Test for selection on polymorphic isozyme genes using the population cage method

Published experimental data of several species of Drosophila using the population cage method were reexamined. The authors of these works have claimed that selection operates on polymorphic isozyme genes. Since selection coefficients (s) estimated by us for the above data, assuming overdominance, or for some cases, additivity between alleles, were too large (s greater than 0.1), we conclude that these experimental results do not reflect the effects of the single loci in question; rather, we suggest that careful experimentation and judgment are required for testing with the population cage method whether or not selection actually operates on single genes.

Linkage disequilibrium in isolated populations of Drosophila melanogaster

In order to test the interaction effect of overdominance and random genetic drift on the formation of linkage disequilibria under the condition of multiplicative gene action, linkage disequilibria between isozyme genes, inter se, and between polymorphic inversions and isozyme genes were tested for the second chromosomes of Drosophila melanogaster sampled from two isolated Pacific islands and one locality of the northern district of the mainlands of Japan. The effective sizes of these populations were known to be approximately 3,000 to 6,000 on the basis of the allelism rate of lethal chromosomes and their frequencies. The following results were obtained: (1) No linkage disequilibrium considered to be induced by epistasis, including the interaction between overdominance and random genetic drift, was detected. (2) Nonrandom association between polymorphic inversions and isozyme genes that are included in the inversions or located in the adjacent region outside the inversions was detected. (3) On the basis of the comparison of chi (2) values, indicating the magnitudes of linkage disequilibrium, between the present isolated populations and the Raleigh, N.C., population (Mukai et al. 1974, 1977), the characteristics of the isolated populations and the nature of these linkage disequilibria are discussed.

Selection of viability at loci controlling protein polymorphisms in Drosophila melanogaster is very weak at most

In an isolated Drosophila population the frequency of chromsomes carrying the second chromosome inversion In(2L)t is about 60%. Three isozyme loci are nearly monomorphic in the inversion-carrying chromosomes but are highly polymorphic in other chromosomes. From known recombination frequencies and the slight polymorphism, it is estimated that the inversion-carrying chromosomes are descended from one or a few identical chromosomes introduced about 1000 generations previously. This is long enougn for lethal and mildly deleterious mutants to reach equilibrium frequencies but not long enough for very weakly selected mutants. Because the difference in viability between homozygous and heterozygous chromosomes is the same, it is inferred that there is at most very weak selection of isozyme loci. This method is sensitive enough to detect selective differences of 0.0005 per locus or less.

Differences in contractile responses to acetylcholine and serotonin in rat stomach fundus strips

The differences in contractile responses to acetylcholine (ACh) and serotonin (5-HT) were studied in the rat fundus strips. At low temperatures, the contractile response to 5-HT was reduced more noticeably than to ACh. The contraction induced by ACh at 4 degrees C was not inhibited by La3+. Removal of extracellular Ca2+ reduced the response to 5-HT more significantly than that to ACh.. La3+, verapamil and D-600 inhibited the 5-HT response more than the ACh response. In Ca2+-free solution the contractile response to ACh of a fundus strip preloaded with Ca2+ was lar ger than that to 5-HT. These results suggest that the Ca2+ movements induced by ACh and by 5-HT are different, and that the difference in Ca2+ movements may cause the difference in contractile responses to the two agonists at low temperatures.

SPONTANEOUS ALLOZYME MUTATIONS IN DROSOPHILA MELANOGASTER: RATE OF OCCURRENCE AND NATURE OF THE MUTANTS

After additional generations of accumulation of allozyme mutants, the 1,000 lines of MUKAI and COCKEFUXAM (1977) were again screened for the same five loci (a-Gpdh, cMdh, Adh, Hex-C and α-Amy), as well as for two new loci (Got-2 and Dip-A). Based on 3,111,598 allele generations: (1) the average mutation rate to new mobility variants with normal function was estimated to be 1.28 x 10-6, and (2) the average mutation rate to null alleles was estimated to be 3.86 x 10-6. A qualitative analysis of the nulls provided evidence that most of the mutants recovered are due to base substitutions. No apparent correlation was observed between structural gene size and mutation rate.

