1. The growth of strains of Drosophila melanogaster selected for large size under different nutritional conditions has been recorded on a variety of different media and compared with that of the unselected population. The experiments were designed to test the inference from earlier work that selection for the same ‘character’, body size, on different diets leads to more or less different changes in growth and metabolism. The inference has been amply confirmed.

2. When compared on a number of deficient synthetic diets, the strains which had been selected either on a low-protein diet or on one in which all the essential nutrients had been reduced, suffered a much smaller reduction in body size than either the unselected population or, especially, a large strain selected on the favourable live yeast medium. Some diets which drastically reduced the body size of the unselected population lead to no change in the size of strains selected on the synthetic media, although development time was prolonged. Hence selection had extended the capacity for maintaining a characteristic adult body size to diets which normally would lead to a decline. This is taken as evidence of improved adaptation to such conditions. There is also some evidence that selection on the synthetic diets had lowered the level of adaptation to the usual live yeast diet, since body size tended to be lower on this medium than on some of the normally sub-optimal diets.

3. To provide comparisons in adverse conditions which are probably more closely related to those commonly encountered by populations in nature or the laboratory, the performance of the strains has been compared in a graded series of competitive conditions on the live yeast medium. By using genetically marked files of the foundation population, which were shown to react in the same way as unmarked flies—in terms of survival, body size and development time—the competitive ability of the different strains has been tested against that of unselected individuals. The latter are generally superior to the selected strains, which differ among themselves, however, in a way which can be related to the conditions in which they were selected.

4. Under such competitive conditions, the strains selected on the synthetic diets suffer a much greater decline in body size than do the unselected individuals. For the strain selected on live yeast, the proportional reduction of body size is about the same for the unselected flies at lower levels of crowding, but is clearly greater under more severe conditions of competition.

5. The low-protein strain has been backcrossed to the unselected stock. When reared on a variety of synthetic diets, the performance of the F1 was generally intermediate between that of the parents.

6. Nutritional variation may be responsible for either a high environmental correlation between the two measures of growth, body size and duration of larval period, or no apparent correlation. Provided the diet is not too unfavourable, body size remains constant although development time may be lengthened to a variable degree. With more adverse conditions, body size is reduced and development time is lengthened more or less proportionately. Such differences in reaction probably depend on the particular stage of larval growth and development primarily affected by the treatment; this problem is being examined further. The inverse relations between body size and development time may represent the operation of a kind of safety mechanism which ensures that the adult reproductive state is attained sooner than would be so if the capacity for maintaining a characteristic body size were more effective in relation to deficient diets. Populations and species adapted to different conditions are likely to differ as to where the balance is struck between effective maintenance of a characteristic adult size, with maximum potential egg production, and the alternative response, according to their ecology. This possibility must be borne in mind when the response to selection for, say, body size is compared in different species.

The mutation rate at seven specific loci was measured among the offspring of male and female mice exposed as 17½-day-old foetuses to 200 r. X-rays. In the female series the mutation rate was lower, by a factor of about four, than the comparable adult rate; in the male series the mutation rate was lower but not statistically significantly lower than in adults.

A system for the screening of reversions in an arginine-dependent strain of Neurospora has been developed.

Reversion resulted from any one of three events:

(a) Back mutation of the original arg gene.

(b) Forward mutation at the pyr locus. This has the pleiotropic effect of suppressing arg while determining a pyrimidine requirement. Interaction with the unmutated nuclei of the same conidium permits growth on minimal medium.

(c) Forward mutation at the pyr locus to alleles of intermediate activity. These alleles suppress arg, but do not create a pyrimidine requirement.

The time in hours from laying to hatching at 17°C. has been measured for eggs from four inbred lines of Drosophila subobscura, and also for F1 eggs, and for eggs obtained by crossing F1 hybrids to unrelated inbred lines and to unrelated F1 hybrids.

