A cytological analysis of the Scutoid mutation of Drosophila melanogaster,and its induced revertants

The Scutoid mutation of Drosophila melanogaster is associated with two small transpositions within region 35 of the polytene chromosome arm 2L. A number of induced revertants of Sco have been recovered and the majority of these are chromosome aberrations. The localisation of at least one breakpoint of these aberrant revertants to the proximal breakpoint of the proximal transposition of Sco, a breakpoint that juxtaposes two loci (noc and sna), normally separated by several loci is evidence that the Scutoid phenotype results from a fusion of noc and sna.     

The suppressor of forked locus in Drosophila melanogaster: genetic and molecular analyses

The suppressor of forked, su(f) locus is one of a class of loci in Drosophila whose mutant alleles are trans-acting allele-specific modifiers of transposable element-insertion mutations at other loci. Mutations of su(f) suppress gypsy insert alleles of forked and enhance the copia insert allele white apricot. Our investigations of su(f) include genetic and molecular analyses of 19 alleles to determine the numbers and types of genetic functions present at the locus. Our results suggest the su(f) locus contains multiple genetic functions. There are two distinct modifier functions and two vital functions. One modifier function is specific for enhancement and the other for suppression. One vital function is required for normal ecdysterone production in the third larval instar, the other is not. We present a restriction map of the su(f) genomic region and the results of an RFLP analysis of several su(f) alleles.     

Actin with tumor-related mutation is antimorphic in Drosophila muscle: two distinct modes of myofibrillar disruption by antimorphic actins

A mutant beta-actin with an amino acid substitution from Gly-245 to Asp has been shown to be related to tumorigenic transformation of a human fibroblast cell line (Leavitt, J. et al. (1987) Mol. Cell. Biol. 7, 2467-2476). To examine the effects of this mutation, we artificially introduced the same amino acid change into the Act88F actin gene of Drosophila melanogaster. The gene (Act88FGD245) was inserted in the Drosophila genome to make transgenic adult flies which synthesize the mutant actin in the indirect flight muscles. The mutant actin was found to be antimorphic with regard to flight and also to cause myofibrillar disruption in transformants even in the presence of two normal alleles. It was initially incorporated into myofibrils and later induced their degeneration from center to periphery. This mode of myofibrillar disruption is distinct from that of previously reported Act88F mutations, where defects are found only in the peripheral region of myofibrils. This indicates that actin functions are altered differently in the two classes of antimorphic mutations.     

Genetic and molecular analyses of vgal: a spontaneous and unstable mutation at the vestigial locus in Drosophila melanogaster

We describe herein, a new unstable mutant of the vestigial locus, isolated from a French natural population. From this mutant vestigial almost (vgal) wild-type flies (vgal+) and extreme vg phenotypes (vge) arose spontaneously without genomic shock. The occurrence of vgal+ or vge alleles depends mostly on the breeding temperature; vgal+ revertants arose principally at low temperature (21 degrees C) and vge at 28 degrees C. These events occur mainly in the male germ line and the phenomenon appears to be premeiotic. Our results with in situ hybridization experiments and Southern blots show that the vgal mutation is due to a 2 kb DNA insertion, which is a deleted hobo element. Genetic and molecular analyses show that two distinct events may underly the wild-type revertants. One is the excision of the resident hobo element, the other a further deletion (about 300 bp in the example characterized herein). The vge mutation is probably due to a deletion of vestigial sequences flanking the hobo insertion.     

The age and evolution of an antiviral resistance mutation in Drosophila melanogaster

What selective processes underlie the evolution of parasites and their hosts? Arms-race models propose that new host-resistance mutations or parasite counter-adaptations arise and sweep to fixation. Frequency-dependent models propose that selection favours pathogens adapted to the most common host genotypes, conferring an advantage to rare host genotypes. Distinguishing between these models is empirically difficult. The maintenance of disease-resistance polymorphisms has been studied in detail in plants, but less so in animals, and rarely in natural populations. We have made a detailed study of genetic variation in host resistance in a natural animal population, Drosophila melanogaster, and its natural pathogen, the sigma virus. We confirm previous findings that a single (albeit complex) mutation in the gene ref(2)P confers resistance against sigma and show that this mutation has increased in frequency under positive selection. Previous studies suggested that ref(2)P polymorphism reflects the progress of a very recent selective sweep, and that in Europe during the 1980s, this was followed by a sweep of a sigma virus strain able to infect flies carrying this mutation. We find that the ref(2)P resistance mutation is considerably older than the recent spread of this viral strain and suggest that—possibly because it is recessive—the initial spread of the resistance mutation was very slow.     