Radiolucent intracranial dermoid Cyst. Report of an unusual case

An intracranial dermoid cyst presenting as a radiolucent mass on the plain radiographs of the skull is described.

Lack of experimental evidence for frequency-dependent selection at the alcohol dehydrogenase locus in Drosophila melanogaster

Two experiments were conducted for testing frequency-dependent selection at the alcohol dehydrogenase locus (Adh; map position, 2--50.1), where two types of alleles, AdhF and AdhS, have been found to be polymorphic in natural populations. First, a competition experiment was carried out between inversion-free AdhF/AdhF and AdhS/AdhS genotypes at five gene frequency levels (those of AdhF = 0.83, 0.67, 0.5, 0.33, and 0.17). First-instar larvae of the two homozygous types in various ratios were placed in a vial, and the frequency of adult flies of each genotype was determined. However, the relative viabilities of these two types did not change with the change in the frequencies of genotypes or genes. The second test was performed by using the results of viability tests with the Cy method of Wallace, using the results of inversion-free crosses only. Although there was variation in the gene frequencies among the vials in testing the viability of individuals consistng of random combinations of two second chromosomes, no difference was found in average viabilities among the vials in which the gene frequencies were different. Thus, no evidence for frequency-dependent selection was obtained.

EFFECTS OF MARKER CHROMOSOMES ON RELATIVE VIABILITY

Viability relative to Cy/Pm as a standard was studied in Drosophila melanogaster. One experiment, E1, consisted of progeny from eleven distinct 7 x 7 factorial mating designs with reciprocals for second chromosomes extracted from a natural population. The other experiment, E2, consisted of two distinct sets of heterozygotes with reciprocals and corresponding homozygotes. It was established from E1 that there are little to no synergistic effects among different genotypes in a vial and that Cy and Pm heterozygotes vary almost as much as would be expected if one chromosome were held constant for wild-type heterozygotes. In wild-type heterozygotes, variances were estimated to be 0.0099 for average chromosomal effects, 0.0054 for interactions of chromosomes, 0.0021 for maternal effects, 0.0079 for paternal effects, and -0.0010 for the remaining interaction effects, all being significantly different from zero except the last. The variances of Cy and Pm heterozygotes, covariance of Cy and Pm heterozygotes, and covariances of Cy and Pm heterozygotes with wild-type heterozygotes, as well as the comparable statistics available in E2, all showed a large paternal component of variance and a smaller maternal component of variance, both unexpected results.—From E2 the variance of homozygotes, excluding error variance, was estimated to be 0.0149, and the covariances of homozygotes with wild-type heterozygotes to be 0.0056 for maternally derived chromosomes common and 0.0126 for paternally derived chromosomes common, again showing the larger paternal than maternal influence. The average genetic regression of heterozygotes on homozygotes of 0.61 was reduced only slightly to 0.56 by correcting for maternal and paternal variances. These genetic regressions, generally utilized as estimators of the average degree of dominance, are larger than any previously reported.—Differential meiotic drive in Cy and Pm parents was shown to be compatible with the large paternal and maternal variances, but other causes cannot be ruled out.—Approximations were developed for translating various variances, covariances, and regressions between single- and double-marker experiments, assuming that marker chromosomes behave as typical wild-type chromosomes in one case and assuming a (partially) recessive model with the population in mutation selection balance in another case. Various features, particularly the estimation of dominance, were compared and discussed between the two cases.

[An evaluation of inhalation and perfusion scintigraphy with 81mKr (author's transl)]

As 81mKr is a radioactive noble gas which has a very short half-life of 13 seconds, it leads no environmental contamination in a laboratory room. Because of its low radiation dose in the human body, it had many merits in routine tests of the inhalation and perfusion scintigraphy, which were very useful extremely in infants, old men and severe patients in need of cooperation. The studies with 81mKr were, either, useful as a load test, since these could repeatedly readily, evaluate the ventilation and perfusion dynamics before and after administration of drugs and show the distribution of the ventilation and perfusion correspond to the various lung volume level.

