Mutants of Aspergillus flavus impaired in aflatoxin production were induced with N-methyl-N′-nitrosoguanidine and analysed by means of the parasexual cycle. The gene symbol afl was assigned to this type of mutation. Diploid complementation tests revealed that most of 14 afl mutants belonged to different complementation groups. One mutant (afl-1) failed to complement or only partially complemented all other mutants. Haploidization of one diploid revealed the independent segregation of two afl mutants. Heterozygous diploids were synthesized between two afl mutants and tester strains genetically labelled on eight linkage groups. Haploidization of the diploids led to the assignment of two aft mutants to linkage groups. Linkage of afl-4 to w+ and afl-1 to leu on linkage groups II and VII, respectively, was demonstrated.

Embryonic tibia growth was measured in large, small and control mouse embryos in situ and in organ culture. In situ tibias of the large line were longer than controls which were longer than tibias of the small line. Relative growth was approximately equal among lines. In culture tibias of the small line grew more than controls and as much as or more than tibias of the large line. Embryonic genotype appears to be more important in regulating tibia growth than uterine environment. Humoral differences among lines may influence tibia growth.

The genetically unrelated chick strains Hy-1 and Hy-2, which have been strongly selected for growth rate, both exhibit hyperplasia of the lens epithelium. These two strains and a control strain N, not selected for growth rate, were compared with respect to incorporation of 3H-thymidine and 14C-uridine by freshly excised lenses in culture at different times throughout a 24-h period. The levels of incorporation of label into the lens cells were found to vary according to the time of day. The pattern of diurnal variation in both thymidine and uridine incorporation was found to be strain specific. Hy-1 and Hy-2 showed a greater degree of synchrony than did normal (N) lenses, and the frequency of the peaks of incorporation was also higher. Autoradiography confirmed that only lens epithelium incorporates thymidine during culture and that the number of labelled nuclei depends on the time of day when the lenses were explanted. These data point to genetic control of the cell cycle.

A mutant, YS17, at the buff spore colour locus in Sordaria brevicollis, when crossed with wild type, gives rise to aberrant asci with a frequency over 10 times that of other buff mutants. Over 98% of the aberrant asci have 6 wild type and 2 mutant spores. From tests with another buff mutant it is concluded that loss of the mutant spore colour when YS17 shows conversion to wild type is associated with loss of the high frequency conversion, and that both characters are caused by the same mutation. A methionine-requiring mutant (met-1) has been obtained that maps 5 units to the left of buff, and this, together with the nicotinamide-requiring mutant (nic-1) 2 units to the right, has provided flanking markers for buff that can be scored with complete reliability. Crosses between YS17 and 28 other buff mutants have revealed close linkage to three of them which map to its right on the basis of flanking marker behaviour, all the others mapping to its left. The frequency of postmeiotic segregation at the sites of buff mutants near to the site of YS17 is greatly increased in the presence of YS17, and occurs in the chromatid showing conversion to wild type at YS17.

From these and other results, obtained largely by ascus analysis, the following conclusions have been drawn.

(1) The YS17 mutation is probably acting as a recognition site for an endonuclease that initiates recombination, with the result that the frequency of heteroduplex DNA within the buff gene is much increased.

(2) The recombination initiated at YS17 is asymmetric (or at least pre-dominantly so), with the YS17 site acting as a recipient of a nucleotide chain from the other parent, not a donor to it.

(3) The frequency of crossing over associated with conversion at YS17 is variable: about 30% in crosses with most of the buff mutants, about half this value in crosses with wild type, and almost zero in crosses with closely-linked buff mutants.

(4) In about one third of the crossover asci in crosses between YS17 and other buff mutants the crossover is not adjacent to the site of YS17 but separated from it by the site of the allele, which shows normal 4:4 segregation.

(5) It seems necessary to revive the idea of more than one recombination event in proximity, a non-crossover conversion event sometimes leading to a second event – a crossover – in the vicinity. It is tentatively suggested that both might be controlled by a single enzyme aggregate.

Thirty-four populations of Aedes aegypti representing the world-wide distribution of the species, were analysed for genetic variation at 19–22 isozyme loci. The species has an average expected heterozygosity of 0·129±0·045 based on 19 loci analysed in every population. Based on this genetic information, two major groups can be defined: the dark, often sylvan, African subspecies formosus and the light domestic subspecies aegypti in Africa and the New World. Asian populations do not fall easily into either group. These results are related to models which have been proposed for the evolution of this species. Although A. aegypti was introduced into the New World c. 350 years ago and has recently recolonized many areas following eradication programmes, no signs of founder effects are evident in this region. Asian populations, on the other hand, do show a significantly lower level of genetic variation compared to other populations. This may be related to the time of introduction of A. aegypti into Asia and historical absence of yellow fever on the Asian continent.

