An approximate expression, is derived for the rate of change in frequency of an inversion introduced at a low initial frequency into a multi-locus system at equilibrium under recombination and selection. It is shown that this expression gives accurate predictions of the rate of progress of the inversion, even if the initial population is perturbed somewhat from equilibrium. Extensions to the cases where there are sex differences in recombination and selection are considered. An implication of the results is that selection pressure for newly arisen inversions depends on the existence of a stable equilibrium with linkage disequilibrium. The expected chance of survival of a new inversion in a large population is shown to be approximately one half the square root of the loss in fitness due to recombination.

1. Sensitivity of Drosophila melanogaster male germ-cells to chromosome breakage by X-rays has been measured by the structural changes found in the salivary gland chromosomes of larvae from irradiated fathers and untreated mothers. The genetical effectiveness of irradiation on the same males was measured by the frequency of sex-linked lethals.

2. Assessed by the overall percentage of germ-cells carrying structural changes, sensitivity follows the well-known pattern: it is highest in spermatids and decreases over spermatozoa and spennatocytes to spermatogonia.

3. This overall sensitivity has been analysed on three levels:

(a) The proportion in successive broods of ‘positive’ males, i.e. males which respond to irradiation. The results indicate that spermatozoa and spermatids in most or all males are equally sensitive to X-rays, while in regard to spermatogonia the males represent a heterogeneous population consisting of positive and negative males. This is fully confirmed by statistical analysis which was carried out by Dr B. Woolf.

(b) The proportion of ‘sensitive’ germ-cells per positive male, i.e. germ-cells carrying at least one structural change. While the proportion of sensitive cells per positive male is about 1:3 for spermatozoa, spermatocytes and spermatogonia, it is higher for spermatids. This is the main reason for the observed increase in overall sensitivity from spermatozoa to spermatids and for its subsequent drop in spermatocytes. On the contrary, the drastic drop in overall sensitivity from spermatocytes to spermatogonia results entirely from a drop in the proportion of positive males.

(c) The distribution of breaks in sensitive cells. The clustering of breaks in sensitive cells is highest in spermatogonia, somewhat lower in spermatids, and lowest in spermatocytes and spermatozoa. It indicates that spermatogonial cells are a heterogeneous population, consisting of cells that are either highly sensitive to X-rays or not sensitive at all.

4. The fact that the sensitivity of spermatogonia varies both between and within individual males is tentatively attributed to the presence or absence of spermatogonial mitoses.

The experiments reported in this paper are primarily addressed to test the hypothesis of overcompensation; i.e. whether polymorphic populations exploit limiting environmental resources better than populations uniform for a single genotype. Overcomposition is an ecological consequence of some models of frequency-dependent selection. Secondarily, the experiments investigate whether overdominance exists at the Mdh-2 locus in Drosophila pseudoobscura.

Two types of experimental populations are established: ‘low-variability’ populations, in which all flies in a culture are offspring from only two laboratory strains; ‘high-variability’ populations, in which the flies in a culture are derived from 20 different strains. However, the overall degree of individual heterozygosity is the same in both types of populations. Three kinds of populations with respect to the Mdh-2 locus are established within each type; two are homozygous for either the 100 or the 112 allele, the third is heterozygous. A fourth kind of population exists among the high-variability populations; namely, populations in which all three Mdh-2 genotypes are present. The experiments are done at two densities; one quasi-optimal, the other highly competitive.

Populations with high overall levels of genetic variation consistently produce more flies than low-variability populations. The differences are significant at the low, but not at the high, density. Moreover, populations polymorphic for the Mdh-2 locus generally produce more flies than populations having only one Mdh-2 genotype. At high density, the Mdh-2 polymorphic populations have greater productivity than populations with anyone of the three genotypes, and the differences are statistically significant when the polymorphic populations are compared with either one of the two homozygotes or with the average of all three genotypes. In brief, overcompensatory effects – which may account for frequency-dependent selection – are observed in the experiment and may be a common phenomenon in nature.

Populations in which all individuals are heterozygous at the Mdh-2 locus produce in every case more flies than populations with only homozygotes for one or the other allele. The superiority of the heterozygotes is statistically significant for all comparisons at low density, but at high density it is statistically significant for the comparison with the homozygote rarer in nature and only in low variability populations.