Isolation and characterization of a plaque-forming lambda bacteriophage carrying a ColE1 plasmid

A plaque-forming lambdaimm434 bacteriophage carrying the entire genome of colicinogenic factor E1 has been isolated and characterized. This phage, lambdaimm434ColE1, can lysogenize as a stable plasmid within a recombination-deficient Escherichia coli cell that lacks the normal attachment site for lambda phage. Furthermore, it has been found that lambdaimm434ColE1 phage carrying amber mutations in the O and P genes of the lambda genome, i.e., lambdaimm434OamPamColE1, behaves as a plaque-forming phage, and this finding suggests that the ColE1 factor DNA permits replication of the DNA of the plaque-forming phage.

Quantitative assessment of ventilation-perfusion mismatch by radioxenon imaging of the lung

By the use of xenon-133 and a scintillation camera with digital data storage and processing system, a topographic relationship between ventilation distribution (V) and perfusion distribution (Q) was examined quantitatively in two groups of normal nonsmokers and one of older smokers, all healthy. In addition, subjects with a variety of cardiopulmonary disease were tested. The fractional regional ventilation (VR) and regional perfusion (QR) were plotted against the V/Q ratio on a logarithmic abscissa for the normal subjects; both were distributed log-normally with a narrow standard deviation, and were dissociated slightly from each other. However, with smoking and with increasing age, the s.d. and the dissociation became wider, suggesting an impairment of gas exchange as estimated by alveolar-atrial gas-pressure differences (A-aD), which were calculated by putting these topographic relationships into a gas-exchange program in a computer. In various cardiopulmonary diseases a good correlation was found between the estimated A-aDO2 thus obtained and the actual A-aDO2 derived from analysis of the blood gases.

INVERSIONS FAIL TO ACCOUNT FOR ALLOZYME CLINES

Allozyme and inversion data from natural populations of Drosophila melanogaster from the eastern United States were analyzed to determine whether the clines at allozyme loci are due to nonrandom associations with common cosmopolitan inversions. All inversions show strong clines. Clines were large and significant for half of the eight allozyme loci. An analysis of the contribution of inversions to clines of allozyme genes revealed three outcomes: the inversion cline (1) enhanced the allozyme cline, but was only partly responsible, (2) reduced the allozyme cline, and (3) had no effect. The allozyme clines were mainly determined by the pattern of allele frequencies within the chromosomal arrangements. Consequently, it was concluded that allozyme clines would exist in the absence of inversion clines.

Purification and properties of deoxyribonuclease from human urine

The DNAase in human urine was purified about 30-fold with a recovery of 28%. This involved DEAE-cellulose and phosphocellulose chromatography steps and gel filtration on Sephadex G-75. The enzyme required divalent cations such as Co2+, Mg2+, Mn2+ and Zn2+ for activity, but Ca2+, Cu2+ and Fe2+ were ineffective. EDTA and G-actin inhibited the reaction. The maximum activity was observed at pH 5.5 in acetate buffer plus Co2+ or Mg2+ and Ca2+. It had a molecular weight of approximately 38 000, estimated by gel filtration on Sephadex G-75 and isoelectric point of around pH 3.9. The enzyme is an endonuclease which hydrolyzes native, double-stranded DNA about 3 to 4 times faster than thermally denatured DNA to produce 5'-phosphoryl- and 3'-hydroxyl-terminated oligonucleotides. The final preparation was free of non-specific acid and alkaline phosphatases, phosphodiesterase and ribonuclease activities.

INVERSION CLINES IN POPULATIONS OF DROSOPHILA MELANOGASTER

Twenty different natural populations of Drosophila melanogaster were sampled to determine the frequencies of inversions. Based on their frequencies and geographical distributions, the inversions could be classified as follows: (1) Common cosmopolitan inversions that are present in many populations in frequencies exceeding five percent and that may exhibit frequency clines over large geographical regions; (2) Rare cosmopolitan inversions that occur throughout the species range but usually at frequencies below five percent and that may be absent in many populations; (3) Recurrent endemic inversions that are found in several adjacent populations in frequencies usually not exceeding one or two percent; and (4) Unique endemic inversions that are found only among the progeny of a single individual and that may represent one aspect of the syndrome termed "hybrid dysgenesis". Four common cosmopolitan inversions that exhibit highly significant clines in populations in the eastern United States are In(2L)t, In(2R)NS, In(3L)P and In(3R)P.

