By directional selection for total leukocyte counts from a hybrid mouse stock we have gradually established two lines of mice, LLC (Low Leukocyte Count) and HLC (High Leukocyte Count), which differ both in total and in differential leukocyte counts. A randombred line (RLC) is also being concurrently maintained. Other variations between these lines of mice are in body weight, in the frequencies of coat color genes, reproductive performance, and resistance to X-irradiation. The LLC line was comparatively low in the latter two physiological parameters, and high in variation of body weight.

Responses to selection for high and low leukocyte counts were asymmetrical. In the first two generations, responses were irregular; thereafter they were large in the low line (LLC) for two or three generations and then became small in comparison with those of the high line (HLC). At eleven generations of selection, the mean leukocyte count of HLC is about three times that of LLC Responses of the different cell types were proportional to their individual percentages of the total counts. There were sexual differences in the counts of total and individual cell types. Selection for total leukocyte counts affected the proportions of the individual cell types. Heritability estimates based on selection differential and response and on sib relationships yielded values ranging from 0·15 to 0·39.

1. The genetic and environmental variation of red eye pigment in individuals of a wild population of Drosophila melanogaster has been studied by extracting and measuring the pigment content of individual flies, which were also scored for eye and body size.

2. Comparison of such variability in the wild population with the individual variation in crosses between inbred lines suggested that 60% of the phenotypic variance is genetic. About 75% of both genetic and environmental variance is due to intrinsic variation of pigment content while the remainder is correlated with eye size, which shows appreciable variation, independent of general body size, as measured by thorax length.

3. Selection for high and low pigment content by both phenotypic and family selection led to 40–50% differences between high and low lines after eight genera tions. The response was asymmetrical and proceeded further and faster with selection for lower pigment content. Crosses between high and low lines showed positive departure from intermediacy, suggesting that more or less recessive effects had contributed to the selection for lower pigment content.

4. There was some evidence of lower viability in the selected lines but no evidence of lower fertility or gamete viability in more extreme individuals of either sex.

5. Comparison of pigment content, eye and body size at different temperatures and under different levels of crowding suggested that the pigment content per ommatidium is subject to a high degree of genetic determination.

6. The average pigment content in strains derived from widely separated localities showed substantial variation, independent of both eye and body size.

7. Inbred lines, derived from the same population, were found to differ greatly in pigment content. Crosses and the exchange of homologous pairs of chromosomes between two of these lines suggested that one or more completely recessive genes were fixed in both the second and third chromosome of one line, while the same second chromosome effect was also fixed in another line. The second and third chromosome difference reduced pigment content by respectively 30% and 50% and combined additively, judging by the effects of single and joint substitutions of homologous pairs.

8. The possibility of combining genetic and biochemical analysis of pigment content is discussed.

1. Populations of Drosophila melanogaster have been adapted to a new, initially unfavourable diet by adding to the food medium the chelating agent, EDTA, which lowers survival, lengthens development and reduces body size, according to the concentration.

2. Six populations were allowed to adapt to the new diet without intervention and compared with two additional populations in which there was either artificial selection for fast development time or in which the effects of variation in development time were minimized and higher egg production was favoured instead.

3. All populations adapted successfully and some were able to grow on medium with EDTA concentrations which were lethal for the original population.

4. Under uncrowded conditions on EDTA-free medium, in seven out of the eight populations, body size was reduced by about 7% below the level of the original population and the larval period was shorter in several instances. But in the population in which higher egg production was favoured, body size was 7% greater than in the original population and 16% greater than the average of the other EDTA-adapted populations. This contrast was attributed either to intense natural selection for shorter development time or to selection for a higher rate of egg production, which is positively correlated with body size when larvae are grown on sub-optimal conditions.

5. Under crowded, competitive conditions, the fitness of the EDTA-adapted and the original populations was reversed according to the presence or absence of EDTA.

6. Genetic differences between one of the EDTA-adapted populations and the original population were studied by using marked inversions to interchange chromosome pairs. Larvae of the alternative genotypes were grown on different diets and adaptation was shown to have involved changes in all major chromosomes and also substantial, complementary interaction between non-homologous pairs. Substitution of the third pair of chromosomes from the original Pacific stock in the background of the adapted strain led to complete sterility of females, on all diets tested, and lethality of both sexes at higher levels of EDTA.

7. The creation of new equilibria, by manipulating the relative importance of components of fitness, in the course of adaptation to a new environment, offers a valuable technique for studying the selective forces which influence the mean value of quantitative characters generally.

1. A test is described for the development of sexual isolation between a wild and a derived population of D. melanogaster adapted to a new diet, containing EDTA. Other experiments had shown that adaptation to the new diet involved genetic changes in all chromosomes. Also fitness was reversed on the alternative diets under crowded competitive conditions.

2. In three replicated trials flies from each population were used to establish paired cage populations, supplied with the medium to which each was adapted, and the pairs of cages were joined to allow restricted immigration between them. The experiment was run for about twenty-five generations.

3. After fifteen and twenty-five generations, flies were collected from each cage to provide eggs which were cultured on the alternative diets to determine how far the members of pairs of populations differed from each other and from the foundation population. There were striking differences between the sub-populations and the parent populations, attributable to immigration between the former. Judged by the differences in performance between the sub-populations, genetic differences persisted but these were minor compared with the differences between the parent populations.

