Viabilities of Third Chromosomes of DROSOPHILA PSEUDOOBSCURA Differing in Relative Competitive Fitness

Arrowhead (AR) third chromosome arrangements of Drosophila pseudoobscura, whose competitive fitnesses had been determined in population cages, were tested for their genetic loads in homozygous, heterozygous (homokaryotypic), and heterokaryotypic (AR/CH) combinations. The results showed that their competitive population cage performances were correlated to their viabilities as homozygotes but were not correlated to their viabilities as heterozygotes or as heterokaryotypes. However, the results do not fit in too simply with the mutational model of population structure, since the improvement of homozygous viability with increased competitive fitness was not accompanied by a significant degree of dominance as measured by the regression of viabilities of heterozygotes on homozygotes. Only the AR chromosomes derived from the population with poorest competitive fitness showed marked partial dominance (h=.35). The viabilities of heterokaryotypes were markedly uniform for all chromosomes tested and produced significantly greater numbers of flies per culture than the homokaryotypes. In general, the results show that the ranking of relative competitive fitnesses of these chromosomes is not a simple extrapolation of their viabilities, although marked changes in the populations tested have occurred. It is proposed that the differences in competitive fitness, homozygous viability, and degree of dominance observed among these chromosomes, arise from differences in genetic variability which enable different linkage relationships to be established for genes affecting these attributes.     

Dominance and overdominance of mildly deleterious induced mutations for fitness traits in Caenorhabditis elegans

We estimated the average dominance coefficient of mildly deleterious mutations (h, the proportion by which mutations in the heterozygous state reduce fitness components relative to those in the homozygous state) in the nematode Caenorhabditis elegans. From 56 worm lines that carry mutations induced by the point mutagen ethyl methanesulfonate (EMS), we selected 19 lines that are relatively high in fitness and estimated the viabilities, productivities, and relative fitnesses of heterozygotes and homozygotes compared to the ancestral wild type. There was very little effect of homozygous or heterozygous mutations on egg-to-adult viability. For productivity and relative fitness, we found that the average dominance coefficient, h, was approximately 0.1, suggesting that mildly deleterious mutations are on average partially recessive. These estimates were not significantly different from zero (complete recessivity) but were significantly different from 0.5 (additivity). In addition, there was a significant amount of variation in h among lines, and analysis of average dominance coefficients of individual lines suggested that several lines showed overdominance for fitness. Further investigation of two of these lines partially confirmed this finding.     

Genetic Load and Viability Distribution in Central and Marginal Populations of DROSOPHILA SUBOBSCURA

Relative viabilities of individuals homozygous or randomly heterozygous for wild O chromosomes derived from a marginal (Norwegian) and a central (Greek) population of D. subobscura were obtained by means of a newly prepared marker strain. In the central and marginal populations 20.8 and 28.8 percent of all chromosomes proved lethal or semilethal in homozygous condition. Mean viability was higher for +/+ random heterozygotes than for +/+ homozygotes. This remained the case for the marginal, but not for the central populations, after exclusion of the detrimental chromosomes from the calculations. The variances of viabilities were higher for homozygotes than for heterozygotes, but the test crosses with chromosomes from the marginal population had generally higher variances than those with chromosomes from the central population. No correlation was found in either populations between the action of a chromosome in homozygous condition with its action in heterozygous condition. This is interpreted as complete recessiveness of genetic load. The results are discussed in terms of the observed reduction of the inversion polymorphism which is not paralleled by a reduction in enzyme and, as shown here, by reduction in viability variation. It is thought that the heterotic effect of inversions is due to their homeostatic action, which depends less on structural genes than on higher orders of organization due to gene interaction or regulation. Whatever the causes, it is very likely that marginal populations differ from central populations with respect to their genetic system.     

