complete. By appropriate correction of the original data for small family size it is shown that the defect is determined not by one but two pairs of autosomal recessive genes. In the terminology of human genetics, ascertainment of affected individuals corresponded to conditions of ‘truncate’ selection in the case of maternal sibships and more nearly to ‘single’ selection in the case of paternal sibships.

The genetic consequences of autogamy have been analysed in the autogamous strain A-25 of E. minuta. During autogamy as well as during conjugation four pronuclei (two genetically identical pairs) are formed in each individual. In conjugating pairs, any two of the four pronuclei of the autogamous conjugant may participate in synkaryon formation. Consequently, an 8:13 segregation ratio of clonal and synclonal mating-type inheritance has been found to occur in the progeny obtained by crossing A-25 animals with non-autogamous animals. During autogamy, synkaryon formation appears to be brought about by a preferential karyogamy of genetically dissimilar nuclei more often than is expected on a random basis. Therefore, heterozygosity is usually maintained after autogamy in spite of the extreme inbreeding characteristics of this sexual process.

Body and liver weight and the number of nuclei in the liver were significantly higher in F1 hybrid than in inbred embryos developing in the same uterus after artificial heterospermic insemination. The size of 13-day-old F1 hybrids corresponds to the size of 13·4-day in bred embryos. No significant differences in cell size were found between F1 hybrid and inbred embryos of the same age. It is concluded that in 13–15-day embryos heterosis takes place through increase in cell number and not cell size.

Using a strain of D. melanogaster carrying an X-4 translocation, comparison between groups of individuals cultured at 14°, 19° and 25°C. showed good correlation between heterochromatization, variegation in malpighian tubules and Notch. All these phenomena are enhanced by low temperatures. Correlation was less good within each temperature group, and considerable variability was observed between individuals within the group, and between nuclei from one individual.

Experiments involving a temperature change during the embryonic period indicate that (1) heterochromatization is especially temperature-sensitive during the early embryonic period but may be increased by low temperature later, (2) larval malpighian tubules are sensitive to temperature only during the early embryonic stage, and (3) N is influenced by temperature during early embryonic life and also during the larval period.

Our observations, in conjunction with those of other workers, could be explained as follows: exposure of individuals to low temperature at a time when a specific system is beginning to differentiate will cause the processes concerned to be blocked in some cells. At least during the early embryonic stages this appears to involve heterochromatization of the relevant locus. Once the processes are established which will lead to the formation of a character, further heterochromatization has no effect on the phenotype. Temperature may affect pupal or adult phenotype in this way or by a direct action on the metabolic processes of cells.

Further experiments showed: (1) the greatest temperature-sensitivity of all three phenomena within the first 6 hours of embryonic life; (2) striking fluctuations of the effect of temperature, especially within the early embryonic period; (3) close correspondence between all three phenomena in time of response to temperature. Some alternative interpretations are considered.

Hair follicle initiation and development was examined in tabby mutants and compared with a previous study of this sequence in normal mice. Initiation commenced at about the 17th embryonic day and continued for 1 and possibly 2 days after birth. Although central primary follicle initiation was of shorter than normal duration, it was more rapid, with the result that the central primaries achieved a greater maximum density, and their total numbers were greater in tabby than in normal mice.

In general structural terms, follicle group development proceeded normally except that no secondary follicles appeared, and for this reason, the mature grouping pattern of follicles was comparatively simple. Central primary follicles, the first to be initiated, were evenly spaced in the skin but the later association of lateral primaries with them was unusually close, and the resulting groups of primary follicles were relatively more compact in the mutants than they were in normal mice.

No simple explanation in terms of the altered timing of follicle initiation or the reduced complexity of the follicle group could be given for the abnormalities of the tabby coat.

Selection for high and low numbers of secondary vibrissae in tabby mice produced some correlated changes in the maincoat follicle population, but there was no apparent alteration to the time of onset of initiation. Evidently selection did not simply modify the effects of the Ta gene.

The effects of acriflavine on the transfer of various episomes and of the bacterial chromosome were studied using acriflavine-sensitive and -resistant strains of E. coli K-12 as donors and recipients. It was found that the inhibition of transfer of these elements by acriflavine was only slight when donor bacteria were resistant to this acridine dye, regardless of whether the recipient bacteria were acriflavine-sensitive or -resistant. This result seems to support the view that acriflavine inhibits the replication of these elements in donor bacteria which is considered essential for their transfer. With colicinogenic factors, in contrast to F and R, the results were affected by the sensitivity or resistance of the recipient and were in support of the killing by acriflavine of the cells which received the colicinogenic factors.

An investigation of mitotic segregation and recombination in A. niger gave the following results:

1. Thirty-one non-allelic markers have been assigned to six linkage groups (containing 11, 9, 6, 3, 1 and 1 markers respectively) by the analysis of haploid mitotic segregants from synthesized diploids.

2. The sequence of nine markers in one linkage group was determined and some of the map intervals were estimated by the analysis of haploids, recombinants for linked markers.

