The presence of a gene designated bncA which produces binucleate and trinucleate conidia in A. nidulans alters the instability of disomics, diploids, and strains with chromosome duplication. In disomics, the gene bncA increases instability. In duplicate and diploid strains, the bncA gene reduces instability by acting as a partial stabilizer. In the strain with chromosome duplication, the bncA gene produces increased percentages of bi- and trinucleate conidia, a fact that may be interpreted to be due to the larger conidial volume of this strain or to the combined effect of bncA and of the strain, which normally already exhibits a small amount of binucleate conidia.

Plants of two rye genotypes, one highly inbred, the other not, were grown with varying amounts of available mineral phosphate. In two experiments, one using culture solutions, the other a field experiment, the results show an effect of phosphate treatments on mean chiasma frequency at first metaphase in meiocytes. Plants given increased amounts of phosphate showed an increase in chiasma frequency. A similar effect of phosphate on chromosome size and mass at mitotic metaphase in meristems is known and there may be a direct link between chromosome size and chiasma frequency.

Some previous attempts to produce tetraploids experimentally have resulted in a proportion of treated embryos becoming 2n/4n mosaics at a frequency which may be as high as 20%, when using cytochalasin B as a fusigenic stimulus and cytogenetic techniques to identify putative tetraploid embryos. To investigate the possible occurrence of 4n/2n mosaicism, tetraploid embryos were produced by electrofusion, a process which allows adjacent blastomeres at the 2-cell stage to fuse following exposure to electric field pulses. Embryos used for electrofusion were hemizygous for a transgene consisting of approximately 1000 copies of the mouse β-globin gene. After in situ hybridization, one hybridization signal is expected per diploid genome. Tetraploid cells in 7·5-, 8·5-, 9·5- and 10·5-day-old conceptuses were distinguished from diploid cells by performing in situ hybridization on histological sections. The frequency of nuclei with two hybridization signals in the ‘hemizygous’ tetraploid embryos was compared to diploid embryos which were either hemizygous or homozygous for the β-globin transgene. Comparison of the frequency of nuclei with two hybridization signals between tissues of ‘hemizygous’ tetraploid conceptuses and homozygous diploid conceptuses showed no significant difference, which implies that the tissues in the tetraploid conceptuses were uniformly tetraploid. No evidence was found to suggest that electrofusion results in 2n/4n mosaicism.

Spleens with black pigment in them were found in 4–57% of mice from 17 stocks, all sublines of C57BL or with significant C57BL ancestry. Splenic lipofuscinosis was absent from 16 stocks, including three C57BL/6By × BALB/cBy recombinant - inbred lines. Progeny testing showed all C57BL/6J mice to be equally likely to develop black spleens. The penetrance of lipofuscinosis differed between sublines but not between the sexes or between laboratories. Susceptibility to lipofuscinosis showed dominant, autosomal, inheritance in F1 hybrids. Observations on backcrosses and on recombinant inbred lines and their intercrosses indicated the existence of two genetic factors. Splenic lipofuscinosis was prevented either by a deficiency of melanin or by homozygosity for a non-C57BL allele close to the c-p region of chromosome 7. The presence of a C57BL allele at a locus on another chromosome is necessary for lipofuscinosis to occur.

R-factor 1818, which is refractory to curing by acridines or ethidium bromide from Escherichia coli K 12 and Klebsiella aerogenes 418 hosts, was eliminated from thymineless mutants of these organisms under conditions of thymine starvation. Neither R-7268 nor R-TEM could be eliminated by this method. The concurrence of R-TEM with R-1818 inhibited the curing of R-1818, whereas R-7268 had no effect on R-1818 elimination

Genetic consequences of non-random segregation of chromosomes are discussed. In the experimental part preferential segregation of two linkage groups in Ascobolus immersus is described. This was established in several crosses involving mutants 164, XXVI and 231 (all in the same linkage group) and mutant 726.

