Mutation-rate determinations based on electrophoretic analysis of laboratory mice

Rate calculations are described for germinal mutations detected by an electrophoretic system as applied to mice in a series of recent experiments. The utility of the electrophoretic system is examined from several comparative viewpoints. The electrophoretic approach has a number of features to recommend it for future applications concerned with genetic hazard assessment. Its advantages include sensitivity, versatility, cost effectiveness and relevance to humans.

A null mutation at the mouse Phosphoglucomutase-1 locus and a new locus Pgm-3

A null mutation at the phosphoglucomutase locus (Pgm-1) was discovered by electrophoretic analysis of the inbred mouse strain C57BL/6J. The null allele (Pgm-1n) was shown to segregate as a Mendelian unit alternative to the Pgm-1a and Pgm-1b alleles. Mice expressing the Pgm-1n allele, either in the heterozygous or homozygous state, are viable, healthy, and fertile. The occurrence of the Pgm-1n mutant revealed a previously unreported genetic locus (Pgm-3) that controls the expression of a third phosphoglucomutase. Two electrophoretically expressed alleles of Pgm-3 (inherited without dominance) are found in the inbred mouse strains C57BL/6J and DBA/2J. Linkage observed between the Pgm-3 locus, the dilute locus (d) and the cytoplasmic malic enzyme locus (Mod-1) has allowed assignment of the Pgm-3 locus to chromosome 9. A striking tissue specific expression of Pgm-1 and Pgm-3 was observed. Products of the Pgm-3 locus were detected in kidney, testes, brain, and heart. In contrast, Pgm-1 controlled isozymes were present in kidney, spleen, ovaries, and erythrocytes.

Electrophoretically detected germinal mutations induced in the mouse by ethylnitrosourea

Male DBA/2J and C57BL/6J mice were treated with ethylnitrosourea (EtNU) at a dose of 250 mg/kg of body weight. After a sterile period of 11 weeks, the treated animals were mated with untreated females; treated DBA with C57 and treated C57 with DBA. A small control group of untreated males was established, and a larger control group from earlier experiments was also used for comparison. Tissue samples removed surgically from the parents and F1 offspring were examined by electrophoresis. Nine newly arisen mutants attributable to induction by EtNU were found in 22,512 loci tested for mutation. No mutations were encountered in 290,252 control locus tests. A preliminary study conducted concomitantly with the electrophoretic analyses for dominant lethal effects and physical malformation showed no increase over background due to EtNU treatment of spermatogonia, which suggests that mutations induced by EtNU tend not to be expressed in this way. The nature of the mutants identified by electrophoresis facilitates follow-up analyses leading to increased understanding of the molecular basis and the physiological effects of induced mutations in mammalian organisms.

A variation in mouse kidney pyruvate kinase activity determined by a mutant gene on chromosome 9

Pyruvate kinase activity was studied in kidney extracts of mice from various sources. One C57BL/6J female among several hundred examined expressed activity approximately one half of the normal level. This animal transmitted the trait, and a single gene responsible for the activity difference was identified and mapped to chromosome 9. Linkage withMod-1anddilutewas established. A spontaneous mutation in the C57BL/6J strain apparently was responsible for the variant pyruvate kinase.

Mouse spermatogonia exposed to a high, multiply fractionated dose of a cancer chemotherapeutic drug: mutation analysis by electrophoresis

Male mice of the DBA/2J strain were injected with procarbazine at a dose of 200 mg/kg body weight twice weekly until an accumulated dose of 2400 mg/kg was reached. A concurrent control group, injected only with the vehicle (saline) was also established. Most of the treated animals died as a result of exposure and all survivors became temporarily sterile. After regaining fertility the few survivors were repeatedly mated with C57BL/6J females over several weeks time to generate a population of F1 animals. The parental animals and the F1 were subsequently analyzed by electrophoresis for the occurrence of newly arisen mutations of spermatogonial origin. A mutation in the gene Pep-3 was found.

The detection of mutants in mice by electrophoresis: results of a model induction experiment with procarbazine

Male mice of the DBA/2J strain were treated with the mutagen procarbazine and mated with C57BL/6J females. Offspring and parents were then analyzed for electrophoretically expressed mutations. A control group, not mutagen treated, was also examined. Two mutants probably due to induction were identified, and several of spontaneous origin were also found.