Eggs laid by females of a given genotype have a characteristic rate of development, which, with one exception, is not greatly influenced by the genotype of the male parent. F1 hybrid females laid eggs which were more uniform in hatching time than eggs laid by inbred females, and which developed more rapidly than eggs laid by three of the four kinds of inbred female studied. The slowest-developing eggs were laid by females from the NFS inbred line, a line originally selected for slow development from egg to adult.

The male parent, and hence the zygotic nucleus, had an appreciable effect on the rate of egg development only for eggs laid by NFS females; these eggs developed more rapidly if fertilized by sperm from an unrelated male.

1. TEM resembles X-rays in inducing dominant lethal mutations in the sperm of rats and mice and sterilizing type B spermatogonia. Beyond this, however, there are several important differences.

2. The relative ease with which TEM and X-rays affect sperm and spermatogonia varies greatly. The X-ray dose which produces 50% dominant lethals in sperm (500 rad) sterilizes spermatocytes and type B spermatogonia, and has such a drastic effect on type A spermatogonia that recovery of fertility is delayed for 2½ months. The TEM dose which produces the same mutation rate in sperm (0·1 mg./kg. rat) has no detectable effect on pre-meiotic stages. Even 1 mg./kg. only sterilizes the most sensitive stage, type B spermatogonia.

3. Taking immature sperm as the standard, mature sperm are more sensitive to X-rays, but less sensitive to TEM, and early spermatids, the most sensitive stage to X-ray-induced dominant lethals, are highly resistant to TEM.

4. Spermatocytes, in which X-rays yield a mutation rate equal to immaturesperm, are highly resistant to TEM.

5. To produce the same mutation rate in immature sperm, mice require twice as much TEM as the rat in mg./kg., though approximately the same X-ray dose.

6. In contrast to mid-stage rat spermatids, which are the most sensitive stage to TEM, mouse mid-stage spermatids are resistant.

The fertility of two unrelated strains of mice (strains N and C) which had both been selected for large and small body size has been studied.

The fertility of pairs of mice in the large or small lines of strain C was unimpaired by selection. In strain N, some of the pairs in the large and small lines, but not in the control line, were sterile. Sterility in the large line was due to the low libido of the males, and not to female infertility. Sterility in the small line was probably due to hypo-functioning of the anterior pituitary of some females: the oestrous cycle was delayed or absent, some mice failed to ovulate after mating, and a high proportion of those mating had no implanted embryos at 12 days' gestation. Oestrus and ovulation could be induced in mice of the small line of strain N by exogenous gonadotrophins, and the proportion of mice with implanted embryos was considerably increased by progesterone supplements.

The number of eggs found after natural mating was considerably higher in large mice than in small mice, and was significantly correlated with body weight in the small line of strain C only. The egg-number/body-weight ratio was higher in the lines of strain C than in those of strain N, though the ratios were similar when carcass fat was subtracted from total body weight.

The amount of endogenous follicle-stimulating hormone secreted by the mice of the five lines was estimated by inducing ovulation with various amounts of exogenous gonadotrophins, and comparing the number of eggs found after each dose with the mean number ovulated after natural mating. Estimates of the amount of follicle-stimulating hormone secreted by mice of strain C were higher than those for mice of strain N.

Differences in the rates of growth and in the numbers of eggs ovulated after natural mating indicate a higher level of pituitary activity in strain C than in strain N.

Six isolate lines of long standing, from the Centre's Brown Leghorn flock, were intermated in all possible directions, providing thirty cross and six pure line samples. Half the birds from each line and reciprocal cross were kept in floor pens and the rest caged in an adjacent battery house.

Sexual maturity, survivors' production to 500 days of age, and November through June egg numbers, all showed two main factors contributing to variations in cross performance: (1) a significant association with parental production levels as measured by mid-parent averages, and (2) an additional hybrid gain. For November to June production in pens, this hybrid gain appeared to approach a constant for the cross groups. Additional sources of variation were present in age at sexual maturity.