An experiment was carried out to test the effect of varying selection intensity on the response to individual selection with a fixed number of individuals scored per generation. The selected trait was egg laying of virgin females of Tribolium castaneum scored from the 7th to the 1 lth day after adult emergence. Five different selected proportions of females were considered (5, 10, 20, 33 and 50%) and each treatment was represented by two replicate lines. Control lines were maintained throughout the experiment. The lines selected at the lowest proportions (5 and 10%) led to the largest initial gains, but the largest final gains were achieved, by the lines where the proportions selected were 10 and 20%, in agreement with theory. Lines where the proportion selected was 50% gave the lowest rate of response over the period considered (32 generations). A good agreement was found between predicted and realized short-term responses to selection. Prediction at later stages of selection deteriorated in the most strongly selected lines mainly due to the levels of inbreeding attained.

Saline loading caused an increase in glomerular filtration rate in RAP mice but not in CBA mice. On the basis of progeny testing of F 2 hybrids, backcrosses to CBA, and inbred lines derived from backcrosses to RAP, it was concluded that the difference between the strains RAP and CBA was probably largely accounted for by a single gene locus. The use of this gene in physiological investigations of the control of glomerular filtration rate is suggested.

Isolation and characterization of five new nuclease (nuc) deficient mutants of Neurospora have been described. The new mutants are unable to utilize nucleic acids as the sole phosphorus source and possess growth characteristics similar to those nuc (nuc-1 and nuc-2) mutants described previously. Two new mutants (nuc-4 and nuc-5) were able to use RNA or predigested DNA (but not intact DNA) as phosphorus source and showed temperature sensitive growth at 37 °C. Based on the data from complementation and genetic analyses the five new nuc mutants (nuc-3, nuc-4, nuc-5, nuc-6 and nuc-7) were found nonallelic to each other and to previously described nuc (nuc-1 and nuc-2) mutants; the new nuc mutants mapped to the right of arg-12 on linkage group II. On biochemical analyses, these nuc mutants were found to possess a lower level of extracellular nucleases and alkaline phosphatase as compared to the wild type strain. The ds DNase activity of the new mutants was only about 2–12% of that of the wild type strain; thus, the low level of these extracellular enzymes in the nuc mutants causes their inability to utilize nucleic acids as the sole phosphorus source. Wild type levels of these enzymes were restored in the complementing heterokaryons capable of full growth on the DNA medium. Data from intercrosses, mutagen sensitivity and spontaneous mutation-frequency studies (as discussed in a subsequent paper) indicated the involvement of the nuc genes in DNA repair and recombination.

Proposals are made for symbols consisting of a primary gene symbol of lower case italic letters referring to the mutant phenotype (three letters for new symbols, old symbols unchanged), an italic capital locus specific letter (or a hyphen in mutants not yet tested for allelism) and an italic mutant number. Optional superscripts are recommended to convey additional information. Further proposals relate to symbols for chromosome aberrations and to strain numbers.

Mutations in at least five different genetic loci cause some degree of resistance to Chloramphenicol, Tetracycline or both antibiotics in Escherichia coli K12. Two have been identified as chromosomal genes: cmlA near λ prophage and cmlB near pyrD. A third, giving mucoid antibiotic-resistant mutants, is probably capR (Ion) near lac. Another group of mucoid mutants gives marked increases in resistance to Tetracycline and in sensitivity to Puromycin; these mutants do not occur at capR. Although all the mutations have rather small effects on resistance level, the patterns of resistance to the three antibiotics appear to be characteristically different for mutations at different loci.

The genetic elements promoting conjugation associated with transmissible drug resistance are of two or more kinds. Experiments with phage specific for F+ bacteria have shown that one large class differs from F essentially only in the production of a cytoplasmic repressor which limits the synthesis of the specific F pilus, whether by the RTF itself or by F when this is present in the same cell.