3. Almost all the haploid segregants were obtained on medium supplemented with the aminoacid analogue, p-fluoro-phenylalanine, the action of which is interpreted as an induction of chromosome losses.

4. The rates of mitotic crossing-over and haploidization are much higher than in the sexual species A. nidulans and the data support Pontecorvo's (1958) suggestion that the parasexual cycle can be a substantial alternative to the sexual cycle.

Interspecific crosses between, different genotypes and species of Nicotiana have revealed S gene polymorphism on a large scale. In the usually yellow-flowered self-compatible species N. glauca a strain occurs that has a self-incompatibility gene which is tightly linked with the bright-red corolla colour gene. In self-incompatible species, there are two kinds of self-incompatibility alleles, SI and SFI, distinguished on the basis of the acceptability of N. langsdorffii pollen. The SI form has a large number of alleles whereas the SFI form has only two, SF10 and SF11. The SI alleles are again divisible into two groups on the basis of the acceptability of N. noctiflora pollen and, on the same criterion, the two SFI alleles, SF10 and SF11, also are distinguishable. N. noctiflora and N. bonariensis plants could be divided into two and three groups respectively on the basis of interspecific compatibility relationships. When plants of each species were crossed among themselves, they formed two and three intra-incompatible and inter-compatible groups respectively, identical with those found on the basis of interspecific compatibility relationships. The significance of this observation is at present not fully understood.

1. Complex hybrids were produced having twenty-nine chromosomes, consisting of one telocentric and twenty complete chromosomes of T. aestivum (2n = 6x = 42), seven complete chromosomes of Ae. speltoides (2n = 2x = 14) and one telocentric chromosome derived from A. elongatum (2n = 10x = 70). The presence of the Ae. speltoides genome permitted pairing between homoeologous chromosomes at meiosis and the behaviour of the two telocentric chromosomes was observed.

2. The A. elongatum chromosome was seen to pair with chromosomes homoeologous to those of group 6. There was no evidence that it paired with chromosomes of any other group.

3. When the A. elongatum telocentric and those of 6A and 6D occurred in the same configuration it was evident that the telocentrics 6A and 6D were for corresponding chromosome arms, and the A. elongatum telocentric for the opposite arm.

4. The average rate of pairing was much lower for the A. elongatum telocentric than for wheat telocentrics. Previous studies had indicated very good genetic compensation of the A. elongatum chromosome for chromosomes 6A and 6D. It was therefore indicated that genetic equivalence and pairing affinity were not closely related in this case. Some implications of this are discussed.

A complicated system of chromosome instability related to gene suppression has been analysed. In addition, unstable genetic events analogous to those previously described in A. nidulans and in other organisms have been detected and one type of unstable variant recovered may be determined by V-type position effect. Furthermore, the selective systems used, clearly offer scope for analysis of genetic instability in general.

1. Five additional phenylalanine (PHE)-requiring mutants have been isolated but they do not add to the number of loci already known which have been designated phenA (phen2 and its alleles) and phenB (phen6).

2. Two pathways for tyrosine (TYR) synthesis in A. nidulans have been proposed: the well-known one by the transamination of p-hydroxyphenylpyruvic acid and an alternative one, as in animals, by the hydroxylation of PHE.

3. Ten allelic partial TYR-requiring mutants (tyrA), presumably blocked in the transamination pathway, have been isolated after N-methyl-N′-nitro-N-nitrosoguanidine (NTG) treatment of bil;phenA3 conidia.

4. Four partial TYR requirers (at another locus—tyrB) have been isolated after NTG treatment of tyr A7, bi1 conidia. They are presumably blocked in an alternative pathway for TYR synthesis, i.e. in the PHE-hydroxylation pathway.

5. tyrA mutants have been found to be p-fluorophenylalanine (FPA)-resistant and allelic to mutants at the fpA locus. tyrB mutants have been found to be very leaky and FPA-sensitive. tyrA;tyrB double mutants have been found to be exacting TYR requirers.

6. Mutants at loci fpA (tyrA) and fpE (anthranilic acid-requiring) have been interpreted to be p-fluorophenylalanine-resistant due to an oversynthesis of PHE.

Heterokaryons were made between a cytoplasmic variant of Neurospora called SG, which shows a slow growth rate, and normal strains. Fusions were made in such a way that the fate of the protoplasmic mixes in individual hyphae could be followed. A semi-quantitative analysis of nuclear and cytoplasmic migration along established hyphae was possible. Apparently, components of SG and normal cytoplasms are able to associate intimately in a single hyphal segment and later segregate intact. More often, however, an interaction of cytoplasms occurs which results in a new stable cytoplasmic type called slow SG. It is postulated the cytoplasmic determinants of SG are particulate and that the slow SG strains are the result of reassortment of the SG factor with other autonomous cytoplasmic entities. Acriflavine in the proper concentration inhibits and kills SG strains on solid medium without affecting normal strains. The acriflavine is most effective against germinating spores. The SG phenotype is redefined on the basis of this sensitivity to acriflavine.