Identification and biochemical analysis of mouse mutants deficient in cytoplasmic malic enzyme

During the biochemical screening of mutant enzymes in mice, individuals with an apparent nonfunctional allele at the locus (Mod-l) responsible for cytoplasmic malic enzyme were identified by starch gel electrophoresis and by enzyme activity measurements. A series of matings and genetic analyses were made, and mice homozygous for the nonfunctional or null allele (Mod-ln) were produced. The mutation appeared to occur spontaneously in the C57BL/6J strain. By double-immunodiffusion and enzyme immunoinactivation assays, the null mutants were shown to express no proteins that cross-react with the antiserum to cytoplasmic malic enzyme (CRM-negative). In liver homogenates of homozygous null mutants, lack of protein components that form complexes with IgG from the cytoplasmic malic enzyme specific antiserum was further demonstrated by passage of the original serum through a mutant liver homogenate--Sepharose column, where the postadsorbed serum retained its titer and specificity. The residual malic enzyme activity (< 10% of the normal) observed in various tissue homogenates of the homozygous null mutants was attributed to that of mitochondrial isozyme of malic enzyme. Assays of enzymes from tissues of different genotypes revealed no significant differences in activities of six other enzymes in the related metabolic pathways. However, in liver from mutant mice, a lower NADPH/NADP+ ratio was consistently observed in comparison to that from control mice. Both the mutant and the control mice of the same age were found to have comparable body weight and lipid content.

GENETIC CONTROL OF MALATE DEHYDROGENASE ISOZYMES IN MAIZE

At least six nuclear loci are responsible for the genetic control of malate dehydrogznase (L-malate: NAD oxidoreductase; EC 1.1 .I .37; MDH) in coleoptiles of maize. Three independently segregating loci (Mdh1, Mdh2, Mdh3) govern the production of MDH isozymes resistant to inactivation by ascorbic acid and found largely or solely in the mitochondria. A rare recessive allele found at a fourth nuclear locus (mmm) causes increased electrophoretic mobility of the MDH isozymes governed by the Mdh1, Mdh2 and Mdh3 loci. -Two loci (Mdh4, Mdh5) govern MDH isozymes that are selectively in- activated by homogenization in an ascorbic acid solution and that appear to be nonmitochondrial (soluble). Mdh4 and Mdh5 segregate independently of each other and independently of Mdhl, Mdh2 and Mdh3. However, there is close linkage between the migration modifier and Mdh4.—Multiple alleles have been found for all of the Mdh loci except the migration modifier, and electrophoretically "null" or near "null" alleles (as expressed in standardized sections of maize coleoptile) have been found for all loci except Mdh4. Duplicate inheritance commonly occurs for Mdhl and Mdh2 and also for Mdh4 and Mdh5.—Inter- and intragenic heterodimers are formed between sub-units specified by the three loci governing the mitochondrial MDH isozymes. The same is true of the alleles and nonalleles at the two loci governing the soluble variants. No such heterodimers are formed by interactions between mitochondrial and soluble MDH isozymes.

Action of N-isopropyl-alpha-(2-methylhydrazino)-p-toluamide hydrochloride (procarbazine hydrochloride) in the germ tissue of mice: dominant lethal effects

A study was carried out on the effects of N-isopropyl-alpha-(2-methylhydrazino)-p-toluamide (procarbazine, Natulan) in the dominant lethal test in the mouse. Male mice were dosed once and mated with fresh virgin females each week. The utilization of sperm, treated as spermatids or testicular sperm with 100-800 mg/kg, resulted in significant post- and pre-implantation death of embryos. Fertility was markedly reduced after the injection of 200 mg/kg of procarbazine and over. This is probably due to a cell killing effect, the most sensitive stages being differentiating spermatogonia, type A sermatogonia and resting primary spermatocytes. Total sterility was induced for several weeks with doses of 600 and 800 mg/kg. Up to 12 weeks after treatment the number of females with implants was still significantly lower than controls indicating a severe depletion of spermatogonial cells. The spectrum of effects correlates well with the drug's effect on nucleic acid and protein synthesis.

Analysis of linkage disequilibria between allozyme loci in natural populations of Drosophila melanogaster

Linkage disequilibria between pairs of 8 polymorphic enzyme loci (αGpdh, Mdh, Adh, Est-6, Pgm, Odh, Est-CandAcph) in some 100 natural population samples ofDrosophila melanogasterwere examined. The estimates of linkage disequilibrium were made from zygotic frequencies. The magnitude of linkage disequilibria are small and similar to those in previous reports. Variation in linkage disequilibrium among related subpopulations was analysed by analysis of variance of the correlation coefficients. Despite the small absolute value of linkage disequilibrium there is a suggestion of a correlation among related subpopulations. The magnitude of linkage disequilibrium was observed to be positively correlated with linkage. Two cage populations were observed to demonstrate large amounts of linkage disequilibrium between closely linked loci in contrast to the situation in natural populations. This is attributable to the finite sizes of these cage populations.

