Seventeen non-motile strains of Chlamydomonas reinhardii isolated by Dr Lewin and thirty-six newly isolated strains have been examined in the electron microscope for structural abnormalities of the flagella. Fourteen of them have straight, paralysed flagella in which the central fibres are replaced by an irregular core of disorganized material. The fourteen mutations map at four unlinked loci. Some of them are leaky; light and electron microscope observations on leakiness are described. In the former case leakiness is measured by the proportion of motile cells, and in the latter by the proportion of intact centre fibres seen in transverse sections. The degree of leakiness is to some extent characteristic of particular loci.

A partial suppressor of some of these mutations has been isolated which acts on all the mutant alleles at two loci and to a much lesser extent on four out of five alleles at a third locus.

A number of Hfr strains have been studied as to the action of radiations on the transmission of their genetic material to female cells. They form two classes, one in which irradiation produces only lesions that prevent transfer, and one in which new modes of chromosome transfer seem to be induced. One group of strains, derived from a common F+ male, appears to give rise to the same induced mode of transfer, while no correlation could be found in the other Hfr's studied.

A technique based upon auxotrophic mutants and recessive suppressor genes has been used to select for somatic recombinants in the dikaryon of Coprinus lagopus. Both haploid and disomic nuclei with a reassortment of chromosomes from the two component nuclei of the dikaryon were found. The presence of the disomics indicates that the recombination process is one of fusion of nuclei with a gradual haploidization by loss of chromosomes at mitotic divisions. An extensive test for crossing-over in the A chromosome failed to give an unambiguous crossover recombinant, but it produced a stock with a normal A chromosome and a fragment of the chromosome.

Curtailed (Tc) is a new T-allele which leads to complete or near-complete absence of the tail in heterozygotes, apart from a small caudal filament. Other heterozygous defects include absence of the axis odontoid process and of the nuclei pulposi, a tendency to have rib and vertebral fusions and a slight decrease in the average number of presacral vertebrae. Tc is lethal when homozygous, causing similar but more extreme defects than T, with absence of all limb-buds and non-closure of the neural folds. A comparison of Tc with other mutants suggests that the anomalies are mainly the result of more severe effects on the notochord-mesoderm system than in T. Differences between the genetic behaviour of T, Tc and Th are discussed.

A close examination of reports on circling mutants of the mouse suggested that the commonly held view that the abnormalities of the inner ear are responsible for the waltzing syndrome was probably without foundation. It was thought that a study of kinky (Fuki/+) mice might resolve the doubt, because this mutant has the widest range of abnormalities of the inner ear and behaviour. The results showed that correlation between the two types of abnormalities, although high, was far from complete. This is interpreted as signifying that they are related collaterally rather than lineally. It is argued that they both originate in some primary abnormality of the central nervous system, and circumstantial evidence is presented in support of this argument. It is further suggested that the mode of gene action is essentially similar in all circling mutants, that is the abnormalities of the inner ear are consequent on some early abnormality of the neural tube and the ganglia in the region of the otic vesicle.

1. This paper describes further investigations of the high sternopleural chaeta-number lines of Drosophila melanogaster established by directional selection by Thoday & Boam (Genet. Res. 2, 161). The lines are vg 4 with a mean of 35·6 and vg 6 with a mean of 39·2 chaetae per fly.

2. Two locatable polygenes, 3a and 3b, distinguish the line third chromosomes from those of Oregon inbred (mean about 20·5, an ancestor of all the lines). These two genes are both located between the markers h and eyg and do not interact.

3. There is one locatable polygene at 41·1 ± 1·7 centiMorgans distinguishing the line second chromosomes from those of Oregon. There is no evidence that this gene is a linked complex, and, if it be a linked complex, it is unlikely to occupy more than 2 map units of the second linkage group. It interacts strongly and positively with the gene 3a.

4. These three genes account for 80% of the genetic variance of the vg 4 × Oregon F2.

5. Two separate regions at 2·4 ± 0·5 and 50·5 ± 0·9 centiMorgans distinguish the vg 6 × chromosome from that of Oregon. They do not appear to interact. Together they interact strongly and positively with gene 3a.

