Colonization of America by Drosophila subobscura: heterotic effect of chromosomal arrangements revealed by the persistence of lethal genes

About 20 years ago Drosophila subobscura, a native Palearctic species, colonized both North and South America. In Palearctic populations lethal genes are not associated in general with particular chromosomal arrangements. In colonizing populations they are not randomly distributed and usually are associated to a different degree with chromosomal arrangements caused by the founder event. The persistence of two lethal genes in the colonizing populations, one completely associated with the O(5) inversion and the other partially associated with the O(3+4+7) arrangement, has been analyzed. In all populations studied (five North American and six South American) the observed frequency of the lethal gene completely associated with the O(5) inversion is higher than expected, the difference being statistically significant in all South American and one North American populations. The observed frequency of the lethal gene partially associated with the O(3+4+7) arrangement is also significantly higher than expected. Taking into account that the O(5) inversion exhibits significant latitudinal clines both in North and South America, an overdominant model favoring the heterokaryotypes seems to be in operation. From this model, a polynomial expression has been developed that allows us to estimate the relative fitness and the coefficient of selection against all karyotypes not carrying the O(5) inversion. The relative fitness of the O(5) heterokaryotypes is higher in South American than in North American populations. Furthermore, the observed frequencies of the lethal genes studied are in general very close to those of the equilibrium. This case is an outstanding demonstration in nature of an heterotic effect of chromosomal segments associated with lethal genes on a large geographic scale.     

Irradiation-resistance conferred by superoxide dismutase: possible adaptive role of a natural polymorphism in Drosophila melanogaster.

The toxic effects of ionizing radiation to DNA are thought to be due to the generation of the superoxide radical, 02-. Superoxide dismutase (SOD), which scavenges 02-., has been invoked as a protecting enzyme against ionizing radiation in viruses, bacteria, mammalian cells in culture, and live mice. We now demonstrate that SOD is involved in the resistance of Drosophila melanogaster against irradiation. The protection is greatest when flies carry the S form of the enzyme (which exhibits highest in vitro specific activity), intermediate when they carry the F form of the enzyme, and lowest when they are homozygous for N, an allele that reduces the amount of the enzyme to 3.5% of the normal level. Natural selection experiments show that the fitness of the high-activity S allele is increased in an irradiated population relative to the nonirradiated control. These results point towards a possible adaptive function of the S/F polymorphism found in natural populations of D. melanogaster.     

Mitochondrial DNA evolution in experimental populations of Drosophila subobscura.

When two mitochondrial DNA (mtDNA) haplotypes of Drosophila subobscura compete in experimental populations with discrete generations, one or the other approaches fixation, depending on the nuclear background with which they are associated. The approach to fixation, however, is strongly dependent on the effective number of females in the population, Nf. Whether or not the ultimate fate of a given mtDNA haplotype is determined by random genetic drift depends on Nf as well as on the relative fitnesses. Our experimental results show that the mtDNA polymorphisms observed in natural populations are affected by interactions among nuclear polymorphisms, random genetic drift, and direct selection on the mtDNA haplotypes.     

Evidence for selection by male mating success in natural populations of Drosophila pseudoobscura.

Gene arrangement frequencies were determined at two stages in the life history of Drosophila pseudoobscura taken from nature. Three populations in the central highlands of Mexico were each sampled twice during 1976. Gene arrangement frequencies were measured in adult males and in larvae that were the offspring of females collected at the same time. The adult males were in all likelihood a representative sample of those who fathered the larvae produced by the wild females. Differences in gene arrangement frequency between these two life stages should indicate the operation of natural selection. One-third of our comparisons of common gene arrangement frequencies in males and in larvae from the next generation were statistically significant, as were one-third of our comparisons of total frequency arrays in the two life stages. We consider the components of selection that could produce such frequency changes and reason that male mating success must be the major one. Gene arrangement frequencies in the Mexican populations fluctuate within wide bounds. Selection must act to retain the polymorphism in the face of this flux in gene arrangement frequencies, and we suggest that male mating success plays an important role.     

Cytotype polymorphism of the P-M system in two wild populations of Drosophila melanogaster.

In Drosophila melanogaster the interactions of the P-M system generate germ-line aberrations (e.g., sterility, mutations) found in certain interstrain hybrids. Two wild populations, from France and Tunisia, were examined in order to determine the distribution of the chromosomal P factor and the extrachromosomal cytotypes. No P factors active for potential GD sterility were found in these populations. But search for the M cytotype, which causes susceptibility to the P factors, and for the P cytotype, which causes resistance to the P factors, showed that both populations are polymorphic for the cytotypes. Such a polymorphism seems to be stable in the wild, at least over a 2-year period. Mutator activity (measured by generated mutations at the sn and ras loci) was found to be present. Some possible interactions between cytotype polymorphism, mutator activity, and the structure of Drosophila populations are discussed.     