Spontaneous mutation rates at enzyme loci in Drosophila melanogaster

In a marked-inversion-balanced lethal system mutations were accumulated at a minimum pressure of natural selection on 2000 second chromosomes of Drosophila melanogaster that originated from 4 stem chromosomes. Five enzyme loci were tested: alpha-glycerol-3-phosphate dehydrogenase (EC 1.1.1.8), malate dehydrogenase (Mdh, EC 1.1.1.37), alcohol dehydrogenase (EC 1.1.1.1), hexokinase-C (Hex-C tentative name), and alpha-amylase (Amy, EC 3.2.1.1). Three band-morph mutants, one at the Mdh locus, one at the Hex-C locus, and one at the Amy locus, were detected out of 1,658,308 allele replications. In addition, 17 null mutants were found. Accepting that the number of structural genes is the same as that of bands in the salivary gland chromosomes, the total mutation rate per generation for all the structural genes in the second chromosomes is estimated to be 0.008-0.040, which is much smaller than that estimated for viability polygenes (0.12-0.17). Thus, it is speculated that most viability and other fitness polygenes are located in controlling regions outside the structural genes.

The genetic structure of natural populations of Drosophila melanogaster XIII. Further studies on linkage disequilibrium

The Raleigh, North Carolina, population of Drosophila melanogaster was examined for linkage disequilibrium in 1974, several years after previous analyses in 1968, 1969, and 1970. alphaglycerol-3-phosphate dehydrogenase-1 (alphaGpdh-1), malate dehydrogenase-1 (Mdh-1), alcohol dehydrogenase (Adh), and hexokinase-C (Hex-C, tentative name, F. M. Johnson, unpublished; position determined by the present authors to be 2-74.5) were assayed for 617 second chromosomes, and esterase-C (Est-C) and octanol dehydrogenase (Odh) were assayed for 526 third chromosomes. In addition, two polymorphic inversions in the second chromosomes [In(2L)t and In(2R)NS] were examined, and the following findings were obtained: (1) No linkage disequilibrium between isozyme genes was detected. Significant linkage disequilibria were found only between the polymorphic inversions and isozyme genes [In(2L)t vs. Adh, and In(2R)NS vs. Hex-C]. Significant disequilibrium was not detected between In(2L)t and alphaGpdh-1, which is included in the inversion, but a tendency toward disequilibrium was consistently found from 1968 to 1974. The frequency of two-strand double crossovers within inversion In(2L)t involving a single crossover on each side of alphaGpdh-1 was estimated to be 0.00022. Thus, the consistent but not significant linkage disequilibrium between the two factors can be explained by recombination after the inversion occurred. (2) Previously existing linkage disequilibrium between Adh and In(2R)NS (the distance is about 30 cM, but the effective recombination value is about 1.75%) was found to have disappeared. (3) No higher-order linkage disequilibrium was detected. (4) Linkage disequilibrium between Odh and Est-C (the distance of which was estimated to be 0.0058 +/- 0.002) could not be detected (chi(2) (df=1) = 0.9).-From the above results, it was concluded that linkage disequilibria among isozyme genes are very rare in D. melanogaster, so that the Franklin-Lewontin model (Franklin and Lewontin 1970) is not applicable to these genes. The linkage disequilibria between some isozyme genes and polymorphic inversions may be explained by founder effect.