Mutation Rate and Dominance of Genes Affecting Viability in DROSOPHILA MELANOGASTER

Spontaneous mutations were allowed to accumulate in a second chromosome that was transmitted only through heterozygous males for 40 generations. At 10-generation intervals the chromosomes were assayed for homozygous effects of the accumulated mutants. From the regression of homozygous viability on the number of generations of mutant accumulation and from the increase in genetic variance between replicate chromosomes it is possible to estimate the mutation rate and average effect of the individual mutants. Lethal mutations arose at a rate of 0.0060 per chromosome per generation. The mutants having small effects on viability are estimated to arise with a frequency at least 10 times as high as lethals, more likely 20 times as high, and possibly many more times as high if there is a large class of very nearly neutral mutations.-The dominance of such mutants was measured for chromosomes extracted from a natural population. This was determined from the regression of heterozygous viability on that of the sum of the two constituent homozygotes. The average dominance for minor viability genes in an equilibrium population was estimated to be 0.21. This is lower than the value for new mutants, as expected since those with the greatest heterozygous effect are most quickly eliminated from the population. That these mutants have a disproportionately large heterozygous effect on total fitness (as well as on the viability component thereof) is shown by the low ratio of the genetic load in equilibrium homozygotes to that of new mutant homozygotes.     

The Maintenance of Single-Locus Polymorphism. IV. Models with Mutation from Existing Alleles

The ability of viability selection to maintain allelic polymorphism is investigated using a constructionist approach. In extensions to the models we have previously proposed, a population is bombarded with a series of mutations whose fitnesses in conjunction with other alleles are functions of the corresponding fitnesses with a particular allele, the parent allele, already in the population. Allele frequencies are iterated simultaneously, thus allowing alleles to be driven to extinction by selection. Such models allow very high levels of polymorphism to evolve: up to 38 alleles in one case. Alleles that are lethal as homozygotes can evolve to surprisingly high frequencies. The joint evolution of allele frequencies and viabilities highlights the necessity to consider more than the current morphology of a population. Comparisons are made with the neutral theory of evolution and it is suggested that failure to reject neutrality using the Ewens-Watterson test cannot be regarded as evidence for the neutral theory.     

The significance of over- and underdominance for the maintenance of genetic polymorphisms. II. Overdominance and instability with random mating

Part I of the present series demonstrates that globally stable polymorphic equilibria may show underdominance in Darwinian fitness. Hence, overdominance in fitness can no longer be conceived of as a necessary condition for the stability of a polymorphism. In the present paper, the question is posed as to whether overdominance is at least sufficient for this stability. A population of randomly mating individuals is considered, where selection operates uniquely through differential fecundities of particular mating types and may generate either a heterozygote excess or deficit relative to Hardy-Weinberg proportions. It turns out that both unstable central overdominance and stable central underdominance are possible and that their occurrence is strongly related to an excess or a deficiency of heterozygotes in the vicinity of the regions of instability or stability. As one consequence, the above suggested sufficiency of heterozygote superiority is not valid, even in random mating populations. Based on the results of both papers of this series, which demonstrate the inadequacy of over- and underdominance as indicators of stability or instability, a modified overdominance principle is discussed. This principle states that a biallelic polymorphism is maintained if the heterozygote is superior in its degree of "heterogamous self-replication" to the degrees of "autogamous self-replication" of the corresponding homozygotes. It is derived with the help of fractional fitnesses, and it is pointed out that certain ratios of these may be more useful for finding evolutionary constants which govern the maintenance of genetic polymorphisms than are ratios of total fitnesses.     

Linkage Modification with Mixed Random Mating and Selfing: A Numerical Study

Although recombination cannot increase under conditions of random mating or complete selfing in regimes of constant selection, with mixed random mating and selfing, selection for increased recombination can occur. For some fitness regimes there may be selection for reduced recombination with both low and high degrees of selfing but selection for increased recombination with moderate degrees of selfing. With some fitness regimes there is a historical effect: depending on which equilibrium a population starts from, there may be selection for either increased or decreased recombination. In other cases the direction of selection may be determined by the present state of individuals within the population. If recombination is already fairly limited, there may be selection for further reduction. If recombination is already fairly frequent, there may be selection for increased recombination. For certain symmetric viability systems there may be an intermediate value of the recombination fraction between 0 and 0.5 toward which the population will evolve. Although it is not yet possible to classify precisely those fitness matrices that can exhibit selection for increased recombination, it does appear that selection for increased recombination can occur only if at least two of the double homozygotes are less fit than would be expected on the basis of a comparison of the fitnesses of the single and double heterozygotes on an additive scale.     