The 500-day records for the penned hybrids exceeded the line average by 50 eggs. Caged samples did less well and showed an advantage of only 34 eggs. In this location the hybrid gain for individual groups decreased with increases in mid-parent level.

Egg weight varied negatively with egg numbers only in the upper half of the wide egg-size range. The best layers fell at the lower end of the covariant range for egg size.

The importance of improving closed strains may not have been lessened by the expansion of the hybrid side of the poultry industry, for the pattern of relationships of line and cross production in this flock is consistent enough to justify the inference that the best hybrids come from the best-producing parents.

Dissections were made of 16-day-pregnant female mice with the object of discovering the developmental stage at which litter size is reduced by inbreeding. Counts were made of the numbers of corpora lutea, implantation sites, and live embryos, and comparisons were made between females with inbreeding coefficients of 50–60% and non-inbred females. Except in one group the embryos were all non-inbred, so that the comparisons showed the effect of inbreeding in the mother of the litter. No influence of inbreeding in the male parent was found.

The only difference found between inbred and non-inbred females was in the number of eggs or embryos lost before implantation. The greater pre-implantation loss in inbred females was enough to account for the smaller number of young born alive in their litters.

There was no difference between the inbred and non-inbred females in the ovulation rate, measured by the number of corpora lutea, or in the post-implantation mortality of the embryos.

There was a positive correlation between ovulation rate and weight at 6 weeks. For reasons explained in the Discussion, the inbred females did not differ in weight from the non-inbred females. If, under other conditions, the weight declined on inbreeding, the ovulation rate would be expected to decline also.

1. The plasma proteins of six inbred strains of mice have been studied, using starch-gel electrophoresis.

2. The existence of two alternative plasma transferrin (β-globulin) phenotypes has been demonstrated. Five of the strains have one of these and one strain has the other. Each of the two transferrin patterns comprises three (or possibly only two) electrophoretic bands. The two patterns differ in all of these bands.

3. The two transferrin types recognized are determined by a pair of allelic, autosomal genes (designated TrfA and TrfB). The TrfA phenotype (CBA strain) is determined by the genotype TrfA/TrfA, and the TrfB phenotype (A, C57BL, JU, KL, RIII strains) by the genotype TrfB/TrfB. The phenotype TrfAB, of the heterozygote (genotype TrfA/TrfB), is distinguishable and shows four (or possibly only three) bands. In this way it closely resembles a mixture of equal parts of TrfA and TrfB plasma.

4. No linkage was detected between the Trf locus and sex, the agouti locus or the haemoglobin locus.

5. The possible molecular basis of the action of the transferrin alleles in the mouse, and the widespread distribution in mammals of polymorphism involving the transferrins, are discussed.

1. The gene for Pintail (symbol Pt) shows no dominance. Heterozygotes are viable and fertile; many homozygotes die early in life, but the survivors appear to breed normally. Pt reinforces the effects of Sd and T on the tail; there is no evidence for interaction between Pt and + /vt.

2. The gene produces a shortening of the tail, but the most marked effect on the adult is in the great reduction of the nucleus pulposus of the intervertebral discs, which may lead to tail kinks in some animals.

3. Pintail embryos can be recognized from the eleventh day stage of development onwards by the possession of a much smaller notochord than normal. This reduced notochord is the result of a reduced rate of cell division which first manifests itself in 10-day embryos. Perhaps this reduced rate is the result of altered relations of the notochord with tail-gut and neural tube in 10-day embryos.

1. The X-ray mutability of wild-type isoalleles of the white-eye loci in Drosophila melanogaster has been determined.

2. No difference in the mutability of the isoalleles was found. However, an appreciable difference in mutation rate of the two halves of the w loci was found.