A sex-linked recessive gene with visible effect will first be detected in the hemizygous sex (male). In lines with equal numbers of males and females, when the gene is initially present in a single female the probabifity of detection falls from 2/3 in single pair lines to 0·54 in large lines. The mean and standard deviation of time to detection are almost independent of population size, being about 4/3 and 2/3 respectively. About 98% of all detections occur within three generations, so a gene detected much later than this after the foundation of a selection line is likely to be a new mutant. Higher initial frequencies and selection favouring heterozygotes increase the chance of detection. The time taken is decreased with higher initial frequencies and increased slightly by selection favouring heterozygotes.

The genetic fine-structure of the niiA niaD gene region of Aspergillus nidulans has been studied using deletion mapping. Deletions were identified as niiA niaD double mutants and comprised 1% of spontaneous chlorate-resistant mutants. All such double mutants were shown to involve deletions and their frequency was not increased by mutagenic treatment with either N-methyl-N′-nitro-N-nitrosoguanidine or with ultraviolet light. Deletion maps of the niaD and niiA genes have been constructed. A further class of mutation was also mapped using the deletions. These crn mutations, which affect a gene whose function is as yet unknown, map on the centromere proximal side of niiA. This analysis of a eukaryote gene cluster will provide a framework upon which to base studies of the regulation of the nitrate assimilation pathway.

A new mutation, Sleek, similar in appearance to mutations at the Ta, cr and dl loci, has been investigated. It is inherited as an autosomal dominant and maps very close to dl on chromosome 13. Allelism with dl seems probable since Sleek interacts with dl but not with cr. The unusual occurrence of dominant and recessive alleles at the same locus which produce a similar mutant phenotype suggests that the locus might either code for a multimeric protein or a regulatory product.

Four recessive amino-acid-analogue-resistant mutants were isolated on a medium containing acetate as the sole carbon source and the amino acid analogues p-fluorophenylalanine and ethionine. None of the mutants showed any growth requirement. Analysis of growth on media containing an amino acid as the sole nitrogen source indicated that two mutants out of the four possess normal systems for utilization of acidic, neutral, basic and aromatic amino acids. The mutants fpa70 and fpa71 showed reduced growth on tryptophan as the sole source of nitrogen. Three new loci, identified after preliminary genetic analysis, were located on three linkage groups: one each on linkage groups I, VI and VIII.

The steps in the parasexual cycle required for genetic analysis by mitotic haploidization were readily accomplished in Aspergillus amstelodami. Balanced heterokaryons were synthesized between strains carrying complementary auxotrophic mutations and heterozygous-diploid colonies were recovered from these heterokaryons at frequencies between 1 × 10−6 and 5 × 10−6 among conidia plated. When grown in the presence of p-fluorophenylalanine or benomyl, heterozygous-diploid strains produced discrete haploid segregants. Examination of the segregation of 12 mutations among haploid segregants from ten diploids indicated that these genes were located in six linkage groups, thereby increasing the number of linkage groups recognized in this species. This haploidization analysis provided a clear distinction between linked and unlinked genes and continuation of this approach should reveal the chromosome number of A. amstelodami. Linkage at meiosis was detected between only one of five pairs of mitotically linked genes tested, emphasising the value of mitotic-haploidization analysis for assigning markers to linkage groups. A. amstelodami is amenable to both mitotic and meiotic genetic analysis and further studies should permit extensive comparisons of the genetic and biochemical organization of this species with that of A. nidulans.

Over 100 temperature-sensitive mutants of mt-h (apogamic) strains of Physarum polycephalum were isolated either by testing clones of mutagenized amoebae (ATS mutants) or by the more laborious method of testing plasmodia derived from such clones (PTS mutants). When amoebae and plasmodia of each mutant were tested for growth temperature sensitivity on different media (to give optimum growth of each phase), only 21% of 73 ATS mutants and 32% of 31 PTS mutants appeared to be temperature-sensitive in both phases, suggesting that the majority of mutants are phase-specific, as concluded from several similar studies by previous authors. When the mutants were tested on a third medium which allows growth of both amoebae and plasmodia, many of the mutants no longer had a temperature-sensitive phenotype in either phase. Among the remainder, 51% of ATS mutants and 67% of PTS mutants were temperature-sensitive in both phases. It was suggested that certain media have a remedial effect on some temperature-sensitive mutants so that the phenotype is apparently normal. Thus, the proportion of phase-specific mutants may be over-estimated if tests of temperature-sensitivity are done on the different media commonly used for culture of amoebae and plasmodia respectively. It was concluded that the most efficient procedure for isolation of temperature-sensitive mutants expressed in plasmodia is to screen clones of amoebae on a medium resembling as closely as possible that which is to be used for testing plasmodia.

