Research Article
Selection for leukocyte counts in mice*
- C. K. Chai
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- Published online by Cambridge University Press:
- 14 April 2009, pp. 125-142
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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.
Selection in natural populations V. Indian Rats (Rattus Rattus)
- Leigh Van Valen, Robin Weiss
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- 14 April 2009, pp. 261-267
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In samples of sixteen populations of Rattus rattus from southern India, the oldest individuals have less variable molar widths than the younger ones. This is probably due to stabilizing selection by mortality. There is no detectable heterogeneity between sexes or teeth or among populations in this selection. Although there is no average difference between age classes in mean tooth width, the difference between age classes is heterogeneous among populations. This heterogeneity may reflect heterogeneity in directional selection or in direct environmental effects. The selection intensity on the variance is about 0·04.
A super-suppressor on the thirteenth linkage group in Saccharomyces*
- Yasuji Oshima, Ikue Oshima
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- 14 April 2009, pp. 1-7
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It has been proved that the suppression of seven different auxotrophic mutant alleles is brought about by a single super-suppressor allele designated S. Super-suppressor S is allele-specific rather than locus-specific and is dominant to its wild-type allele.
Suppressible mutant alleles show no linkage relationships either among themselves or with the suppressor S.
Super-suppressor S has been shown to be closely linked (about 5 stranes) to one of the centromeres other than those of linkage groups I to XII. This establishes a new thirteenth linkage group.
The possible mechanism of action of the super-suppressor is discussed.
Quantitative inheritance of red eye pigment in Drosophila melanogaster
- Narendra S. Chauhan, Forbes W. Robertson
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- Published online by Cambridge University Press:
- 14 April 2009, pp. 143-164
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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.
The interpretation of complementation data
- Oliver J. Gillie
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- 14 April 2009, pp. 9-31
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The evidence for complementation maps being linear is examined by analysis of all known complementation maps in micro-organisms, and by constructing maps from mutants randomly sampled from amongst those at the leu-2 locus in Neurospora with known complementing properties. Eleven loci out of thirty-five examined in six micro-organisms have non-linear complementation maps. Two linear maps, his-3 and ad-3b (having 25 and 35 complementation groups respectively) have a sufficiently large number of groups for it to be likely that if they do not remain linear on testing further mutants, they will at least have a lower frequency of mutants exceptional to linearity than known non-linear loci. On the basis of maps made from mutants sampled from the leu-2 data, it seemed unlikely that non-linearity would be observed with less than 24 complementing mutants or 13 complementing groups in the sample, and therefore many loci with linear maps are likely to be found to have non-linear maps when larger samples of mutants are tested. This conclusion is important in attempting to correlate the structure of complementation maps with recombination maps and with functional data concerning enzyme activities.
The relationship between the number of complementing mutants, number of groups and number of units at the leu-2 locus is described and a statistical method of determining the total number of groups at a locus is discussed.
Known complex complementation maps have been replotted according to consistent rules, and are illustrated in a shorthand form. The form of the complex maps is discussed in relation to current hypotheses concerning the interpretation of complementation maps. In particular an interpretation of the ‘circular’ leu-2 map is given in terms of the theory of complementation proposed by Crick & Orgel (1964).
The effect of linkage on limits to artificial selection
- W. G. Hill, Alan Robertson
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- 14 April 2009, pp. 269-294
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(i) A computer simulation study has been made of selection on two linked loci in small populations, where both loci were assumed to have additive effects on the character under selection with no interaction between loci. If N is the effective population size, i the intensity of selection in standard units, α and β measure the effects of the two loci on the character under selection as a proportion of the pheno-typic standard deviation and c is the crossover distance between them, it was shown that the selection process can be completely specified by Niα, Niβand Nc and the initial gene frequencies and linkage disequilibrium coefficient. It is then easily possible to generalize from computer runs at only one population size. All computer runs assumed an initial population at linkage equilibrium between the two loci. Analysis of the results was greatly simplified by considering the influence of segregation at the second locus on the chance of fixation at the first (defined as the proportion of replicate lines in which the favoured allele was eventually fixed).