Marginal Overdominance in Drosophila

A reanalysis of Drosophila viability data was undertaken to determine the role of genotype-environment interactions in the maintenance of polymorphism. Between-replicate variances of viabilities in chromosomal homozygotes and heterozygotes with the same mean fitnesses were compared, with the expectation that if the heterozygote variance were on the average greater, conditional overdominance would be prevalent; if it were less, partial dominance would be prevalent; and if it were the same, marginal overdominance of the type considered by Wallace (1968) would be the prevalent type of variation. In fact, heterozygote variance was slightly less. The work of Dempster (1955) and of Gillespie and Langley (1974) is cited to show that this situation can still lead to balanced polymorphisms. Their general model for genetic variation in populations, consistent with the viability data, is reinforced.     

European ACP1*C allele has recessive deleterious effects on early life viability

The acid phosphatase locus (ACP1) is a classical polymorphism that has been surveyed in hundreds of human populations worldwide. Among individuals of European ancestry, the ACP1*C allele occurs with an average frequency of approximately 0.05, whereas it is nearly absent in all other human populations. It has been hypothesized that this allele is maintained by overdominant selection among European populations. Here, we analyze ACP1 protein polymorphism data from more than 50,000 individuals previously surveyed in 67 populations across Europe as well as inheritance data from more than 6,000 European parent-offspring pairs to assess the signature of natural selection currently acting on this allele. Although we see a significant excess of ACP1*C heterozygotes relative to Hardy-Weinberg expectations, we find no evidence that natural selection favors ACP1*C heterozygotes. Instead, ACP1*C appears to have a strongly deleterious and recessive fitness effect. We observed only 48.9% of expected homozygous offspring from heterozygous parents and significantly fewer homozygotes than expected within populations. Because parent-offspring pairs indicate a significant deficiency of ACP1*C homozygotes, we infer that viability selection is acting on ACP1*C homozygotes very early in life, perhaps before birth. We estimate that approximately 1.2% of all couples of European ancestry are composed of individuals who both carry the APC1*C allele. As such, selection against ACP1*C homozygotes may represent a nonnegligible contribution to the overall number of spontaneous abortions among women of European ancestry and may cause substantial fertility reductions among some combinations of parental genotypes.     

The Effect of Level of Heterozygosity on the Performance of Hybrids between Isogenic Lines of Barley

Sixteen "isogenic" lines of Atlas 46 barley differing in one to four short chromosome segments, and 16 heterozygotes obtained by crossing these lines to male-sterile Atlas, were used to study the effect of level of heterozygosity on performance. In field tests conducted in four environments (two planting dates in two years) significant differences were found among the homozygous isogenic lines for the traits seed yield, kernel weight, tiller number, plant height, and heading time; thus each of the marked chromosome segments carries genes which, when homozygous, affect these quantitative characters. It was also found that heterozygotes produced more and heavier kernels and were taller and earlier than homozygotes but there was no clear indication that the degree of heterosis increased as the number of heterozygous segments increased from one to five. Degree of heterosis was, however, strongly affected by the environment, by allelic state at each segment (especially the segment marked by the two-row, six-row spike locus), and also by genotype for other marked segments. These results indicate that heterosis in barley has a more complex structure than can be adequately represented by simple models, such as the multiplicative model in which fitnesses are the product of fitnesses at individual loci, or threshold models in which optimum fitness is approached asymptotically as the number of heterozygous loci increases.     

