Activated cysts of Didinium nasutum continue excystment after treatment with actinomycin D or puromycin. Cysts treated at developmental stages 19–20 emerge holociliated and lacking the seizing organ. No effect of the antibiotics at any other stages was found, suggesting that it is at stage 19–20 that the initiation of normal ciliation and of mouth formation occurs. Treatment with other metabolic inhibitors either blocks excystment completely or has no effect on normal development depending upon dosage, suggesting that the action of the antibiotics is specific.

An attempt was made to modify a near-limit of selection response in the egg production of a closed poultry flock which had been under selection for 31 years; to this end the laying hens were subjected to weekly periods of 24-hour feed deprivation, and selected for egg number under these conditions.

In two replicate populations genetic gains of about 20 eggs were made under shock treatment to 40 weeks of age, over 5 and 6 generations of selection. These gains were also manifest under normal management.

Selection, under 24-hour starvation per week, also improved the ability of the selected populations to lay under weekly starvation periods of 33 and 48 hours respectively. Differences between selected and unselected lines increased with the more severe stress treatments. Also it was found that the difference in average egg production between 33-hour and 48-hour treatments was larger in unselected lines than in the selected ones, indicating the existence of genotype-environment interactions.

Correlated responses of reduced egg-weight and body-weight, and somewhat earlier maturity of selected lines correspond with what might be expected under normally effective selection for egg number. These adaptations however varied considerably between replicate lines.

1. Chromosome 6B of T. aestivum, to which a small segment of an Ae. umbellulata chromosome had been translocated, was examined both by interference microscopy and in stained preparations.

2. In the stained preparation a constriction was observed in the long arm of chromosome 6B and also a terminal knob that was also observed on chromosome A of Ae. umbellulata.

3. These features were also observed by interference microscopy and in addition the portion of the chromosome 6B of Transfer distal to the constriction had different phase-retarding properties.

4. It is concluded that the part of chromosome 6B distal to the constriction is the part of Ae. umbellulata chromosome A which carries the gene for the leaf-rust resistance of that species and of the wheat variety into which it was translocated.

The histidine decarboxylase (HDC) gene complex, [Hdc], comprises the structural gene for mouse kidney HDC and closely linked regulatory elements which determine enzyme concentration and its response to hormones. One of these regulatory elements, Hdc-e, determines the response (induction or repression) of kidney HDC to oestrogen. HDC is oestrogen-inducible in C57BL/10 and oestrogen-repressible in DBA/2 and C57BL/6; alleles of Hdc-e segregate in crosses between C57BL/10 and DBA/2 and between the C57BL substrains. Two different haplotypes of[Hdc] have been defined previously, B.10 (Hdc-sb, Hdc-cb, Hdc-eb) in C57BL/10 and D (Hdc-sa, Hdc-cd, Hdc-ed) in DBA/2. C57BL/6 represents a third haplotype (B.6) (Hdc-sb, Hdc-cb, Hdc-ed) which differs from both B.10 and D. Hdc-e may therefore be a component of the complex independent of Hdc-s and Hdc-c.

Recombination in a short genetic interval spanning the proximal region of chromosome 3 was studied in the regions st-in-ri-eg2-Ki-pp. Crossover frequencies in this region varied considerably in different genetic backgrounds; however, in all genotypes, the following observations were made: (1) an excess of multiple recombinant chromosomes indicative of high negative interference, was detected; (2) among the multiple recombi-nants, a positive correlation of simultaneous exchange in the most proximal and shortest adjacent genetic intervals was noted; (3) several classes of reciprocal products were not equally recovered. Three possible explanations for these results are: pre-meiotic exchange, chromatid interference and gene conversion.

The observed dependence of lacuna counts on the growth phase of colicinogenic cultures suggests a relationship between the probability of colicin synthesis, and the number of colicin factors, per cell.

The inducibility of three enzymes (β-galactosidase, tryptophanase and D-serine deaminase) has been measured at various times during the cell cycles of three strains of Escherichia coli (K12 58–161 F−, B/r F– and 15T−). In each strain sharp increases in inducibility of these enzymes occurred at characteristic periods in each cell cycle. Such increases depend on DNA replication and therefore probably reflect synchronized gene replication. It is inferred that chromosome replication in these F− strains is sequential from a fixed origin.

Infection with F′Lack+ results in an extra period of increase in inducibiity of β-galactosidase in each cell cycle. It is concluded that the F′ episome replicates once in each cell cycle at a time soon after cell separation.

Genetic mapping of suppressors of the ilv-188 mutation is described. Suppressors of this mutation have been mapped at four ochre suppressor loci: sup L, sup M, sup N and sup O. The ochre suppressor described by Brenner & Beckwith is shown not to suppress ilv-188 whereas the suppressor described by the same authors suppresses ilv-188 and may represent the same locus as supO suppressors. The mapping by P1 transduction of the sup L, sup M and sup N loci is described.