Regional lung function in pulmonary hypertension

Utilizing 133-xenon gas and a scintillation camera, we examined the distributional relation between ventillation (V) and perfusion (Q) in patients with pulmonary hypertension (PH) comprising mitral stenosis (MS) and left to right shunt disease (shunt). In the case of MS, Q decreased at the dependent part of lung in proportion to the degree of PH, but preserved normal V and resulted in high V/Q value at this region, which was estimated to cause and physiological dead space effect on the gas exchange function. In the case of shunt disease, even with a high degree of PH, reduction of Q at the dependent lung region was not observed with normal preservation of V, thus obviating any type of the V/Q imbalance. Since the washout of xenon gas by resting tidal volume ventilation was observed to be delayed from the dependent lung region, latent ventilatory impairment in this region due to small airway closure at tidal breathing was also suspected.

Cloning bacterial genes with plasmid gammadv

recombinant molecules recovered from transformed E. coli cells consisted of a biologically functional bacterial DNA fragment and, excep

HIGH RATES OF OCCURRENCE OF SPONTANEOUS CHROMOSOME ABERRATIONS IN DROSOPHILA MELANOGASTER

Spontaneous mutations were accumulated for 40 generations in 140 unrelated second chromosomes with the standard gene arrangement. These were extracted from the same population by using the marked inversion technique, and the following findings were obtained: (1) In 42 out of the 140 chromosome lines, chromosome aberrations were detected by examining the salivary gland chromosomes: 40 paracentric and 15 pericentric inversions, 2 reciprocal translocations between the second and the third chromosomes, and 6 transpositions. (2) In 63 out of the 90 originally lethal-free lines, recessive lethal mutations occurred. (3) There were only 3 lines that acquired chromosome aberrations (inversions) with no lethal effects in the homozygous condition. (4) In a comparison of these results with those of the (CH), (PQ), and (RT) chromosomes in which no chromosome aberrations occurred after accumulating mutations for 22058 chromosome·generations (Yamaguchi and Mukai 1974), it was concluded that some of these 140 chromosomes carried a kind of mutator. (5) The frequency of mutator-carrying chromosome lines was estimated to be 0.66 on the basis of the distribution of the break-points on the chromosome lines and the frequency of lines that acquired neither recessive lethal mutations nor chromosome aberrations. Thus, the average number of breaks per mutator-carrying chromosome was estimated to be about 0.19/generation.

On the basis of these estimates, the nature of the mutator factor was discussed.

Determination of liver volume by the use of a gamma camera connected with 1600 or 4096 channel analyser in diffuse liver diseases

A method of in vivo calculation of liver volume using a gamma camera and a 1600 or 4096 channel analyser is described. Topographically, the maximum thickness giving the highest counts in matrix data in the right anterior oblique projection is measured as the maximum breadth in the right posterior oblique projection. Then the total counts given by the liver can be transformed to the volume. The necropsy criteria indicated that the errors were between --11-9 +/- 14-7% in 13 cases by the use of 198Au-colloid. In 20 selected normal control subjects, the mean of measured liver volumes was 779 +/- 99 cm3/m2 of surface area. The volumetric calculation using 99mTc-Sn-colloid or 99mTc-phytate was compared with 198Au-colloid in the same patients. The results with 99mTc-Sn-colloid were well correlated with those with 198Au-colloid, but the results with 99mTc-phytate and 198Au-colloid were almost equal. It was concluded that, excluding rare severe cases of liver cirrhosis, liver volume can be measured with reasonable accuracy.

The genetic variability of third chromosomes in a local population of Drosophila melanogaster