6. These five genes account for 87·5% of the chaeta-number difference between vg 6 and Oregon.

7. The locatable polygenes on chromosomes II and III each have qualitatively distinguishable developmental effects.

8. It is pointed out that, though the genetics of these lines may be unusually simple, the results indicate that attempts to locate specific genes and study their individual effects should be made more often by students of continuous variation. Since the location of the polygene in chromosome II was done using marker genes 45 map units apart, such studies may be practicable even in species whose linkage groups are much less well marked than those of Drosophila melanogaster.

1. The breeding behaviour of the three tetraploid Rubus species R. caesius, R. calvatus and R. laciniatus (2n = 28) has been investigated.

2. Megaspore mother cells of all three species always undergo a normal meiosis and embryo-sac formation is of the Polygonum type. Egg cells have fourteen chromosomes.

3. There is no evidence for the production of either aposporic or diplosporic embryo-sacs as has previously been assumed.

4. The proportion of sexual and apomictic progeny differs in the three species and, in R. laciniatus, varies according to the chromosome number of the pollinating parent.

5. The apomictic progeny are produced by diploidization of the reduced egg cells. These diploidized egg cells can subsequently be fertilized in R. laciniatus.

6. The versatility in the breeding behaviour of these species is explained on the basis that only one type of embryo-sac is formed and that the developmental behaviour of the egg cell is conditioned by the chromosome number of the pollinating parent. Apomixis in these species is not a consequence of a breakdown of meiosis.

1. The rate of hydrolysis by sheep plasma of some carboxylic and phosphate esters has been determined for a random flock, and for a flock previously selected for its ability to hydrolyse di-(2-chloroethyl) aryl phosphates.

2. A discontinuous variation in hydrolysis rate was found with all substrates tested and, using combinations of substrates, six types of plasma could be distinguished, each type having a different pattern of esterase activity.

3. The most useful substrates for distinguishing between phenotypes were 1-naphthyl acetate and 4-ethoxycarbonylcoumarin-7-yl acetate. Three rates of hydrolysis were possible for each of these esters, and the highest rate for one was invariably combined with the lowest rate for the other, although the converse did not apply.

4. To explain these results, and those of Lee (1964), it has been postulated that the quantitative production of esterase hydrolysing 1-naphthyl acetate is governed by the presence of an allele, termed Esa, at a particular gene locus. Similarly, the production of esterase hydrolysing 4-ethoxycarbonylcoumarin-7-yl acetate is determined by allele Esb, and where neither substrate is attacked the presence of a third allele, Esc, is proposed.

5. The hydrolysis rates of haloxon, 1-naphthyl butyrate and 4-nitrophenyl butyrate varied in the same way as that of 1-naphthyl acetate, whereas the hydrolysis of indophenyl acetate followed the same pattern as that of 4-ethoxycarbonylcoumarin-7-yl acetate. The variation in hydrolysis rate of Coroxon could be explained by assuming that Esa and Esb are equal in this respect.

6. A mating experiment produced results which were in accordance with the genetic hypothesis, but were too few in number to provide confirmation.

7. The genetic marking of six types of sheep is possible, utilizing the variation in plasma A-esterase activity.

1. Six initially identical populations of Drosophila pseudoobscura have been maintained in population cages for 7 years. Two populations have been kept at 16°C, two at 25°C, and two at 27°C.

2. One and a half years after the start, there was no significant genetic divergence in body size among the populations. When the populations were about 6 years old, a striking genetic divergence in body size was found. The genetic difference between the populations having the smallest and the largest mean sizes is over half the total phenotypic change in size between the two extreme temperatures at which the populations were kept. The populations kept at the lower temperature have genetically larger flies than the populations kept at the higher temperatures.

3. Accompanying the changes in body size were changes in the time of develop ment from egg to adult, the faster developers being the larger flies.

4. The F1 hybrids from crossses between Vetukhiv's populations showed non-additivity of the genes for body size, the F1's in most cases being significantly larger than the midparent. There was no change in variability of body size in the F1 or F2 hybrids.

5. The temperature-directed selection for body size found in Vetukhiv's experimental populations may well be similar in kind to that which has produced temperature-oriented geographic gradients for body size in natural populations of several species of Drosophila.

Data from three experiments bearing on the relative stability of the four mating systems required to test the restrictions against inbreeding and unequal family size were examined in relation to the results given by Robinson & Bray (1965). Tribolium castaneum was the experimental animal used in these experiments.