Segregation of sperm abnormality level and H-2 haplotypes was investigated in F2 hybrid males obtained from reciprocal crosses involving two B10.congenic strains carrying H-2 and the Y chromosome of Japanese wild mice: B10.MOL-OHM (H-2ωm4, 23·1% of sperm abnormalities) and B10.MOL-OKB (H-2ωm8, 11·1% of sperm abnormalities). In both types of crosses mean levels of abnormal spermatozoa were significantly higher for males typed as H-2ωm4/H-2ωm4 than for heterozygous H-2ωm4/H-2ωm8 or homozygous H-2ωm8/H-2ωm8. These results suggest that the gene for high sperm abnormality is linked to H-2 complex of the B10.MOL-OHM strain.

The tetracycline-resistance determinants of R-factors from different compatibility groups have been tested in Escherichia coli K12 and phenotypically classified into two major classes. Class I determinants confer high-level resistance to tetracycline (> 100 μg/ml) and moderate resistance to minocycline (5–25 μg/ml) while those of Class II gave moderate resistance to tetracycline (50–70 μg/ml) and low resistance to minocycline. Each class was subdivided because of variation in resistance profiles and in the abilities of tetracycline and minocycline to induce increased resistance. Strains carrying two compatible Tetr R-factors of the same or different phenotypic groups did not show increased tetracycline resistance.

An experimental evaluation of Jódar & López-Fanjul's (1977) theoretical treatment of the optimum proportions to select when the numbers of males and females scored are unequal has been carried out for sternopleural bristle number in Drosophila melanogaster. Three different values of the sex-ratio (c) were considered (c = 1, 4 and 10) for the same total number of individuals scored per generation. For each c value two types of line were selected with proportions theoretically maximizing the response to be attained after 10 or 20 generations, respectively. Thus, there were six types of lines and each type was replicated sixfold. A good qualitative agreement was found between the observed and the expected rankings of the different types of selected lines at the designated generations.

The total genomic copy numbers of ten families of transposable elements of Drosophila melanogaster in a set of ten isogenic lines derived from a natural population were estimated by slot-blotting. The numbers of euchromatic copies of members of each family were determined for each line by in situ hybridization of element probes to polytene chromosomes. Heterochromatic numbers were estimated by subtraction of the euchromatic counts from the total numbers. There was considerable variation between element families and lines in heterochromatic abundances, and the variance between lines for many elements was much greater for the heterochromatin than for the euchromatin. The data are consistent with the view that much of the β-heterochromatin consists of sequences derived from transposable elements. They are also consistent with the hypothesis that similar evolutionary forces control element abundances in both the euchromatin and heterochromatin, although amplification of inert sequences derived from transposable elements may be in part responsible for their accumulation in heterochromatin.

The genotypes are given of over 4500 asci showing aberrant segregation at the grey (g) spore colour locus in Sordaria fimicola from crosses, with outside markers present, between g locus mutants and wild type, or involving pairs of g locus mutants that differed in spore colour (g locus mutants range from grey to hyaline). This colour difference enabled asci with aberrant segregation of the mutant spore colours to be detected in the two-point crosses, as well as those with wild-type (black) spores. From a total of 21 one-point crosses involving altogether seven primary and two secondary g locus mutants, 2700 aberrant asci have been grouped into the 14 ascus genotypes that can be recognized, and in the two-point crosses 1830 asci (1044 fully scored) from 10 pairwise combinations of the mutants have been classified into the 150 ascus genotypes that can be distinguished.

Testis size was investigated in two inbred strains of mice, BALB/c/Ola and CBA/Gr, at different ages. BALB/c mice were found to have the larger testes from day 14 of embryonic development onwards. Body weights of the two strains differed to a lesser extent. The differences in testis weights were analysed post-natally at 2, 4, 6 and 8 weeks in the two strains, F1s, F2s and backcrosses. Testis size was found to be affected by the origin of the Y chromosome, the X chromosome, the autosomes (and/or pseudoautosomal regions) and by maternal factors. At 8 weeks of age, the Y-chromosomal effect was at its strongest, and the X-chromosomal effect was at its weakest, while the maternal effect had vanished. It is postulated that non-Y-chromosomal factors which modify testis size may affect the gonads in both sexes. The possibility is discussed that loci affecting gonad size may be identical with testis-determining factors.

1. The effect of genetic sampling, when this sampling is without replacement, on variation in gene frequency is studied, and equations describing the genetic drift are derived. The effective size turns out to be about one greater than under sampling with replacement.

2. The relation between ‘spread of genes’ and genetic drift is worked out.

3. The University of Queensland control poultry flock is analysed by these methods.

4. The design of control populations is discussed with particular reference to the relative importance of genetic drift and phenotypic sampling.