(ii) The effects of linkage are sufficiently described by Nc. The relationship between chance of fixation at the limit and linkage distance (expressed as 2Nc/(2Nc + 1)) was linear in the majority of computer runs.
(iii) When gene frequency changes under independent segregation were small, linkage had no effect on the advance under selection. In general, segregation at the second locus had no detectable influence on the chance of fixation at the first if the gene effects at the second were less than one-half those at the first. With larger gene effects at the second locus, the chance of fixation passed through a minimum and then rose again. For two loci to have a mutual influence on one another, their effects on the character under selection should not differ by a factor of more than two.
(iv) Under conditions of suitable relative gene effects, the influence of segregation at the second locus was very dependent on the initial frequency of the desirable allele. The chance of fixation at the first, plotted against initial frequency of the desirable allele at the second, passed through a minimum when the chance of fixation at the second locus was about 0·8.
(v) A transformation was found which made the influence of segregation at the second locus on the chance of fixation at the first almost independent of initial gene frequency at the first and of gene effects at the first locus when these are small.
(vi) In the population of gametes at final fixation, linkage was not at equilibrium and there was an excess of repulsion gametes.
(vii) The results were extended to a consideration of the effect of linkage on the limits under artificial selection. Linkage proved only to be of importance when the two loci had roughly equal effects on the character under selection. The maximum effect on the advance under selection occurred when the chance of fixation at both of the loci was between 0·7 and 0·8. When the advance under selection is most sensitive to changes in recombination value, a doubling of the latter in no case increased the advance under selection by more than about 6%. The proportion selected to give maximum advance under individual selection (0·5 under independent segregation) was increased, but only very slightly, when linkage is important.
(viii) These phenomena could be satisfactorily accounted for in terms of the time scale of the selection process and the effective size of the population within which changes of gene frequency at the locus with smaller effect must take place.
Genetical and embryological studies of the jt form of syndactylism in the mouse
- Elizabeth M. Center
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- 14 April 2009, pp. 33-40
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1. A form of syndactylism (jt) involving two or three digits on one or more feet has been found in the descendants of Swiss-albino mice treated with HN2. The fusions are in a proximo-distal direction and usually involve only the skin and soft tissues; digits 1 and 5 are unaffected.
2. This type of syndactylism is apparently due to a single recessive gene, jt, whose penetrance is influenced by ‘genetic background’. Apparently the identical mutation has occurred independently in stock maintained at Bar Harbor and Stanford. Linkage tests indicate that jt is not linked with c or lu.
3. Embryological studies of the developing footplate reveal a loss in both preaxial and postaxial footplate tissue, primarily the latter, prior to and during the condensation of mesenchyme into the precartilage of the digital elements in the jt/jt limb.
4. The importance of the shape and amount of footplate tissue to the resultant morphology of the foot is apparent. It is evident that genes such as jt are responsible for changes in footplate development, but the immediate underlying factors which cause these alterations remain an unsolved problem.
‘Nude’, a new hairless gene with pleiotropic effects in the mouse
- S. P. Flanagan
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- 14 April 2009, pp. 295-309
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1. Nude is a new recessive gene causing hairlessness in the mouse. It is linked to rex and trembler in linkage group VII. The order of the three loci and the recombination frequencies are as follows:
2. In addition to hairlessness the new gene causes reduced body growth rate, very low fertility and a liver disease causing death. Nude mice may be classified at birth by the absence of vibrissae.
3. The hairlessness is due to abnormal keratinization of hair in the follicles. The skin histology resembles that of naked mice. The hair follicles were found to be deficient in free sulphydryl groups.
4. The majority of nude mice die of general body weakness within 2 weeks of birth. The survivors grow slowly and may live for a considerable period. But all nude mice eventually die, usually between 3 and 14 weeks of age.
5. The livers of dead or moribund nude mice are covered with lesions and scars. The defect has been traced histologically to its initial stage, namely, necrosis of small areas of tissue.
6. Attempts to relate the deficiency of sulphydryl groups in the hair follicles to abnormal sulphur metabolism in the liver were unsuccessful.
7. Pseudo-cysts of a parasitic protozoan, Toxoplasma gondii, were identified in the liver and brain of nude mice. In one case the free form of the organism was found.
