Parameters necessary for predicting direct and correlated responses for large and small 13-day larval weight in T. castaneum on two levels of nutrition were estimated in the base population. Larval weight in the GOOD environment was approximately twice that observed in POOR. Heritabilities (estimated from the ratio of sire component to total phenotype variance) of larval weight on the two rations were similar, 0·21 ± 0·06 and 0·19 ± 0·05 for GOOD and POOR, respectively. Heritabilities based on dam-offspring covariances were similar to these, but those obtained from full-sib covariances were much larger (0·97 ± 0·07 for GOOD and 0·69 ± 0·07 for POOR). This suggested that considerable dominance rather than maternal effects were present. The genetic correlation between growth on GOOD and growth on POOR was estimated as + 0·60 ± 0·21.

The selection experiment was replicated four times with each replication extending over eight generations. Good agreement between predicted and observed values for direct selection was observed for large selection in both environments and small selection in POOR. However, response to small selection in GOOD was significantly greater than predicted in all four replications and was associated with increased selection differentials. Realized heritabilities were approximately the same for both directions in GOOD yet asymmetrical responses occurred for all replications due to unequal selection differentials. On the other hand, realized heritabilities were asymmetrical in POOR. Those observed for small selection were more than twice the size of those calculated for large lines. However, the responses in POOR were symmetrical since the selection differentials varied inversely with the realized heritabilities.

Because of the asymmetry observed for heritabilities and selection differentials, correlated responses were poorly predicted. The average effective genetic correlation between growth in GOOD and growth in the POOR environment agreed remarkably well with the base estimate, yet asymmetry of the genetic correlation was a consistent phenomenon with values for the large lines being less than the base parameter while small lines were uniformly larger.

Asymmetries of the various genetic parameters were not anticipated from base estimates. They were not caused by sampling or chance fluctuations since all four replications were remarkably consistent. Asymmetry for any one genetic parameter (e.g. heritability) was associated with a particular environment or direction of selection while other genetic parameters reacted asymmetrically in populations exposed to a different set of environmental treatments.

For maximum performance in a single environment, these results show that selection should be practiced in that environment. With regard to mean performance in GOOD and POOR environments, selection for large size in POOR gave some 25% more gain than selection in GOOD. Selection for small size in either environment was equally effective in obtaining minimum size in both environments.

In C57BL/10 and the majority of other strains of mice, females have about 20-fold higher kidney histidine decarboxylase levels than males; in DBA/2 mice, however, HDC in females is only 3- to 4-fold higher than males. The low ratio HDC phenotype of DBA/2 animals is due to decreased sensitivity of the HDC gene complex to repression by testosterone in males. From conventional crosses and by the use of the BXD recombinant inbred lines we conclude that the C57BL/ 10: DBA/2 difference, in testosterone sensitivity of HDC, is due to an allelic difference in the regulatory gene Hdc-a of the HDC gene complex, [Hdc], on chromosome 2; DBA/2 contains a third allele of this gene, Hdc-ad.

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.

Previous theoretical studies of the evolution of the selfing rate have shown that mixed mating systems are not evolutionary stable states. Such models have, however, not included the effects of population structure and thus biparental inbreeding together with the evolution of selfing rates and inbreeding depression. In order to examine selection on selfing rates in structured populations, a stochastic model simulating a finite population with partial selfing and restricted pollen and seed dispersal has been developed. Selection on the mating system was followed by introducing modifiers affecting the selfing rate. The major result was that, with density dependent recruitment, a process which maintains the population structure necessary for biparental inbreeding to occur, a mixed mating system could be maintained. This result was associated with an increase of the mutation load with high selfing rates, and the selected selfing rate depended on the degree of population structure rather than on the initial selfing rate. With low dominance of deleterious alleles, complete allogamy can be selected for. Further studies showed that the more general condition of spatial heterogeneity of recruitment can lead to similar results, the most important condition being the maintenance of genetic structure within populations. A brief survey of the empirical literature shows that a positive relationship between the magnitude of inbreeding depression and the inbreeding coefficient within populations has been observed, in support of the present model.

Selection and mapping of mutations affecting cysteine synthesis in Aspergillus nidulans was carried out. A new locus, cysE, is described, the mutants of which are deficient in in vivo conversion of O-acetylserine to cysteine, a step mediated by cysteine synthase. Three loci (cysB, C and E) were thus found to control this step in vivo, apparently without affecting the enzyme activity in vitro. By scoring for propargylglycine sensitivity of cys mutants, chromosomal map positions were obtained for all five cysteine loci (A, B, C, D and E).

The frequencies of genetically determined electrophoretic variants of two enzyme systems in the parthenogenetic crustacean D. magna have been followed in two isolated populations. In both populations a marked excess of hetero-zygotes was found in the later samples. It is concluded that the observed changes in gene and genotypic frequencies are due to natural selection as both migration and genetic drift can be excluded.

The pleiotropic effect of the rose-comb gene (R) on fertility when combined with artificial selection against the single-comb type (r) results in an interesting example of operational over-dominance. An evaluation of the equilibrium frequency of the rose-comb gene based on this over-dominance concept provides a plausible explanation for the relatively high frequency of single-comb birds appearing in the Wyandotte breed of fowls.