Five hundred and two third chromosomes were extracted from a large cage population of Drosophila melanogaster initiated two months after collection of the progenitors near Raleigh, North Carolina in 1970.---Salivary gland chromosomes of 489 chromosome lines were examined and 54 chromosomes were found to carry inversions. The inversions were classified into three polymorphic types [In (3L)P, In (3R)P, and In (3R)C] and two unique types. The polymorphic inversions were found in frequencies of 0.012, 0.88, and 0.010, respectively.--Viabilities of homozygotes and heterozygotes were examined. Chromosomes with lethals occurred with a frequency of 0.495: 0.537 in the group of inversion-carrying chromosomes and 0.490 in the group of inversion-free chromosomes. The average homozygote viability computed on the basis of an average heterozygote viability of 1.0000 was 0.3235 if lethal lines were included and 0.6290 if they were excluded. The detrimental load to lethal load ratio (D:L ratio) was 0.70 (=0.4636-0.6650). The average viability of lethal heterozygotes was significantly larger than that of lethal-free heterozygotes. It appears, however, that lethal genes in heterozygotes have deleterious effects on fitness as a whole.--The average degree of dominance for viability polygenes was estimated to be about 0.3-0.4 in lethal-free individuals and nearly zero in lethal heterozygotes. Overdominance or some form of balancing selection was suggested at some loci. The difference between the values obtained for average degree of dominance due to genetic backgrounds and superior vibaility of lethal heterozygotes (but not fitness as a whole) suggest that some epistasis or coadaptation occurs.--The results described above are similar to those obtained for the second chromosomes.

Functional imaging of intrarenal blood flow using scintillation camera and computer

In order to obtain spatial distribution of an index for regional blood flow at each element on a scintigraphic image of the kidney, we attempted the construction of the so-called functional image. After injecting a single bolus of 133Xe into a renal artery by means of a catheter, this objective was accomplished using digital computer processing for a sequence of scintillation camera recordings of the following washout process from the kidney. This is expressed in a form of matrix of disappearance rate constant. Calculation for the rate constant, the flow index of the functional image, was done using either the least squares (LS) method or height-over-area (H/A) method. Although the former method was considered to be theoretically suitable without undue participation of background activities, the latter was preferred because of stable results for image construction. On reviewing the functional image thus obtained from 22 patients representing a variety of renal diseases, the H/A gave specific information concerning regional distribution of the perfusion integrity mainly related to the cortical part of the kidney. This is often difficult to accomplish utilizing the conventional method of compartmental analysis of the xenon washout curve or selective renal angiography.

In vitro construction of different oligomeric forms of lambdadv DNA and studies of their transforming activities

Plasmid lambdadv1, which is in a dimeric form, was converted to a linear monomer duplex by the action of EcoRI restriction endonuclease that incises at a unique site in this plasmid genome. The resulting products were then joined by Escherichia coli DNA ligase to produce molecules with various oligomeric forms, and from these monomeric, dimeric, or trimeric circular molecules were purified. By transformation of cells with these DNAs, clones were obtained that carried lambdadv1 in a monomeric or dimeric form. The former type of clones have not been generated in vivo, except for one in a different host strain, and carriers of timeric or tetrameric lambdadv1's have not been obtained so far. It was observed that a considerable fraction of these oligomeric circular DNAs were converted to lower oligomers (e.g., from trimer to dimer) during transformation. The characteristics of the monomeric lambdadv1 carriers obtained were compared with those of dimeric lambdadv1 carriers. The stabilities of the plasmids of the two forms were the same. However, the monomeric plasmid carriers were less tolerant to lambdavir phage infection and perpetuated about 30% less plasmid genomes in monomer units. Furthermore, dimeric plasmid carriers appeared spontaneously and accumulated in cultures of the monomeric lambdadv1 carriers.

Mutator factors and genetic variance components of viability in Drosophila melanogaster

In the process of testing whether or not the independent-locus selection model holds true with previously estimated genetic parameters (cf. MUKAI and MARUYAMA 1971) in D. melanogaster collected near Raleigh, North Carolina, we found an abnormal phenomenon: an unusually large increase in dominance variance for viability in comparison with additive variance with the accumulation of mutations on 140 randomyl sampled, inversion-free second chromosomes. Mutations were accumulated only through males heterozygous for the Pm-carrying chromosome [In(2LR)bwV1] and the extracted second chromosomes, and the genetic variance components were estimated by using a partial diallel cross method.--Further investigations clarified that chromosome abberations occurred at a frequency of 0.0114 per second chromosome per generation (inversions: 0.0098; transpositions: 0.0011; translocation: 0.0004), and recessive lethal mutations occurred at an average rate of 0.031 per second chromosome per generation.--From these results and from the amount of change in the homozygous load, it was speculated that about 60--70% of the second chromosomes used had a kind of mutator which induced chromosome and/or chromatid breaks at a minimum rate of 0.18 per second chromosome per generation. These breaks resulted in recessive lethal mutations at a rate more than ten times higher than the normal rate. Also these breaks were most probably the cause of male recombination.--The above unusual increase in dominance variance can be explained by assuming that chromosome segments, introduced into the extracted "wild" chromosomes by male recombinations (double crossover) from the marker chromosomes [In(2LR)bwV1], showed heterosis and linkage disequilibria with deleterious mutations and possibly with other introduced segments.--Finally, the nature and possible significance of mutator factors are discussed.