8. The possible relationship between the liver disease and the pathogenic organism is discussed.
The ecological genetics of growth in Drosophila 8. Adaptation to a New Diet
- Forbes W. Robertson
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- 14 April 2009, pp. 165-179
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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.
Genetical analysis of proline mutants and their suppressors in Aspergillus nidulans
- P. Weglenski
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- 14 April 2009, pp. 311-321
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1.Complementation between thirteen proline auxotrophs in A. nidulans was studied. Two groups of mutants with different complementation pattern were found. These two groups could also be distinguished on the basis of recombination tests.
2. The spontaneous reversion rate of proline mutants was established. In all cases studied the reversions were due to suppressor mutations. Dominant, semi-dominant and recessive suppressors were distinguished.
3. Complementation between recessive suppressors was studied. Only a few of the suppressors obtained could be located in different complementation groups.
4. Three suppressor loci were mapped, two of them, su-2 and su-6 in chromosome III linked to the phen-2 locus, respectively 22 and 26 map units distant, and the third in chromosome I, linked to the ad-9 locus (1·9 map units). su-2 is a mutant at the Su-4 pro locus already identified by Forbes (1956).
5. The action of these suppressors is thought to consist in affecting the pathway of arginine synthesis by one of three mechanisms: (1) accumulation of an inter mediate (ornithine); (2) increased activity of ornithine δ-transaminase; and (3) a third, as yet, unclear process possibly involving feed-back regulation of arginine synthesis or the regulation of arginine breakdown to ornithine.
The Dosage compensation of Drosophila and mammals as showing the accuracy of the normal type*
- H. J. Muller, W. D. Kaplan
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- 14 April 2009, pp. 41-59
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Examination, in the salivary gland nuclei of D. melanogaster males, of four cases of translocation between the X and fourth chromosome, involving breaks of the X in widely different positions, disclosed no influence of the rearrangement on the width, morphology or chromatin-staining of either of the separated parts of the X, or on the fourth chromosome. Both parts of the single X were distinctly narrower than were the double major autosomes, as is true for the X of normal males but not for the double X of females. At the junction between a portion of a translocated single X and the double fourth chromosomes the transition in width, morphology and staining was abrupt and striking. As in structurally normal males, however, the parts of a single X, here removed from one another by the translocation, did appear to be somewhat swollen, as compared with half of a double chromosome, but to be correspondingly paler in stain, so as to indicate an unaltered amount of chromatin (see also Offermann, 1936; Rudkin, 1964; Pavan & Frota-Pessoa, 1964).
The above evidence of the regional autonomy in characteristics of the different parts of the X studied by us, and also of the fourth chromosome, is in contrast to the lack of such autonomy found in translocation studies on mammals, where the X chromosome and that joined with it are subject to an influence diffusing along them and thus acting ‘wholesale’, rather than ‘piecemeal’. Likewise, a re-examination of the earlier genetic evidence on dosage compensation in Drosophila leads back again to a decidedly ‘piecemeal’ interpretation of its operation and evolution, according to which most genes in the X, and sometimes even different phases of the action of the same gene, have their own system of separately evolved, scattered compensators, which are also located in the X.
The fact that two so differently working compensation mechanisms as those in Drosophila and mammals have evolved independently to serve the same function emphasizes the importance of that function. That is, it points up the survival value of having the effectiveness of normal genes regulated to a very exact level. For the compensation enables the single representative of the X in the male cell to become equivalent to the two representatives of the X in the female cell. Moreover, this equivalence is of a considerably finer grade than that already afforded by the phenomenon termed ‘dominance’, which has evolved to meet the same basic need (that of phenotypic stabilization), and which has, incidentally, made even the uncompensated effects of one and two doses of either sex-linked or autosomal normal genes not readily distinguishable in most cases.
