Ross J. MacIntyre

An Analysis of Gene-Enzyme Variability in Natural Populations of Drosophila melanogaster and D. simulans

Nine populations of D. melanogaster and two populations of D. simulans were analyzed for polymorphism in 10 gene-enzyme systems by the technique of gel electrophoresis. In the eight natural populations of D. melanogaster, an average of 54% of the enzymes were polymorphic, and the average heterozygosity was 22.7%. An experimental population of D. melanogaster, which has been maintained in a laboratory cage for 20 years, showed levels of polymorphism equivalent to those of natural populations. The D. simulans populations had much less variability. The possible factors involved in maintaining these polymorphisms are discussed.

Analysis of glycolytic enzyme co-localization in Drosophila flight muscle.

SUMMARY In Drosophila flight muscles, glycolytic enzymes are co-localized along sarcomeres at M-lines and Z-discs and co-localization is required for normal flight. We have extended our analysis of this phenomenon to include a set of six glycolytic enzymes that catalyze consecutive reactions along the glycolytic pathway: aldolase, glycerol-3-phosphate dehydrogenase (GPDH), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), triose phosphate isomerase, phosphoglycerate kinase and phosphoglycerol mutase (PGLYM). Each of these enzymes has an identical pattern of localization. In mutants null for GPDH, localization of none of the other enzymes occurs and therefore is interdependent. In optimally fixed preparations of myofibrils, accumulation of the enzymes at M-lines is much greater than at Z-discs. However, localization at M-lines is more labile, as shown by loss of localization when fixation is delayed. We have begun to analyze the protein—protein interaction involved in glycolytic enzyme co-localization using the yeast two-hybrid system. We have identified two pair-wise interactions. One is between GPDH and GAPDH and another is between GPDH and PGLYM.

Concerted Evolution Within the Drosophila dumpy Gene

We have determined by reverse Southern analysis and direct sequence comparisons that most of the dumpy gene has evolved in the dipteran and other insect orders by purifying selection acting on amino acid replacements. One region, however, is evolving rapidly due to unequal crossing over and/or gene conversion. This region, called “PIGSFEAST,” or PF, encodes in D. melanogaster 30–47 repeats of 102 amino acids rich in serines, threonines, and prolines. We show that the processes of concerted evolution have been operating on all species of Drosophila examined to date, but that an adjacent region has expanded in Anopheles gambiae, Aedes aegypti, and Tribolium castaneum, while the PF repeats are reduced in size and number. In addition, processes of concerted evolution have radically altered the codon usage patterns in D. melanogaster, D. pseudoobscura, and D. virilis compared with the rest of the dumpy gene. We show also that the dumpy gene is expressed on the inner surface of the micropyle of the mature oocyte and propose that, as in the abalone system, concerted evolution may be involved in adaptive changes affecting Dumpys possible role in sperm–egg recognition.

Low specific activity of rare allozymes of |[alpha]|-glycerophosphate dehydrogenase in Drosophila

ELECTROPHORETIC surveys of Drosophila species1 show that many gene–enzyme systems are essentially monomorphic. Very often, however, one or more populations may contain rare alleles, that is, at frequencies less than 5%. Are these alleles selectively neutral and rare because of historical accidents2 or are they deleterious and maintained in populations only through mutation? The dimeric enzyme α-glycerophosphate dehydrogenase (αGPDH) is a good example of an essentially monomorphic Drosophila gene–enzyme system. Out of the 101 species of Drosophila surveyed thus far for electrophoretic variation of αGPDH (refs 3–5), only D. melanogaster is polymorphic. This polymorphism seems to be maintained by selection with temperature being, or strongly correlated with, the selective agent6–8. Microcomplement fixation experiments indicate that this enzyme is evolving slowly among Drosophila species and has accepted relatively few changes in its structure9. We propose that the rare alleles of the αGPDH locus found in some species (see ref. 5) may be at low frequencies because the products of these alleles (allozymes) are catalytically less efficient than their ‘wild-type’ counterparts.

The α-Glycerophosphate Cycle in Drosophila melanogaster. III. Relative Viability of "Null" Mutants at the α-Glycerophosphate Dehydrogenase-1 Locus

The description of genetic variation in natural populations has involved three different general approaches: (1) the detection, by means of special genetic techniques, of genes in Drosophila which affect viability, fertility, or developmental rates (e.g., Dobzhansky and Spassky 1953) ; (2) the measurement of chromosomal inversion polymorphism in Drosophila (Dobzhansky 1970, chap. 5); and (3) the electrophoretic monitoring of gene-enzyme (allozyme) variation in a number of species from many different animal and plant phyla (e.g., Lewontin and Hubby 1966; OBrien and MacIntyre 1969; Allard 1971; Prakash, Lewontin, and Hubby 1969; Selander and Yang 1969). The first approach uncovers cryptic variation with respect to various components of fitness; such variation is revealed by techniques that render chromosomes homozygous. The other two approaches reveal genetic variation virtually by direct observation. The relation of this variation to fitness is not at all immediately obvious; indeed, there are cogent arguments that much allozyme variation is selectively neutral (King and Jukes 1969; Kimura and Ohta 1971). The first method of measuring relative viability of wild chromosomes has provided a wealth of information to the field of population genetics mainly through the efforts of Dobzhansky and his colleagues (for references, see Dobzhansky 1970, p. 118; Wallace 1968, p. 36). Relative viability is determined through the use of invested balancer chromosomes which contain a dominant marker mutation and a recessive lethal. In these studies, male and female flies, heterozygous for the balancer chromosome and the same sampled chromosome, are mated to each other and their progeny are scored. The ratio of homozygous wild-type offspring to balancer heterozygous offspring has been determined for large numbers of wild chromosomes from natural populations of several Drosophila species. This ratio can be related to the ratio obtained by comparing the relative viability of heterozygotes for different wild chromosomes with that of the balancer heterozygotes in control cultures. It is striking that in the majority of analyses reported, regardless of which chromosome, population, or species is sampled, the frequency distribution of chromosome viability followed a uniquely distinct

The α Glycerophosphate Cycle in Drosophila melanogaster V. Molecular Analysis of α Glycerophosphate Dehydrogenase and α Glycerophosphate Oxidase Mutants

Two enzymes, alpha glycerophosphate dehydrogenase (GPDH-1) in the cytoplasm and alpha glycerophosphate oxidase (GPO-1) in the mitochondrion cooperate in Drosophila flight muscles to generate the ATP needed for muscle contraction. Null mutants for either enzyme cannot fly. Here, we characterize 15 ethyl methane sulfonate (EMS)-induced mutants in GPDH-1 at the molecular level and assess their effects on structural and evolutionarily conserved domains of this enzyme. In addition, we molecularly characterize 3 EMS-induced GPO-1 mutants and excisions of a P element insertion in the GPO-1 gene. The latter represent the best candidate for null or amorphic mutants in this gene.