An analysis of variance indicated that both restrictions were probably effective (P approximately 0·10) in reducing the phenotypic variability of control population means. It seems likely that the apparent gain in stability obtained when the inbreeding restriction was used in addition to the restriction against unequal numbers is due to non-random genotypic proportions which would affect estimates of genetic variability based on assumptions of random mating.

1. Lines of Drosophila melanogaster, which differ greatly in body size and/or development time, have been created by selecting for either large or small body size or longer or shorter development time on different chemically defined axenic media. The ten lines studied here have been selected either on the optimum medium or on media deficient in either protein, ribonucleic acid or choline.

2. The lines show a variable degree of correlation between adult size and the duration of the larval period. Selection for one trait may involve a positively correlated change in the other or have no such effect. There is a fairly regular association between the composition of the diet and the presence or absence of evidence for genetic correlation.

3. The biochemical composition of the various lines, and also the unselected stock from which they were derived, has been compared at successive stages from egg to adult in animals grown on the optimum axenic diet. Records of wet weight, protein, RNA and DNA content per individual have revealed differences in composition which can be releated to characteristic differences in the response to selection.

4. Comparisons between the individual content of the various constituents at the critical size, i.e. at the time in early third instar when larvae can complete development even if no longer allowed to feed, and in newly emerged adults indicate a high positive correlation between DNA content and the time to reach the critical size and also the time to pupation. Protein and RNA content, on the other hand, may vary within wide limits without alteration of the development time. The association between DNA content and development time which underlies the correlation between body size and development time, is related to the rate of DNA synthesis in early larval life, such that the rate of synthesis tends to be negatively correlated with the absolute DNA content at the critical size and in the adult.

5. Since the protein/DNA ratio is comparatively unchanged in lines with correlated changes in body size and development time, but varies widely in lines in which development time is unaltered, the contrasts may reflect, in adults at least, differences in respectively cell number or cell size.

6. An earlier hypothesis of the origin of correlated and uncorrelated changes in body size and development time has been modified. It appears that the absolute critical size is positively correlated with the size of the adult, whether or not changes in development time have occurred. The intrinsic differences in growth operate from very early stages of development. This has been demonstrated in the differences in RNA/DNA ratio in early larval life and also in a high correlation between the time to complete development in the egg and the time to reach critical size after hatching from the egg.

7. There is also evidence, in the relative deviations from unselected of the various estimated constituents, of partial independence of growth between early and late larval life, and this may reflect differences in the growth of the imaginal discs.

8. The data are discussed in relation to earlier experimental evidence relating to the growth of Drosophila. The point is made that judicious selection for appropriate parameters of growth will often create differences great enough to reveal developmental interrelations which would otherwise be difficult to detect.

When mating pairs of Escherichia coli K12 are separated by physical means, e.g. by violent agitation, the Hfr cell is unable to mate with a second F− cell. This effect can be simulated by agitation of the Hfr cells prior to mating and, in addition, such cells can be shown to have an impaired ability to adsorb the male-specific coliphage μ2. Inhibitory concentrations of chloramphenicol block the repair of the damage.

The locus of the gene striated, Str, on the mouse X-chromosome, was previously reported to be on the side of tabby (Ta) away from bent-tail (Bn). Results given in the present paper show this report to have been incorrect, and that the order of loci is Bn-Str-Ta-(Mo-Blo). In addition, the position of the translocation break in Searle's translocation (T1GH) has been confirmed, the order of loci with respect to T16H being Bn-T16H break-Ta-Blo.

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.

Growth of K(P1) bacteria under conditions which lead to a reduction in the level of nucleases also leads to a reduction of their ability to restrict the growth of λ.C. Experiments designed to estimate the time after adsorption at which restriction takes place indicate that phage DNA is probably restricted by a nuclease while passing through the periplasm.

The sex factor F is itself exceptional among conjugation factors in being freely expressed in all the bacteria of a culture, while related factors which are largely unexpressed are widespread. These can be recognized by the presence of a small proportion of cells sensitive to phage specific for F+ bacteria, and may sometimes constitute the conjugation factors associated with colicinogeny. Moreover, F itself can mutate to the repressed state so that derivatives of E. coli K12 which appear F− include some in which F is still present but rarely expressed.

The restriction of phage λ.C by K(P1) cells is reduced when the cells are subjected to an EDTA cold-wash treatment which has been shown to remove surface-localized enzymes. We conclude that a surface-localized enzyme plays an essential role in host-controlled restriction.