Taken by itself, the ‘piecemeal’ mechanism of Drosophila provides far stronger evidence for this conclusion than does the ‘wholesale’ mechanism in mammals. For the former must have required the establishment of far more numerous mutational steps and, taken individually, each of these steps was of correspondingly lesser survival value. Since they nevertheless affected fitness enough to become established, it also follows that usually a normal gene—or at any rate one of the kinds whose mutants have usually been studied—confers a significantly higher fitness when not heterozygous for such a mutation in it, despite the seeming recessiveness of most mutations. Thus, the expression ‘normal gene’ continues to have a very high validity.
A test of sexual isolation in Drosophila
- Forbes W. Robertson
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- 14 April 2009, pp. 181-187
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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.
Studies on mitotic gene conversion in Ustilago
- Robin Holliday
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- Published online by Cambridge University Press:
- 14 April 2009, pp. 323-337
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In order to develop a system for the study of the mechanism of intragenic recombination in Ustilago, mutants lacking nitrate reductase activity were isolated, and five alleles were combined in pairs in ten vegetative heteroallelic diploids. The diploids have the mutant phenotype, i.e. inability to utilize nitrate as sole source of nitrogen, but they will recombine to produce wild-type cells much more frequently than the back-mutation rates of haploids or homoallelic diploids. The spontaneous rate of recombination can be enormously increased by low doses of UV light, particularly if treatment is during the period of DNA synthesis in the mitotic cycle. By means of half-tetrad analysis it has been shown that this process of intragenic recombination, as in other fungi, is due to gene conversion rather than reciprocal exchange. It has also been shown that the frequency of UV-induced conversion under standard conditions gives a rough measure of the distance between two mutant sites, since it was possible to use these frequencies to make a linear fine structure map of the gene. These results are discussed in relation to a hybrid DNA model for gene conversion slightly modified from that previously suggested for meiotic recombination.
Compatibility and stability of diploids in Coprinus lagopus
- Lorna A. Casselton, D. Lewis
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- 14 April 2009, pp. 61-72
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Artificially selected diploids of Coprinus lagopus when mated in compatible combinations, either together or with haploids, produce dikaryotic mycelia which are typical of normal haploid-haploid dikaryons. In a diploid-haploid dikaryon, the diploid nucleus is not as stable as when alone in a monokaryon but it can persist through repeated sub-culturing into a fruiting body and eventually through meiosis into the basidiospores. In a diploid–diploid dikaryon either one or the other nucleus becomes haploid so that fruiting bodies with two diploid nuclei are never formed. This fact constitutes a restriction on diploidy in nature and a useful method of reducing diploids to the haploid state.
Matings that might be considered to be incompatible at the B mating gene show a significant difference which is related to the number of B alleles common to the mating colonies. Matings with one B allele in common, e.g. B3B6+B2B3 produce fully compatible and normal dikaryons. Matings with two B alleles in common, e.g. B3B6+B3B6 have, at first while the diploid nuclei still persist, the appearance of an incompatible common B haploid heterokaryon. This indicates that the B incompatibility system is based not on a complementary action between different B alleles but on an oppositional action between the same alleles neutralizing the B gene product which is necessary for dikaryon formation.
F-prime factor formation in E. coli K12
- John Scaife
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- 14 April 2009, pp. 189-196
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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.
Snell's waltzer, a new mutation affecting behaviour and the inner ear in the mouse
- M. S. Deol, Margaret C. Green
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- Published online by Cambridge University Press:
- 14 April 2009, pp. 339-345
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A new recessive gene affecting behaviour and the inner ear in the mouse has been discovered. It was named Snell's waltzer, and assigned the symbol sv. It is in linkage group II, about two map units from short ear (se) on the side opposite to that of luxoid (lu). The behaviour of sv/sv mice closely resembles that of other members of the shaker-waltzer group. The abnormalities of the inner ear consist in degeneration of certain parts following normal morphogenesis. The entire neuro-epithelium —that is, the organ of Corti, the two maculae and the three cristae—is affected, and this feature distinguishes it from other degenerative type mutants of this group.
Lack of evidence that inactivation of the mouse X-chromosome is incomplete
- Mary F. Lyon
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- 14 April 2009, pp. 197-204
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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.