Mode of replication of plasmid lambda-dv-1

About 20 copies of plasmid lambda-dv are perpetuated per host chromosome in Escherichia coli K12 cells. The mode of DNA replication of this plasmid lambda-dv was studied, using the density-labelling technique followed by banding in DsCl. It was shown that all the copies of plasmid lambda-dv are potentially capable of replicating roughly once a cell generation. Their replication occurs by random, that is a copy of the plasmid is taken out at random for replication from a pool, to which the two replicas resulting from replication are returned. Chloramphenicol did not inhibit the initiation of a new round of replication of the plasmid molecules, indicating that selection from the replication pool does not require concomitant protein synthesis.

Variation of spontaneous occurrence rates of chromosomal aberrations in the second chromosomes of Drosophila melanogaster

After accumulating mutations by the aid of marked inversions, spontaneous occurrence rates of chromosome aberrations were estimated for 1148 chromosome lines that originated from five stem line second chromosomes of Drosophila melanogaster. In chromosome lines originating from three stem chromosomes (CH, PQ, and RT), mutations were accumulated for 7550, 7252, and 7256 chromosome generations, respectively, but no structural change was detected. For the chromosome lines that originated from the other two stem chromosomes, the situation was different: Twenty aberrations (19 paracentric inversions and 1 translocation between the second and the third chromosomes) during 45990 chromosome generations took place in the 500 chromosome lines derived from stem line chromosome (AW), and 92 aberrations (83 paracentric inversions, 6 pericentric inversions, 2 translocations between the second and the third chromosomes and 1 transposition) arose during 45006 chromosome generations in the 500 chromosome lines derived from stem line chromosome (JH). For the AW group the occurrence rate becomes 0.00043 per chromosome per generation for all aberrations and 0.00041 for inversions. For the JH group the corresponding rates are 0.00204 and 0.00198, respectively.-A non-random distribution of the breakpoint on the salivary gland chromosome was observed and the breakpoints were concentrated in the regions 26, 29, 33, and 34.-The cytoplasms and the chromosomes (other than the second chromosomes) were made approximately uniform throughout the experiments. Thus, this remarkable variability in the occurrence rate is most probably due to the differences in one or more chromosomal elements on the original five stem chromosomes. The mutable chromosomes (AW and JH) appear to carry a kind of mutator factor such as hi (Ives 1950).

The genetic variance for viability and its components in a local population of Drosophila melanogaster

Two hundred and ninety second chromosomes extracted from a natural population of Drosophila melanogaster were analyzed to estimate the genetic variance of viability and its components by means of a partial diallel cross (Design II of Comstock and Robinson 1952). The additive and dominance variances are estimated to be 0.009 and 0.0012. Using the dominance variance and the inbreeding depression, the effective number of overdominant loci contributing to the variance in viability is estimated to be very small, a dozen or less. Either the actual number of loci is small, or the distribution of viabilities is strongly skewed with a large majority of very weakly selected loci. The additive variance in viability appears to be too large to be accounted for by recurrent harmful mutants or by overdominant loci at equilibrium with various genetic parameters estimated independently. The excess might be due to frequency-dependent selection, to negative correlations between viability and fertility, or possibly to the presence of a mutator. The selection for viability and fertility, or possibly to the presence of a mutator. The selection for viability at the average polymorphic locus must be very slight, of the order of 10(-3) or less.