Twelve inbred lines derived from an M′ strain of Drosophila melanogaster were used to study the repression of P-element-mediated hybrid dysgenesis. Initial assessments indicated that the lines differed in the ability to repress gonadal dysgenesis, and that this ability was highly correlated with the ability to repress snw hypermutability. Later assessments indicated that most of the lines with low or intermediate repression potential evolved to a state of higher repression potential; however, Southern analyses failed to reveal significant changes in the array of genomic P elements that could account for this evolution. In addition, none of the lines possessed the incomplete P element known as KP, which has been proposed to explain repression in some D. melanogaster strains. One of the lines maintained intermediate repression potential throughout the period of study (52 generations), indicating that the intermediate condition was not intrinsically unstable. Genetic analyses demonstrated that in some of the lines, repression potential was influenced by factors that were inherited maternally through at least two generations; however, these factors were not as influential as those in a classic P cytotype strain. Additional tests with a dysgenesis-inducing X chromosome called T-5 indicated that repression itself was mediated by a combination of maternal effects and paternally inherited factors that were expressed after fertilization. These tests also suggested that in some circumstances, the P transposase, or its message, might be transmitted through the maternal cytoplasm.

The duration of meiosis and its stages has been estimated in Hordeum vulgare variety Sultan grown at 20·0 °C with continuous illumination. The results are in good agreement with a previous estimate of the duration of meiosis and some of its stages in barley. The duration in barley does not differ greatly from that predicted from the previously calculated regression of DNA amount on meiotic duration in diploid plant species. As in other cereal species, meiosis is comparatively short.

Analysis of the inheritance of the three bar-properties of bacteriophage T4: exclusion of T2 from the progeny of crosses, glucosylation of the hydroxymethylcytosine (HMC)moiety of the DNA according to T4, and plating with large plaques on E. coli K strains, was carried out by means of marker rescue from T4 by T2 on E. coli K (λh) as a selective indicator. Five of the strains isolated plated with large plaques on K (λh), but did not exclude T2 and showed T2 glucosylation; plating on E. coli K (λh) was found to segregate from the other two bar-properties. The sixth isolate showed, in addition to plating with large plaques on K, partial non-excludability by the parental T4 and T4 glucosylation of HMC. If partial non-excludability is the result of T4 glucosylation, the role of the additional glucose substitutions might be a protective effect on the DNA against the exclusion factor of T4. This proposal is supported by the analysis of the progeny from a single burst from a cross of T4 and T2. The following T2 genes were partially excluded: host-range, no exclusion of parental T2, sensitivity to ultraviolet, and limited plating efficiency on E. coli K (λh). The exclusion factor of T4 is not transmitted to all progeny and does not behave like a bar-property. Only resistance to exclusion and T4 glucosylation were transmitted to all twenty-seven progeny of the single burst. The elimination of sensitivity to exclusion and T2 glucosylation is explained by assuming that the recombinant class with the exclusion factor of T4 and T2 α-glucosylation will exclude itself and be suicidal upon infection of a new host. Exclusion and differential glucosylation are discussed with regard to restriction and modification, respectively.

Evidence for close genetic linkage between the structural loci for serum albumin (Alb) and serum vitamin D binding protein (Gc) in chicken is presented. The results are based on a study of a single sire family comprising 36 informative offspring. No recombinants have been observed. It is concluded that this linkage in the chicken is homologous to the close linkage of the albumin and Gc loci reported in man and the horse. Thus, this linkage group has most probably been conserved for at least 300 million years.

Michie's hypothesis of ‘affinity’ is invoked to explain how fertility might theoretically be achieved in a female mule or hinny. Chromosomally balanced haploid gametes could be produced in these inter-specific hybrids by movement of centromeres of similar ancestry to opposite poles at anaphase I of meiosis. Male hybrids could probably never be fertile on account of the low numbers of spermatozoa produced; females, on the other hand, might occasionally achieve fertility by the ovulation of a chromosomally balanced egg.

Male mice carrying one complete t haplotype transmit it to virtually all offspring. This implies that either the sperm carrying the t haplotype have an advantage in fertilization, or that the sperm not carrying a t haplotype are dysfunctional. To distinguish between these possibilities, epididymal sperm from tw32/ + and congenic + / + males were artificially inseminated into the uterus of T / + females, and the transmission ratio determined from the phenotype of the offspring. After artificial insemination (AI) of sperm from tw32/ + males, the mean ratio was 0·95 (3 experiments, 5 litters), demonstrating that the transmission ratio remains high after AI into the uterus. After AI of a mixture of equal numbers of motile sperm from tw32/ + and congenic + / + males, the mean ratio was 0·22 (6 experiments, 8 litters). These results suggest that sperm carrying a t haplotype from t/ + males are not superior to sperm from + / + males, and therefore imply that sperm not carrying a t haplotype from t/ + males are dysfunctional.

Several genetic mosaics for larval body marking of the silkworm, Bombyx mori, have been induced by X-ray irradiation. It is hypothesized that the occasional loss of chromosomal fragments carrying the genes for body marking during development may give rise to this type of mosaicism. Using pulsed field gel electrophoresis (PFGE), we find that a DNA molecule of about 2·5 megabases (Mb) is present in one type of mosaic (mottled striped strains pSm788 and pSm872), and not in any other strain. This DNA fragment hybridizes strongly with some chorion genes which are less than 6·9 cM away from the ps locus, and hence it corresponds to a chromosomal fragment containing genes for both striped marking (ps) and the chorion. In the non-mottled ps strain, the phenotype before X-ray irradiation, no band was detected either on a PFGE gel or after hybridization with the chorion probe. These results suggest that the mottled ps strains carry short chromosome fragments which are lost differentially during cell divisions.