Quantitative genetic variation of enzyme activities in natural populations of Drosophila melanogaster

The genetic component of variation of enzyme activity in natural populations of Drosophila melanogaster was investigated by using two sets of chromosome substitution lines. The constitution of a line of each type is: i₁/i₁;+₂/ +₂;i₃/i₃ and i₁/i₁;i₂/ i₂;+₃/+₃, where i refers to a chromosome from a highly inbred line and + refers to a chromosome from a natural population. The + but not the i chromosomes vary within a set of lines. By use of a randomized block design to test and estimate components of variance, 50 of the second- and 50 of the third- chromosome substitution lines have been screened for variation in the activity levels of seven enzymes. Six of the seven enzymes show a significant genetic component in at least one set of lines, and five of the seven enzymes show activity variations attributable to factors that are not linked to the structural gene. These unlinked activity modifiers identify possible regulatory elements. Analyses of covariance show that most of the genetic variation of enzyme activities cannot be accounted for by genetic variation of live weight or protein content. These results and the lack of strong correlations between the genetic effects on the activities of different enzymes indicate that the effects are mainly specific for individual enzymes.     

Enzyme null alleles in natural populations of Drosophila melanogaster: Frequencies in a North Carolina population

A Raleigh, NC, population of Drosophila melanogaster was sampled for the presence of enzyme null alleles at 25 loci. No nulls were found at any of five X-linked loci. Nulls were recovered at 13 of 20 autosomal loci; the weighted mean frequency for all 20 autosomal loci was estimated to be 0.0025. A consideration of the effects of these null alleles on viability strongly suggests that, although they may contribute to so-called polygenic variation, they are not representative of the entire genome.     

Linkage disequilibrium over space and time in natural populations of Drosophila montana.

The previously described allelic frequencies and linkage disequilibrium among the active and null alleles of four tightly linked loci coding for the alpha-esterases were found to be maintained by one population for 5 years, and were found to be present in two other populations which were shown to be genetically distinct from the first. It appears that enzyme variants coded by these highly polymorphic loci are being maintained in the populations by selective forces.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Causes of natural variation in fitness: Evidence from studies of Drosophila populations

DNA sequencing has revealed high levels of variability within most species. Statistical methods based on population genetics theory have been applied to the resulting data and suggest that most mutations affecting functionally important sequences are deleterious but subject to very weak selection. Quantitative genetic studies have provided information on the extent of genetic variation within populations in traits related to fitness and the rate at which variability in these traits arises by mutation. This paper attempts to combine the available information from applications of the two approaches to populations of the fruitfly Drosophila in order to estimate some important parameters of genetic variation, using a simple population genetics model of mutational effects on fitness components. Analyses based on this model suggest the existence of a class of mutations with much larger fitness effects than those inferred from sequence variability and that contribute most of the standing variation in fitness within a population caused by the input of mildly deleterious mutations. However, deleterious mutations explain only part of this standing variation, and other processes such as balancing selection appear to make a large contribution to genetic variation in fitness components in Drosophila.Advances in DNA sequencing methods have enabled geneticists to measure the amount of genetic variability in natural populations at the most basic level: the frequencies of variants in nucleotide sequences. This achievement has ended one component of a debate on the extent and causes of genetic variability that was initiated in the 1950s by Hermann Muller and Theodosius Dobzhansky (1, 2); we now know that DNA sequences are highly variable within the populations of most species (3). It has, however, been much harder to provide a definitive answer to the other component of this debate, which concerns the nature and intensity of the evolutionary forces that influence the frequencies of genetic variants within populations (1, 2, 4, 5). Are these variants mostly selectively neutral (6), with the fates of new mutations determined by random fluctuations in their frequencies (genetic drift)? Is selection on variants that affect fitness mostly purifying, so that mutations with harmful effects are rapidly removed from the population (1)? Or do many loci have variants maintained by balancing selection (2)? What fraction of newly arisen variants cause higher fitness and are in the process of spreading through the population and replacing their alternatives? How strong is the selection acting on nonneutral variants, and how much variation in fitness among individuals within populations is contributed by such variants? Does the existence of wide variation in fitness among individuals imply a genetic load that threatens the survival of the species (1)?These questions are very broad, and this paper deals only with one aspect of them. It focuses on the question of how recent inferences concerning the strength of purifying selection, derived from genome-wide surveys of DNA sequence variability, can be connected with the results of statistical studies of genetic variation in components of Darwinian fitness such as viability and fertility. I will refer to these two approaches as population genomics and quantitative genetics, respectively. The first approach sheds light on the general nature of the fitness effects of the DNA sequence variants found in natural populations, but says little about how these fitness effects are caused. The second tells us how much genetic variability exists for fitness traits, the rate at which it arise by mutation and something about the type of selection involved, but is silent about the nature of the underlying sequence variants.Surprisingly little attention has been paid to integrating these two lines of inquiry, except for ref. 7. I largely confine myself to results from studies of the fruitfly Drosophila, because this has been the most useful model organism for investigating these problems, especially by quantitative genetics methods. Current information derived from population genomics studies will first be reviewed, followed by an analysis of the results of quantitative genetics experiments on both mutational and standing variation. I show that the quantitative genetics results can only be explained if there is a significant input of new mutations with much larger effects on fitness than those inferred from population genomics. There also appears to be too much genetic variation in fitness components in natural populations to be explained purely by mutation selection balance, so that additional processes such as balancing selection must make an important contribution.     