4. Tests of preferential mating on the part of flies from paired sub-populations were entirely negative.

5. Fourteen generations of selection for positive assortative mating failed to provide evidence of sexual isolation between the two basic populations, adapted to different diets.

6. From these and other experiments it is inferred that sympatric divergence is improbable in a species like D. melanogaster.

A model for F-prime formation is presented. It predicts that an Hfr strain giving rise to an F-prime factor would acquire a deletion corresponding to the chromosomal fragment carried by the episome. Genetic studies have confirmed this prediction. Concomitant transfer to the episome of a gene determining a function vital to the cell has permitted selection of derived Hfr strains in which the episomal fragment has been translocated to various sites on the bacterial chromosome.

The fact that the X-linked genes scurfy (sf) and sparse-fur (spf) of the mouse do not produce a mosaic effect in heterozygotes had been taken, by other workers, together with results from X-Autosome translocations, as evidence that inactivation of the mouse X was incomplete. In this paper it is argued that absence of a mosaic effect is not adequate evidence that a gene is not inactivated. The argument was backed by an experiment in which the spf gene was introduced heterozygously into females carrying an X-linked translocation resulting in non-random X-inactivation with the same X active in all cells. When the mutant (spf) allele was on the active X its effect was fully expressed, indicating that the normal allele on the structurally normal inactive X was undergoing inactivation. Argument is further presented that results from X-Autosome translocations do not indicate the degree of completeness of inactivation in a structurally normal X. Hence, there is no evidence that inactivation of the mouse X is incomplete, although evidence from XO females does suggest that it may be incomplete in man.

This paper is concerned with three related aspects of the behaviour of populations under artificial selection for increased scutellar bristle number: (i) the pattern of response on the probit scale; (ii) the homeostatic behaviour of the selection lines on relaxation of artificial selection; and (iii) correlated responses in generation interval, reproductive capacity and competitive ability. The study was designed so that linkage would be a comparatively unimportant factor in promoting correlated responses to selection, and the effects of genetic sampling from generation to generation were also reduced to a low level.

Progress from the base mean of 4·05 bristles in females to a level of almost 8 bristles has been shown to involve two distinct phases with realized heritabilities of 0·34 and 0·10 respectively, the zone of transition corresponding closely to the position of the 6/7 threshold on the underlying scale. In addition to an apparent average reduction of about 25% in the additive genetic standard deviation in phase II by comparison with phase I, the loss in response due to the opposition of natural selection has been shown to reach a maximum near the zone of separation of the two phases.

The pattern of behaviour of the populations under artificial and natural selection has suggested the presence in the base population of genes of large effect on both bristle number and reproductive fitness. There is also evidence of additional genetic variation in bristle number which is effectively neutral with respect to fitness. Continued selection for increased scutellar bristle number in large populations has been shown to reduce mean competitive ability by more than 80%.

Derivatives of E. coli K12 which had received a number of different drug resistance factors by conjugation from Salmonella strains were found also to have acquired insensitivity to colicins. The insensitivity associated with some R factors was similar in character to immunity conferred by colicinogeny, while that given by others more closely resembled the higher level of resistance due to absence of the colicin receptor. It was, however, not complete. Many of the R factors also restricted the growth of various phages.

This paper describes the aspects of fertility that had been affected by selection on litter size. For twelve generations previously the mice used as parents were chosen because they had been born in large or small litters. At the end of this time, litters in the fertile strain averaged 11·1 young born alive, while the less fertile strain averaged 5·5.

It was found that male fertility and inherent viability of the young had nothing to do with the response although neither was excluded by the method of selection. Several contributions, however, were made by the females, who were affected not only in ovulation rate, but also in their control of pre-implantational losses, foetal mortality and mortality of the newly born.

Females from the less fertile strain were particularly prone to pre-implantational loss of eggs. It remains to be shown whether these were due to fertilizational or implantational failure.

The incidence of earlier and later embroyonic losses in females of the same strain were uncorrelated—Utters that were depleted early were neither more nor less inclined to be depleted later.

Argininosuccinase has been purified from wild-type Neurospora crassa, strain ST.A. The purified enzyme, which is homogeneous by the criteria of analytical centrifugation and starch-gel electrophoresis, has a molecular weight of about 175,000. The enzyme has also been partially purified from a heterokaryon between the arg-10 mutant stocks B317–9–9a and 402–3a.

The reaction kinetics of the two enzymes were compared in several respects, and they were found to be indistinguishable. The enzymes were also indistinguishable by starch-gel electrophoresis, and sedimented at the same rate through a sucrose gradient. It seems likely, however, that the enzymes do differ physically since they showed different affinities for both calcium phosphate gel and hydroxylapatite during purification.

In Cattanach's X-autosome translocation a piece of autosome of linkage group I has been inserted into the X-chromosome and a piece of X may have been reciprocally translocated to the autosome (Cattanach, 1961; Ohno & Cattanach, 1962). The present communication reports investigations to locate the autosomal insertion in the X-chromosome linkage map and provide evidence pertinent to the question of the possible reciprocal nature of the rearrangement; a brief summary of the results has already been reported (Cattanach & Isaacson, 1965).

A formula for effective population size when fertility is inherited is worked out. It is shown that the effective size decreases as the heritability of fertility or progeny number increases.