Restricted trandsuction by bacteriophage P22 in Salmonella typhimurium
- P. F. Smith-Keary
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- Published online by Cambridge University Press:
- 14 April 2009, pp. 73-82
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1. In the transduction pro-401 (×) + some of the transductants are surrounded by several hundred small wild-type satellite colonies; these transductants spontaneously release phage which transduces pro-401 to wild-type at high frequency (HFT phage).
2. When the HFT phage is used to infect pro-401 at very low multiplicities of infection, most of the transductants are defective lysogens and segregate proline-requiring phage-sensitive derivatives; these transductants are apparently heterogenotes. At higher multiplicities of infection, or with lysogenic recipients, a higher proportion of satellited transductants is found.
3. The HFT phage preparations transduce only the proline region of the donor genome.
4. The existence is inferred of a defective P22 particle specifically incorporating the proline region of the Salmonella chromosome; these defective particles can establish themselves as prophage and confer immunity upon the infected cell, but are unable to replicate unless a normal prophage is also present. Satellited transductants are lysogenic both for a normal and defective (proline region carrying) phage, and so on lysis release transducing phage.
5. This system is compared with the λdg-gal and P1-dl-lac systems in E. coli.
A genetical study of thymineless mutants of E. coli K12
- S. I. Alikhanian, Tamilla S. Iljina, Eza S. Kaliaeva, Svetlana V. Kameneva, V. V. Sukhodolec
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- 14 April 2009, pp. 83-100
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The 150 independently isolated thy− mutants of E. coli K12 Hfr 3.OSO were studied genetically and phenotypically. Variants were found among the mutants in respect to the lag period of thymineless death, and temperature sensitivity. The latter correlates with mutations located at a specific site on the genetical map.
The thy locus is located between the cys and ser/gly genes, and is a linear structure where 134 thy mutants are distributed over more than 17 sites. The site distribution of the mutants is not regular: about a half of them (62) are localized within one site and all these are temperature-sensitive.
Two further genes involving utilization of thymine—tlr and td—were found. Mutations of tlr lead to a reduced thymine requirement (0·5 μg./ml. instead of 20 μg./ml.). A mutation of td results in thymidine sensitivity.
This latter character is expressed when the td-s allele is transferred into E. coli K12, prototrophic for thymine, by conjugation. Thymidine inhibition can be reversed by the addition of any riboside to the growth medium. Both genes map at the proximal end of the Hfr 3.OSO chromosome and are linked with the thr gene. The most probable gene order is: tlr-td-thr.
The following results have been obtained from 14C-thymine incorporation experiments with wild-type cells, as well as with thy−tlr+ and thy−tlr− cells: (1) Wild-type cells incorporate exogenous thymine extremely poorly, but incorporate thymidine better. (2) The thy−tlr+ mutants are able to incorporate thymine only when high concentration are used, but can utilize a low concentration of thymidine. (3) The thy− mutants are able to incorporate exogenous thymine as well as thymidine at low concentration. (4) The tlr mutation is a thymine-specific one.
The limits to artificial selection for body weight in the mouse I. The Limits Attained in Earlier Experiments
- R. C. Roberts
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- 14 April 2009, pp. 347-360
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1. The results of some selection experiments for body weight in the mouse, conducted in the past in this laboratory, have been examined from the point of view of the limits ultimately reached.
2. The limits that are apparently attained do not necessarily remain stable over prolonged periods of time; two large lines showed marked decreases despite continued selection for high body weight.
3. Selection for high body weight reached a limit in the region of 30 g. at 6 weeks of age; small mice reached their limit at around 12 g.
4. The time taken to reach the limit may vary from ten to thirty generations, even for this one trait.
5. The total response for unidirectional selection was between two and six times the phenotypic standard deviation, or three to twelve times the additive genetic standard deviation.
6. Consideration of the half-life of the selection responses excluded the likelihood of the chance fixation of alleles unfavourable to the direction of selection.
7. The loci contributing to the response could each have an effect amounting to anything from one-half to one phenotypic standard deviation in the base population.
8. This indicated that up to twenty loci had contributed to the response.
9. The intensity of selection practised was close to the optimum for obtaining the maximum total response.
10. The rule of parsimony would indicate the exhaustion of the additive genetic variance as an adequate explanation of the limits attained.