A population with u deleterious mutations per genome per generation is considered in which only those individuals that carry less than a critical number of k mutations are viable. It has been shown previously that under such conditions sexual reproduction is advantageous. Here we consider selection at a locus that determines recombination frequency of the whole genome. The value v = u/ √ k has been found to play the decisive role. When v < 0·35 the direction of selection for recombination may be different for different cases, but the intensity of selection is always very small. The advantage of recombination becomes considerable when v > 0·5, its growth under increasing v being approximately linear. If v > 2 no less than 95% of the progeny are bound to die because of the selection against deleterious mutations. Since this seems to be too great a mutation load, we may assume 0·5 < v < 2·0 for any sexual population if mutation really maintains crossing-over. Results on selection at a locus which controls mutability provide evidence that v is located within the specified interval if the physiological cost of a twofold reduction of the mutation rate is within the range 10–80%. A number of consequences of this hypothesis about the mechanism of selection for sex and recombination are discussed.

The effects of acriflavine on the transfer of various episomes and of the bacterial chromosome were studied using acriflavine-sensitive and -resistant strains of E. coli K-12 as donors and recipients. It was found that the inhibition of transfer of these elements by acriflavine was only slight when donor bacteria were resistant to this acridine dye, regardless of whether the recipient bacteria were acriflavine-sensitive or -resistant. This result seems to support the view that acriflavine inhibits the replication of these elements in donor bacteria which is considered essential for their transfer. With colicinogenic factors, in contrast to F and R, the results were affected by the sensitivity or resistance of the recipient and were in support of the killing by acriflavine of the cells which received the colicinogenic factors.

At the white eye colour locus, there are a number of alleles that have altered expression between males and females. To test these regulatory mutations of the white eye colour locus for their phenotypic expression in metafemales (3X; 2A) compared to diploid females and males, eleven alleles or transduced copies of white were analysed. Two alleles that exhibit dosage compensation between males and females (apricot, blood) also exhibit dosage compensation in metafemales. White-ivory and white-eosin, which fail to dosage compensate in males compared to females, but that are distinct physical lesions, also show a dosage effect in metafemales. Two alleles with greater expression in males than females (spotted, spotted-55) exhibit even lower expression in metafemales. Lastly, five transduced copies of white carrying three different lengths of the white promoter, but that all exhibit higher expression in males, show reduced expression in metafemales, exhibiting an inverse correlation between the level of expression and the dosage of the X chromosome. Because these alleles of white respond to dosage compensation in metafemales as a continuum of the male and female responses, it is concluded that the same basic mechanism of dosage compensation is involved and that the dosage of the X chromosome conditions the sexually dimorphic expression.

Mice were selected for growth from 3 to 9 weeks of age on a normal protein diet (N) containing 19·3% protein and a reduced protein diet (R) containing 5·1% protein. On each diet there were 3 high (H), 3 low (L) and 3 unselected control (C) lines. After 6 generations of selection, half of the mice in each line were tested on each diet. Responses were obtained when selecting for both increased and decreased growth on both diets. The realized heritabilities from within-family selection were 33 and 26% for the divergences on the normal and reduced protein diets, respectively. Consistent genotype-environment interactions were found when all lines were tested on both diets in generation 7. Performance on each protein level was best improved by selection on that protein level. Further, the correlated response was significantly less than the direct response when selecting on both diets. The estimates of the genetic correlation between growth on the two protein levels were low, rN = 0·16 from selection on the normal protein diet and rR = 0·51 from selection on the reduced protein diet. Selection resulted in a change in environmental sensitivity in the lines, dependent on the diet and direction of selection. The average of the divergences on the two diets was not dependent on the selection environment.

The level of liver catalase activity in substrain C57BL/6 mice is only approximately half that in substrain C57BL/He. That this difference was due to a single major gene with low level of activity dominant to high was indicated by analysis of the F1, F2, and backcross hybrids and confirmed by progeny tests of first back-cross male segregants. The gene symbol Ce is suggested.

The absence of any difference between reciprocal hybrids indicated no extra-chromosomal maternal influence.

There was no evidence that this gene controlling liver catalase had any influence on the level of kidney catalase activity for this did not vary between the two parent substrains or the hybrid groups.

Males consistently had higher liver and kidney catalase activity than females. The difference was more pronounced in respect to kidney catalase, males having about twice the activity found in females.

The wild-type N group plasmid RN3, which is phenotypically Res+ Mod+(RII) is not eliminated by thymine starvation of its bacterial host. Derivatives of RN3 selected for the Res− phenotype are eliminated. The presence or absence of the RII modification specificity does not affect thymineless elimination of RN3 Res− plasmids. A Res−Mod(am) RN3 mutant is not eliminated by thymine starvation from either amber suppressing or non-suppressing hosts, suggesting that it carries a cryptic mutation in a novel genetic locus required for elimination. Thymineless elimination is shown to be recA+-dependent and the presence of the X group plasmid R6K significantly inhibits elimination of RN3 Res−Mod+. However, since R6K has no effect on two other plasmid-mediated functions of UV protection and UV-induced mutagenesis, which are also recA+-dependent, it would appear that elimination is determined by a separate plasmid gene than that encoding the UV functions. This is confirmed using derivatives of another N group plasmid R390, which eliminate but which have lost the ability to increase UV-induced mutagenesis in their host.