When grown on high-phosphate medium, the wild-type strain 74A of N. crassa synthesized two acid phosphatases, as shown by DEAE -cellulose chromatography. These purified enzymes showed heterogeneity on PAGE, low specific activities towards PNP-P, molecular weight values of at least 300000, no deviation from Michaelian behaviour, and great stability in 50 mM sodium acetate buffer, at pH 5·4, when kept at 54 °C. These acid phosphatases were synthesized in reduced amounts or not at all when the mould was grown under conditions of phosphate starvation, indicating that the level of phosphate also regulates the synthesis of the high molecular weight enzyme forms. When grown on high phosphate medium, the pho-3 mutant strain also synthesized two acid phosphatases, whose purified enzymes showed no pronounced differences when compared to those synthesized by the wild-type strain in terms of electrophoretic analysis, specific activities towards PNP-P, molecular weight values, and Michaelian behaviour. However, one enzyme form had a higher Km value and a lower heat stability than the corresponding enzyme of the wild-type strain. Even though the pho-3 locus might not be responsible for an alteration in the primary structure of the repressible acid phosphatase, it seems clear that the enzymes synthesized by the mould grown on low-or high-phosphate medium must share some structural features. Thus, the drastic differences observed in the molecular properties of the enzymes synthesized by the mould grown under conditions of phosphate starvation as opposed to phosphate repression might be due to an effect exerted by the level of inorganic phosphate in regulating the translation, post-translational modifications and/or excretion, but not necessarily the gene-directed synthesis of distinct mRNAs.

Wild-type strains of Coprinus lagopus are sensitive to para-fluoro-phenylalanine and ethionine. Resistant mutants to these two analogues are known but all these mutants are recessive in a heterozygous dikaryon except for F7 (pfpr-3) which is semi-dominant. Resistance to two other analogues, however – canavanine sulphate and azetidine-2-carboxylic acid – were found to be wild-type features. One strain of C. lagopus sensitive to canavanine was identified. Selection for canavanine resistance in monokaryons always yielded only dominant resistance, while selection for para-fluorophenylalanine resistance in monokaryons gave only recessive resistance. Canavanine-resistant mutants were due to a single gene mutation which, like the wild-type resistance, were dominant in heterozygous dikaryons. The wild-type resistance was also dominant in a diploid but the mutant resistance was recessive. Selection for resistance to para-fluorophenylalanine in auxotrophically balanced dikaryons resulted in the identification of two new loci (pfpr-10 and pfpr-ll), and two specific dominance modifiers (mod+-10 and mod+-ll). In the absence of the specific modifier, pfpr-10 and pfpr-ll were recessive while, in the presence of even one dose of the specific modifier, resistance was dominant in the dikaryon. The pfpr-10 and pfpr-ll even in the presence of two doses of modifier were fully recessive in the diploid. The action of the modifier genes and the reversal of dominance in dikaryon and diploid is discussed in terms of negative complementation in an oligomeric product of the pfpr gene and localized translation of the relevant mRNA in the cell with the modifier acting as a reinforcer of localization.

The esterase 6 (Est-6) locus in Drosophila melanogaster is located on the third chromosome and is the structural gene for a carboxylesterase (E.C.3.1.1.1) and is polymorphic for two major electromorphs (slow and fast). Isogenic lines containing X chromosomes extracted from natural populations and substituted into a common genetic background were used to detect unlinked factors that affect the activity of the Est-6 locus. Twofold activity differences of esterase 6 (EST 6) were found among males from these derived lines, which differ only in their X chromosome. These unlinked activity modifiers identify possible regulatory elements. Immunoelectrophoresis was used to estimate quantitatively the levels of specific cross-reacting material in the derived lines. The results show that the variation in activity is due to differences in the amount of EST 6 present. The data are consistent with the hypothesis that there is at least one locus on the X chromosome that regulates the synthesis of EST 6 and that this regulatory locus may be polymorphic in natural populations.