Establishment and characterization of B cell lines from individuals with various types of adenine phosphoribosyltransferase deficiencies

Patients with 2,8-dihydroxyadenine urolithiasis are either completely or partially deficient in adenine phosphoribosyltransferase activities. Patients with partial enzyme deficiencies, all of whom have been found among Japanese, are homozygotes having a unique mutant adenine phosphoribosyltransferase gene (APRT*J) in double dose (Japanese type deficiency). We have established B-cell lines from heterozygotes and homozygotes of complete and Japanese type adenine phosphoribosyltransferase deficiencies as well as normal individuals. Characterization of the cell lines indicated that all homozygous cells were deficient in adenine phosphoribosyltransferase function while all heterozygous and normal cells had functional adenine phosphoribosyltransferase.     

Genetic load and viability variation in korean natural populations ofDrosophila melanogaster

Summary Natural populations of Drosophila melanogaster from Anyang and Susac (suburbs of Seoul) have been analyzed with respect to viability variation on the second chromosome. Homozygotes as well as random heterozygotes for wild chromosomes were studied. The frequency of lethal factors was about 16 per cent, that of drastics 26 per cent. The average viability of homozygotes was 0.650 including lethal lines and 0.858 for quasinormals; that for random heterozygotes was 1.125. Allelism tests have been performed for the lethals. The allelism rate turned out to be as high as 0.036 and 0.0214, respectively. Using a formula by Nei, the effective population size can be estimated from these data. Korean D. melanogaster populations proved as small as 2000 to 3000 individuals. No correlation between homozygous and heterozygous viabilities could be found. According to these observations, along with the fact that partly big clusters of identic lethals could be found in the allelism tests, it is concluded that in Korean populations quite a large part of the hard genetic load is balanced. The connection between population size, population structure and associative or genuine overdominance is discussed.     

Biology of a duplicate gene system with glucose 6-phosphate dehydrogenase activity in Drosophila melanogaster: Genetic analysis and differences in fitness components and reaction to environmental parameters among Zw genotypes

There are two structural forms of glucose 6-phosphate dehydrogenase activity in Drosophila melanogaster. Whether one or the other or both show in vitro (and probably in vivo) activity depends on the genotype of a sex-linked locus (Zw). In this article, the relative fitnesses of heterozygotes (with both electromorphs active) and homozygotes (with activity demonstrable for only one or the other electromorph) for the Zw locus are described. It is shown that the relative fitness of heterozygotes increases with increase in population density, or degree of crowding and trophic stress, and that the mean development times of Zw heterozygotes are lower than those of the Zw homozygotes. In addition, and perhaps accounting for the fitness and viability excess of the heterozygotes, one set of evidence strongly suggests that they are better buffered against trophic stress than the homozygotes.     

Chromosome Interactions in DROSOPHILA MELANOGASTER. II. Total Fitness

In a large experiment, using nearly 200 population cages, we have measured the fitness of Drosophila melanogaster homozygous (1) for the second chromosome, (2) for the third chromosome, and (3) for both chromosomes. Twentyfour second chromosomes and 24 third chromosomes sampled from a natural population were tested. The mean fitness of the homozygous flies is 0.081 ± 0.014 for the second chromosome, 0.080 ± 0.017 for the third chromosome, and 0.079 ± 0.024 for both chromosomes simultaneously. Assuming that fitnesses are multiplicative (the additive fitness model makes no sense in the present case because of the large selection coefficients involved), the expected mean fitness of the homozygotes for both chromosomes is 0.0066; their observed fitness is more than ten times greater. Thus, it appears that synergistic interactions between loci are considerable; and that, consequently, the fitness function substantially departs from linearity. Two models are tentatively suggested for the fitness function: a "threshold" model and a "synergistic" model.—The experiments reported here confirm previous results showing that the concealed genetic load present in natural populations of Drosophila is sufficient to account for the selective maintenance of numerous polymorphisms (of the order of 1000).     