Multiple paternity in two natural populations (orchard and vineyard) of Drosophila.

Male mating success is an important fitness component in Drosophila. The seminal fluid conveyed with the sperm inhibits the proclivity of the female to remate and reduces her fitness. Nevertheless, females may remate before they have exhausted the sperm from the first male and consequently use sperm from both males. We have studied concurrent multiple paternity (CMP) in two Drosophila melanogaster populations, from an apple orchard and a vineyard just after harvest. CMP is high in both populations, somewhat greater than 50%; but it is not significantly higher in the vineyard, where the population density is much greater than in the orchard. Population density had been thought to be an important determinant of CMP incidence. We have used four gene loci coding for enzymes as independent markers for detecting CMP.     

Nature screen: an efficient method for screening natural populations of Drosophila for targeted P-element insertions.

The efficiency of molecular techniques is making it increasingly necessary to rely on reverse genetics to understand the function of genes. Tissue-specific libraries allow one to identify numerous genes that can be cloned, sequenced, and mapped and whose temporal and tissue-specific pattern of expression are well characterized but whose function remains unknown. In such cases, it is desirable to generate targeted mutations to examine the phenotype of loss-of-function lesions. Here we describe a method for identifying naturally occurring variants of Drosophila melanogaster with specific genes tagged by a nearby P element. Imprecise P-element excision can then be used to generate a series of small deletions in or near the gene. In the method described here, large numbers of wild-caught males were crossed to balancer females, and inserts were identified in pooled samples by the polymerase chain reaction with one primer from each target gene and one primer from the P-element terminal repeat. We present the calculations for the probability of successfully tagging a gene and show that it is greatly improved by simultaneously screening inserts into several genes. If a large natural population is available, a nature screen is faster and easier than inducing P-element transposition in the laboratory, but the resulting lines, being genetically heterogeneous, may require more subsequent work to isolate. Using this method to screen the genomes of approximately 10,400 males, we found P-element inserts in close proximity to 3 of 10 genes that were screened.     

Stable heteroplasmy for a large-scale deletion in the coding region of Drosophila subobscura mitochondrial DNA.

Due to the extremely economic organization of the animal mitochondrial genome, large-scale deletions are rarely found in animal mtDNA. We report the occurrence of a massive deletion in the coding region of mtDNA in Drosophila subobscura. Restriction mapping and nucleotide sequence analysis revealed that the deletion encompasses six protein genes and four tRNAs. All individuals of an isofemale strain proved to be heteroplasmic for normal and deficient mtDNA molecules. This type of heteroplasmy resembles one observed in patients with mitochondrial myopathies but differs in that the fitness of heteroplasmic flies is not significantly reduced even though the mutant mtDNA constitutes 50-80% of total mtDNA in most of the individuals studied. The heteroplasmic strain is genetically stable: despite extensive screening not a single homoplasmic fly was observed since the foundation of the line.     

Enzyme polymorphism of the malaria vector, An. balabacensis (Diptera: Culicidae) revisited--why sample natural populations?

Two separate observations from recent electrophoretic studies of the systematics and population genetics of laboratory-reared populations which had a long history of colonization in various laboratories, were found to be inconsistent with the present study which used wild-caught populations from East Malaysia. Reanalysis of the two data sets generally indicated a low amount of genetic variation in laboratory colonies. The latter is characterized by higher frequency of monomorphic loci, low average heterozygosity values and, in one extreme case, no variability at two loci. However, natural populations of An. balabacensis and An. leucosphyrus showed more protein variability by the use of polyacrylamide gel electrophoresis. Since laboratory-maintained mosquitoes are genetically and phenotypically different from those in the field, results of laboratory studies on the systematics and population genetics of Anopheles species complexes may be biased.     

Transfer of genetic information in the rp49 region of Drosophila subobscura between different chromosomal gene arrangements.

Nucleotide variation in the region including the ribosomal protein 49 (rp49) gene was investigated by direct sequencing of 10 alleles of Drosophila subobscura from chromosomes differing in gene arrangements. Fifty-six nucleotide and seven length polymorphisms were detected over a 1.5-kb region. Of the 20 nucleotide polymorphisms present more than once in the sample, 13 were segregating in O[3 + 4], 9 in O[3 + 4 + 8], and 4 in O[st] chromosomal classes. Several of these polymorphisms were segregating in more than one chromosomal class, a strong indication of genetic transfer between different chromosomal gene arrangements either by double crossover or gene conversion. Given the probable role played by gene conversion in the history of the rp49 region in D. subobscura, estimates of nucleotide diversity within chromosomal class indicate that the O[3 + 4] chromosomal gene arrangement is older than the O[st] arrangement.