The isolation of a temperature sensitive pts I mutant which fails to utilize lactose provides strong evidence that Klebsiella strain CT-1 utilizes lactose via a phosphoenolpyruvate dependent lactose-phosphotransferase system (PTS-lac). We designate this lactose utilization system elu for evolved lactose utilization. Analysis of a series of Lac− mutants identifies two genes, eluA and eluB, whose function is required for lactose utilization by this pathway. The functions specified by these genes are not known, but neither locus specifies the hydrolytic enzyme phospho-β-galactosidase. A mutant of CT-1, strain RPD-2, exhibits a half-maximal growth rate at a lactose concentration 40 fold lower than that of strain CT-1; and it has a Km for lactose uptake that is 40 fold lower than that of strain CT-1. That mutation defines the locus eluC, which is assumed to specify the enzyme Il(lac) of the PTS-lactose system. From the observations that (i) cellobiose induces the phospho-β-galactosidase enzyme, (ii) pregrowth in cellobiose dramatically reduces the growth lag when cells are shifted into lactose minimal medium, (iii) eluB mutants exhibit a growth lag when shifted into cellobiose minimal medium, and (iv) lactose induces a phospho-β-glucosidase enzyme; we speculate that the phospho-β-glucosidase enzyme is the same enzyme as the phospho-β-glucosidase that normally functions in cellobiose metabolism. We conclude that the original mutation that allowed CT-1 to utilize lactose was a regulatory mutation that permitted inducible expression of the eluC gene.

The position and orientation of an interchange quadrivalent in flattened lateral views of metaphase I were studied in inflorescences of an interchange heterozygote of Allium triquetrum showing high, medium and low alternate orientation frequencies. As the overall frequency of alternate orientation changes in these different inflorescences so the positioning of the alternately oriented quadrivalents changes, in that it becomes more like that of the adjacent quadrivalents. The present data argue for the biological significance of the position/orientation data, and imply the presence of two components of alternately oriented quadrivalents with different positional characteristics.

The normal response to injection of noradrenaline (NA) in newborn lambs is an increase in metabolic rate and rectal temperature, due to the stimulation of non-shivering thermogenesis. In a previous study 6 out of 7 lambs born to a sire previously selected for low resistance to cold failed to show this characteristic response and were termed non-responders. The sire, 2 of his male offspring and 6 sires selected randomly from the flock were mated to several ewes and the response to NA stimulation recorded in 116 newborn lambs. Control sires produced only normal responder lambs whereas the remaining sires all produced both responder and non-responder lambs. The proportion of non-responders was not significantly different from 0·50, which is consistent with a dominant major gene. Analysis of the quantitative traits, peak metabolic rate and peak rectal temperature following NA injection confirms that a major gene is segregating in the study population, but is unable to distinguish between dominant and recessive modes of inheritance. Absence of non-responders in other studies suggests that dominance is more likely. This new genetic resource could aid our understanding of brown adipose tissue metabolism and the effect of catecholamines on metabolic pathways.

A new developmental mutant is described in Xenopus laevis (Amphibia, Anura): muscle opacity (mo). Homozygotes die at larval stage 48. The underlying defect, visible at stage 47, is a degeneration of cephalic musculature. The mo gene is linked to the rusty locus, with a map distance of 6·1%.

Contrary to previous reports, evidence has been presented which indicates that Xan-b61, a Xantha mutant of barley, does not have an auxotrophic requirement for leucine. It has been shown that this mutant, when grown under appropriate conditions in the absence of leucine, will synthesize leucine and will continue to grow for an extended period of time.

A large outbred population of Drosophila melanogaster was subjected to artificial selection on lipid and glycogen storage. In three separate experiments, two replicates underwent sib selection for both increased and decreased storage. In the first study, flies were selected on the basis of total triacylglycerol for ten generations. This experiment resulted in no significant direct response, but there was a significant change in total body weight, underscoring the importance of concern for the allometric relationship between body weight and lipid content. In the second study, selection was performed for 15 generations on the percentage of body composition that was triacylglycerol. A significant direct response was obtained, and the two replicates revealed heritability estimates of 0·40 and 0·43. The third study selected glycogen content for 15 generations, and produced a significant response with heritabilities of 0·25 and 0·31. A series of 12 biochemical and enzyme kinetic traits was examined at five generation intervals in all experiments, and a number of correlated responses were detected. The results are interpreted with respect to the evolutionary constraints on energy storage evolution and the genetic basis of the allometric relationship between body weight and fat content.