Homozygous and Hemizygous Viability Variation on the X Chromosome of DROSOPHILA MELANOGASTER

We report here a study of viability inbreeding depression associated with the X chromosome of Drosophila melanogaster. Fifty wild chromosomes from Mt. Sinai, New York, and 90 wild chromosomes from Death Valley, California, were extracted using the marked FM6 balancer chromosome and viabilities measured for homozygous and heterozygous females, and for hemizygous males, relative to FM6 males as a standard genotype. No statistically significant female genetic load was observed for either chromosome set, although a 95% confidence limit estimated the total load <0.046 for the samples pooled. About 10% of the Death Valley chromosomes appear to be "supervital" as homozygotes. There is little evidence for a pervasive sex-limited detrimental load on the X chromosome; the evidence indicates nearly identical viability effects in males and homozygous females excluding the supervital chromosomes. The average degree of dominance for viability polygenes is estimated between 0.23 to 0.36, which is consistent with autosomal variation and implies near additivity. We conclude that there is little genetic load associated with viability variation on the X chromosome and that the substantial reduction in total fitness observed for chromosome homozygosity in an earlier study may be due largely to sex-limited fertility in females.     

A Mutation Affecting the Lactate Dehydrogenase Locus Ldh-1 in the Mouse. II. Mechanism of the Ldh-a Deficiency Associated with Hemolytic Anemia

A procarbazine hydrochloride-induced mutation at the Ldh-1 structural locus encoding the A subunit of lactate dehydrogenase (LDH) was used to study the molecular and metabolic basis of severe hemolytic anemia due to LDH-A deficiency in the mouse. The mutant allele designated Ldh-1(a-m1Neu) codes for an enzyme that as homotetramer differs from the wild-type enzyme by a marked instability, acidic shift of the pH profile, increased K(m) for pyruvate and altered inhibition by high concentrations of this substrate. Except for the latter, all these altered properties of the mutant protein contribute to the diminished LDH activity in heterozygous and homozygous mutant individuals. Impaired energy metabolism of erythrocytes indicated by a relatively low ATP concentration is suggested to result in cell death at the end of the reticulocyte stage leading to the expression of hemolytic anemia with extreme reticulocytosis and hyperbilirubinemia. Despite the severe anemia, affected homozygous mutants exhibit approximately normal body weight and do not show noticeable impairment of viability or fertility. To date no such condition is observed in man. This discrepancy is likely due to the fact that in human erythrocytes both LDH-A and LDH-B subunits are expressed such that homozygotes for a LDH-A or LDH-B deficiency would not result in a comparably extreme LDH activity deficiency.     

Stochastic selection in large and small populations

We describe two models of stochastic variation in selection intensity. In both models the arithmetic mean fitness of all genotypes is equal; in both models the geometric mean fitness of the heterozygous genotype is greater than that of both homozygous genotypes. In one model the correlation between the fitnesses of the homozygous genotypes is +1; in the other it is −1. We show that the expected time to absorption of an allele in a finite population is significantly retarded for all initial gene frequencies in the former model. The expected time to absorption of an allele in the latter model is retarded only at extreme initial gene frequencies; at intermediate initial gene frequencies the expected time to absorption is accelerated. We conclude that the criterion for polymorphism based on the geometric mean of the heterozygote being greater than that of both homozygotes provides only limited information about the fate of gene frequency.     

Evidence for an elevated frequency of in vivo somatic cell mutations in ataxia telangiectasia.

Somatic cell mutation frequency in vivo was measured in individuals with high cancer risk who were from ataxia telangiectasia (A-T) families. The assay for somatic mutation measures the frequency of variant erythrocytes which are progeny of erythroid precursor cells with mutations that result in a loss of gene expression at the polymorphic glycophorin A (GPA) locus. Samples from 14 of 15 A-T homozygotes showed high frequencies of GPA gene expression-loss variant cells with normal expression of only one of the two alleles at the GPA locus (i.e., GPA hemizygous variant cells). The mean elevation of the frequency of hemizygous variant cells over those in normal controls and unaffected family members was 7-14-fold. A-T homozygotes also showed an increase in the frequency of cells in which one allele at the GPA locus had lost expression and in which the remaining allele was expressed at a homozygous level (i.e., GPA homozygous variant cells). Family members who are obligate A-T heterozygotes did not appear to have a significantly elevated frequency of GPA hemizygous or homozygous variant cells. These indications of elevated in vivo frequencies of variant erythrocytes in A-T homozygotes support a causal link between susceptibility to somatic mutation and susceptibility to cancer.     