The importance of adrenocortical steroids in the pathogenesis of ketosis in diabetic guinea pigs (Caviae porcellus)

The roles of adrenocortical steroids in the pathogenesis of diabetic ketosis were investigated in guinea pigs. Groups of normal and Streptozotocin-induced diabetic animals were either given saline as a control, methylprednisilone as an exogenous steroid, or adrenocorticotrophin to stimulate endogenous steroid production. Plasma and urine ketone determinations were made daily for six days at which time animals were necropsied. The qualitative procedure used failed to detect ketonemia, and ketonuria was only found in diabetic guinea pigs treated with steroids. The results support the hypothesis that the absence of ketosis in spontaneously diabetic guinea pigs may be the result of a relative glucocorticoid insufficiency secondary to fatty degeneration of the adrenal glands, a common lesion in spontaneously diabetic guinea pigs.

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.

Variance component analysis of allozyme frequency data from eastern populations of Drosophila melanogaster

Gene frequency variation at eight polymorphic allozyme loci in Drosophila melanogaster populations in North Carollina and the east coast of the United States were analyzed utilizing the variance component estimation procedures suggested by Cockerham (1969, 1973). These variance components were used to estimate correlations of genes within small geographic regions. The average (over loci) correlation between genes in the same individual within subpopulations was estimated to be 0.033. That between genes in the same subpopulation in different individuals was estimated to be very small, although significantly different from zero. The macrogeographic variation measured by the correlation of genes sampled from the same local region was large for some loci and smaller for others. this variation was also analyzed by correlation with latitude and longitude. Several previously recognized clines were identified as were several new clines.--These results were interpreted as indicating either some degree of nonrandom mating and local breeding unit isolation or a low frequency of null alleles. The geographic and temporal variation has no simple interpretation.

Genetic control and racial variation of beta-glucosidase isozymes in maize (Zea mays L.)

beta-Glucosidase (beta-D-glucoside glucohydrolase, E.C. 3.2.1.21, beta-Glu) isozyme variants were studied in a large number of inbred lines, crosses, and races of maize (Zea mays L.). The pattern of Mendelian inheritance demonstrated for beta-GLU variants indicated that they are under nuclear gene control. Twenty-two allelic forms at a single locus were identified in the materials studied by starch gel electrophoresis. Genetic data indicate that beta-GLU in maize is functionally a dimer. Variation of beta-GLU isozymes in 51 racial collections of maize from Mexico showed little correlation with morphological or geographical data. In 39 collections from Central America, variation patterns appeared to have some association with altitude.

Isozyme variability in species of the genus Drosophila. VIII. The alcohol dehydrogenase polymorphism in North Carolina populations of D. melanogaster

Adult Drosophila melanogaster flies collected from populations broadly dispersed over ecological and geographic strata of North Carolina, and over a period of 4 years, were analyzed for alcohol dehydrogenase phenotypes by gel electrophoresis. Gene frequencies in spring-summer-fall field collections were remarkably stable over all strata. Two winter collections exhibited contrasting gene frequency changes. In one case the results are interpreted in terms of long-distance migration from Florida, while the other is explicable by assignment of a causal role to environmental factors which accompany the winter season.

The alcohol dehydrogenases of Drosophila melanogaster: frequency changes associated with heat and cold shock

Laboratory populations of Drosophila melanogaster were subjected to heat and cold shock of sufficient duration and intensity to cause about 90 percent mortality. Alcohol dehydrogenase phenotypes were determined in survivors and controls. Significant changes in phenotypic frequencies occurred as a result of both treatments such that relative fitness appeared to be dependent on temperature. The alcohol dehydrogenase phenotype most frequently surviving heat under laboratory conditions is the same phenotype as that previously found to be higher in frequency in natural populations in areas with higher average temperature.

Isozyme allelic frequencies related to selection and gene-flow hypotheses

Significant correlations between allelic frequencies and environmental variables in a number of insect species have been demonstrated by multivariate techniques. Since many environmental variables show a strong relationship to geographic location and since gene flow between populations can also produce patterns of gene frequencies which are related to the geographic location, both selection and gene-flow hypotheses are consistent with the observed correlations. The genetic variables can be corrected for geographic location and so for linear gene-flow patterns. If, after correction, the genetic variables still show significant correlations with similarly corrected environmental variables, then these correlations are consistent with hypotheses of selection but not of gene flow. The data of Johnson and Schaffer (1973) have been reanalyzed using the method of canonical correlation after correction for geographical location by means of multiple regression. Five of the nine loci studied exhibit significant canonical correlations. These results, under the assumption of linear gene flow, support hypotheses of selective action of environmental variables in the genotype-environment relationships observed.

Isozyme genotype-environment associations in natural populations of the harvester ant, Pogonomyrmex badius

Harvester ants (P. badius) were collected from 49 localities in the southeastern U.S. Amylase and naphthylamidase activities were assayed by gel electrophoresis, and the frequencies of alleles controlling electrophoretic variation were calculated. Soil samples were analyzed and plotted against allelic frequencies. Climatographic and genetic data were analyzed by the method of principal components. Statistically significant correlations were found to occur between the patterns of genetic variation, and between the genetic and environmental patterns. These correlations are consistent with a selective basis for the maintenance of isozyme polymorphisms.