Plasmodial formation in the Myxomycete Physarum polycephalum is controlled by a mating type (mt) locus. There are a number of different heterothallic mt alleles; and also a variant, mth, that allows plasmodial formation in pure clones. This paper reports an analysis of this differentiation system. The strain CL (mth) forms plasmodia in pure clones (i.e. it selfs). CL was mutagenized with NMG and 21 mutants unable to differentiate into amoebae were isolated. The mutants, together with CLd, fell into two complementation groups, twelve in difA and ten in difB. Both complementation groups are closely linked or allelic to the mt locus. difA represents a gene essential for plasmoclial formation, but it is suggested that difB represents a class of revertants to the mt2 heterothallic state. A model of the control of plasmodial formation is proposed in which the mt locus is suggested to code for the repressor of the difA gene. Genetic control is explained in terms of the dilution of allele-specific repressors.

Alloimmunization in sheep revealed an antigen specificity B1 which is also to be found in cattle sera. The molecular weight of the protein carrying B1 was greater in sheep than in cattle, whilst in both cases its isoelectric point was at pH 6·2. The electrophoretic position of the antigen suggests that the molecule carrying B1 might be a β-globulin. Family studies indicated that in both species B1 was inherited in a simple Mendelian manner, as if it were a product of a dominant allele B1 at an autosomal locus B.

The expression of the antennal homoeotic mutant, Nasobemia, is shown to be strongly dependent on nutrition. Dietary deficiencies in any of the following: thiamine, calcium pantothenate, nicotinic acid or RNA, cause a curing of the homoeotic defect. It is demonstrated that the effects of thiamine and pantothenate are through limiting acetyl-CoA synthesis and that fatty acids are required for high penetrance of the Ns gene. Similar nutritional effects are described for other Antennapedia and aristapedia alleles. The period of sensitivity to nutritional changes is defined for Ns and compared with the period of temperature sensitivity. These results suggest the hypothesis that the homoeotic mutations Antennapedia and aristapedia share some common metabolic requirements which involve synthesis of acetyl-CoA.

Some sampling properties related with the mean and variance of the number of alleles and single locus heterozygosity are derived to study the effect of variations in mutation rate of selectively neutral alleles. The correlation between single locus heterozygosity and the number of alleles is also derived. Monte Carlo simulation is conducted to examine the effect of stepwise mutations. The relevance of these results in estimating the population parameter, 4Neν, is discussed in connexion with neutralist-selectionist controversy over the maintenance of genetic variability in natural populations.

Five clones of Chinese hamster ovary cells with increased resistance to bleomycin have been isolated following ethylmethanesulphonate mutagenesis. Resistance was stable in three of the clones, but unstable in the other two. One of the stably resistant clones was cross resistant to unrelated drugs, and in contrast to the parental cells, its response to bleomycin was potentiated by tween 80. These two observations suggested a membrane alteration in the resistant clone. There was no significant difference in bleomycin-inactivating enzyme activity between the parental and resistant clones.

Three methods of artificial induction of autogamy in Paramecium caudatum were described: (1) treatment with KCl+papain, (2) treatment with KCl and then with KCl + papain and (3) ordinary mating reaction and then treatment with papain. As expected, one-to-one segregation ratios were obtained in the progeny from the parents heterozygous for the two loci: mating type and lactate dehydrogenase. A high rate of autogamy is induced by method (1), but its use is restricted to only a few clones. Autogamy is also induced at a high rate by method (2), by which the induction is more stable. Autogamy is induced at a lower rate by method (3), but this method can be widely applied to every species of Paramecium which has complementary mating types. Some exautogamous progeny become completely sterile through successive autogamy. The cause of this sterility is discussed.

Macronuclei assorting simultaneously for H, Chx, Mpr, and co, and containing only one or two copies of the HD allele produced several combinations of phenotypes at the other loci, instead of only one or two such combinations. It follows that macronuclear subnuclei, if they exist at all, must frequently exchange parts. Models involving somatic recombination, transient subnuclei, or progressive chromosome fragmentation are discussed as possible explanations.

New experimental designs have detected unexpectedly large variations in the time at which macronuclear assortment begins, and in the ratios of the stabilized products. Variation is detected between strains, and, within strains, between conjugating pairs. The Chx locus gave results ranging from late assortment (40–60 fissions) to early assortment with skewed input, indicating the existence of some relation between the parameters of input ratio and time of determination.

A mechanism for RNA–RNA splicing is proposed. A species of RNA (‘splicer’ RNA) hybridizes to precursor mRNA across the splice point. This hybridization can be with intron or exon sequences or both. The double-stranded RNA structure precisely indicates to the splicing enzymes the exact location for exon ligation.

A model for the control of gene expression is presented. The regulation of synthesis of different splicer RNAs will also control which precursor mRNA molecules are spliced. The removal of intervening sequences from a precursor mRNA molecule could be both a signal for that molecule to be transported to the cytoplasm and a means of allowing gene expression.