The Genetic Structure of Natural Populations of DROSOPHILA MELANOGASTER. Xi. Genetic Variability in a Local Population

Six hundred and ninety-one second chromosomes were extracted from a Raleigh, North Carolina population, and the following experimental results were obtained: (1) Salivary gland chromosomes of all lines were observed and the number of inversion-carrying chromosomes was 130, among which 76 carried In(2R)NS, 36 carried In(2L)t, 4 carried In(2L)t and In(2R)NS, and 14 carried different kinds of rare inversions. (2) Viabilities of homozygotes and heterozygotes were examined. The frequency of lethal-carrying chromosomes was 275/691 (or 0.398):70/130 (or 0.538) in inversion-carrying chromosomes and 205/561 (or 0.365) in inversion-free chromosomes. The former is significantly higher than the latter. The average homozygote viability was 0.4342 including lethal lines and 0.7163 excluding those, the average heterozygote viability being 1.0000. The detrimental load to lethal load ratio (D:L ratio) was 0.334/0.501 = 0.67. The average viability of lethal heterozygotes was less than that of lethal-free heterozygotes, significantly in inversion-free individuals but not significantly so in inversion-carrying individuals. Inversion heterozygotes seem to have slightly better viability than the inversion-free heterozygotes on the average, but not significantly so. (3) The average degree of dominance of viability polygenes was estimated to be 0.293 +/- 0.071 for all heterozygotes whose component chromosomes had better viabilities than 0.6 of the average heterozygote viability, 0.177 +/- 0.077 for inversion-free heterozygotes and 0.489 +/- 0.082 for inversion heterozygotes. (4) Mutation rates of viability polygenes and lethal genes were estimated on the basis of genetic loads and average degrees of dominance of lethal genes and viability polygenes. Estimates were very close to those obtained by direct estimation. (5) Possible overdominance and epistasis were detected, but the magnitude must be very small. (6) The effective size of the population was estimated to be much greater than 10,000 by using the allelism rate of lethal-carrying chromosomes (0.0040) and their frequency.-On the basis of these findings and the comparison with the predicted result (Mukai and Maruyama 1971), the mechanisms of the maintenance of genetic variability in the population are discussed.     

Genes Affecting Productivity in Natural Populations of DROSOPHILA MELANOGASTER

Two hundred second chromosomes were extracted from a Japanese population in October of 1972, and the viabilities and productivities of homozygotes and heterozygotes from them were examined. Viability was measured by the Cy method and productivity by the number of progeny produced per female. The frequency of lethal-carrying chromosomes was 0.315. When the average heterozygote viability was standardized as 1.000, the average homozygote viability was 0.595 including the lethal lines, and 0.866 excluding them. The frequency of recessive sterile chromosomes among 131 non-lethal lines was 0.092 in females and 0.183 in males. There were two instances in which homozygosis for the second chromosome caused sterility in both sexes, which was close to the number expected (2.2) on a random basis of 0.092 x 0.183 x 131. When the average heterozygote productivity of 200 lines was standardized as 1.000, the average homozygote productivity was 0.532 including female steriles, and 0.584 excluding them. The ratio of detrimental load to lethal load was 0.383, while the ratio of partial sterility load to complete sterility load was 5.767. The average viability of lethal heterozygotes was slightly, but not significantly, lower than that of lethal-free heterozygotes, while the average productivity of lethal heterozygotes was significantly lower than that of lethal-free heterozygotes. There was a significant association of sterility in either sex with low viability of homozygotes. However, no statistically significant differences in viability and productivity were detected between sterile heterozygotes and non-sterile heterozygotes. The heterozygous effects of viability and productivity polygenes were examined by regressions of the heterozygotes on the sum of corresponding homozygotes. The regression coefficients were slightly positive for both viability and productivity if lethal and sterile chromosomes were excluded. The correlation between viability and productivity in homozygotes was significantly positive when sterile chromosomes were included, but the significance disappeared when the sterile chromosomes were excluded. In the heterozygotes there were no detectable correlations between them.