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.

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.

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.

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.

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.

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.

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.

The heritability (h2) and frequency of the neural tube closure defects, cranium bifidum (CB) and spina bifida (SB), have been estimated for a population of 9-day-old hamster embryos through half-sibling analysis. The average frequency of the total affected embryos per litter is approximately 17% while the pooled estimate for h2 based on between sires and between dams within sires components was 4%. This value points to the importance of environmental factors in contributing to the variance in defect frequencies observed within this population.

A map of the seventeen R.SmaI sites within the 33·4 kb transfer region of plasmid F has been obtained. The number of R.SmaI sites contrasts sharply with the few sites for other restriction endonucleases with hexanucleotide specificity. Location of the R.SmaI sites in γtra transducing phages allowed physical location of lambda insertions into the traB, traF and traH genes.