3. The bearing of these data on the problems of isoallelism and pseudoallelism are discussed.

In haploid strains of Aspergillus nidulans, asci arise from croziers. The two nuclei of the young ascus (the penultimate cell of the crozier) fuse, and the zygote nucleus immediately undergoes meiosis. At diakinesis and first metaphase eight bivalents are seen: three large (one, Chromosome 2, with a satellite), two medium sized (Chromosomes 4 and 5), two small (Chromosomes 6 and 7) and one very small (Chromosome 8). The perithecia of haploid strains are packed with asci and have very few sterile hyphae.

Diploid strains (heterozygotes made by Roper's technique) are very different from haploids in that the perithecia contain many sterile hyphae with little cytoplasm and coiled hyphae with dense cytoplasm made up mainly of uninucleate cells, and there are few asci in a perithecium. Croziers are absent: some of the cells of the coiled hyphae become asci. At diakinesis and first metaphase, the same chromosome configurations are seen as in a cross between the haploid strains from which the diploid was synthesized; the chromosomes are bivalents. The young ascus thus has a single (diploid) nucleus which undergoes meiosis, and asci develop apogamously. No evidence of nuclear fusion in the young ascus and of a tetraploid meiosis was obtained. Cultures from ascopores isolated by micro-manipulation from perithecia of diploids were all haploid. Meiosis appears to proceed normally to first metaphase, but typical later stages, and asci with spores, are rarely seen. First anaphase frequently fails to occur, and the chromosomes clump together.

In the strain ad2 y, obtained from a normal strain by X-irradiation, nine bivalents are present, one a very small fragment. In crosses between ad2 y and a normal strain, and in a diploid made from them, the fragment pairs with Chromosome 6. Asci in the cross between ad2 y and a normal haploid are highly irregular, often with less than the usual eight spores.

Diplotene and diakinesis chiasma frequency in oöcytes of the mouse cannot be studied successfully with the present technique. Metaphase chiasmata have been examined in thirty-nine oöcytes. It is deduced that the total diplotene map length in females is about 2300 cM. compared with 1950 cM. in males. There is sexual dimorphism in the frequency of chiasmata, which is paralleled by similar dimorphism in frequencies of crossing-over, measured genetically.

The two sexes differ in the duration of various stages of meiosis. In adult males the pachytene stage, lasting for about 7 days, is directly followed by diplotene and diakinesis, after which the metaphase stage sets in. The sex bivalent in males develops visible chiasmata much earlier than do the autosomes and it precedes them in anaphase separation. Quick terminalization of chiasmata in it leads in a fair proportion of cases to precocious separation and in less than 1% of cases to cytologically detectable non-disjunction of sex chromosomes.

In females the pachytene stage appears in oöcytes of the embryo and is followed by the dictyotene stage, which last still ovulation, i.e. between 35–40 days and several months. Since in the oöcyte chiasmata are formed and move during the dictyotene stage, it follows that stainable materials of the chromosomes are not necessary for the formation and movement of chiasmata and are concomitant with pairing and anaphase separation. It follows also that the time for chiasma formation and movement is in females at least five to six times longer than in males. In old oöcytes in which time is available for maximum terminalization of chiasmata, non-disjunction may appear with detectable frequency. This mechanism may also operate in cases of Mongolism in man, where non-disjunction of an autosome has been recently cytologically established and higher frequency of incidence of the condition for old mothers has been known for some time.

It is possible that the differences in duration of various stages of gametogenesis are connected with the period at which gametic selection is operating: in spermatogenesis after the second meiotic division, in oögenesis prior to first meiotic metaphase.

It has long been believed that partially paired chromosomes observed at early meiotic prophase represent synapsis caught in the act and that crossing over must follow or accompany this stage. In contradiction to this it has often been suggested that certain observations (e.g. the occurrence of 3: 1 ratios from individual tetrads of yeast, Neurospora and Aspergillus) are most easily explained if it is assumed that crossing over occurs at the time of reduplication which in turn is now thought to occur during the premeiotic interphase (Taylor, 1957). Another phenomenon, which has been reported in Neurospora, Drosophila and bacteriophage, and apparently must be fitted into any successful model of recombination, is localized negative interference.