Scutoid mutation of drosophila melanogaster specifically decreases olfactory responses to short-chain acetate esters and ketones

A molecular-genetic approach has been taken to identify genes involved in olfactory transduction in Drosophila melanogaster. Two independent lines of research led to the finding that the dominant Scutoid (Sco) mutation causes a diminshed extracellular electroantennogram response to the odorants ethyl acetate (EtAC) and acetone (AC). Sco flies showed about 4- and 2.5-fold reduced responses to EtAC and AC, respectively, compared to Canton-S wild-type and sibling control flies lacking the Sco mutation when electroantennogram recordings were made from the proximal anterior third antennal segment. The responses to five other odors from three different chemical classes were unaltered. The maximum response to either EtAC or AC was decreased with no change in apparent affinity. Responses to short-chain (but not long-chain) acetate esters and ketones were dramatically affected at all antennal locations tested. Only in the proximal quadrants were responses to ethyl acetoacetate also reduced. Most Sco revertants tested had a normal olfactory response; duplications of the region including no-ocelli partially suppress the Sco bristle as well as olfactory phenotypes. Sco adults had an impaired behavioral response to EtAC but not to banana or propionate. There was no effect of the mutation on larval chemosensory behavior or extracellularly recorded adult compound eye and ocellar visual responses. These findings suggest the involvement of Sco in an olfactory pathway in adults which is specific for short-chain acetate esters and ketones. © 1995 John Wiley & Sons, Inc.     

The molecular genetics of early neurogenesis in Drosophila melanogaster

The extent of neurogenesis in Drosophila is under the control of the so-called neurogenic genes, named for their mutant phenotype of causing neural hyperplasia. Their wild-type products appear to be responsible for a signal chain that decides the fate of ectodermal cells in the embryo. Various kinds of data, from cell transplantation experiments as well as from genetic and molecular analyses, suggest that the proteins encoded by the genes Notch and Delta may act at the membrane of the signal-transmitting cells to provide a ligand to a still unknown receptor molecule; in contrast, the locus of Enhancer of split codes for several functions related to the transduction and further processing of the signal.     

The mutation rates of di-, tri- and tetranucleotide repeats in Drosophila melanogaster

In a recent study, we reported that the combined average mutation rate of 10 di-, 6 tri-, and 8 tetranucleotide repeats in Drosophila melanogaster was 6.3 x 10(-6) mutations per locus per generation, a rate substantially below that of microsatellite repeat units in mammals studied to date (range = 10(-2)-10(-5) per locus per generation). To obtain a more precise estimate of mutation rate for dinucleotide repeat motifs alone, we assayed 39 new dinucleotide repeat microsatellite loci in the mutation accumulation lines from our earlier study. Our estimate of mutation rate for a total of 49 dinucleotide repeats is 9.3 x 10(-6) per locus per generation, only slightly higher than the estimate from our earlier study. We also estimated the relative difference in microsatellite mutation rate among di-, tri-, and tetranucleotide repeats in the genome of D. melanogaster using a method based on population variation, and we found that tri- and tetranucleotide repeats mutate at rates 6.4 and 8.4 times slower than that of dinucleotide repeats, respectively. The slower mutation rates of tri- and tetranucleotide repeats appear to be associated with a relatively short repeat unit length of these repeat motifs in the genome of D. melanogaster. A positive correlation between repeat unit length and allelic variation suggests that mutation rate increases as the repeat unit lengths of microsatellites increase.      

Deletion of an insulator element by the mutation facet-strawberry in Drosophila melanogaster

Eukaryotic chromosomes are thought to be subdivided into a series of structurally and functionally independent units. Critical to this hypothesis is the identification of insulator or boundary elements that delimit chromosomal domains. The properties of a Notch mutation, facet-strawberry (fa(swb)), suggest that this small deletion disrupts such a boundary element. fa(swb) is located in the interband separating polytene band 3C7, which contains Notch, from the distal band 3C6. The fa(swb) mutation alters the structural organization of the chromosome by deleting the interband and fusing 3C7 with 3C6. Genetic studies also suggest that fa(swb) compromises the functional autonomy of Notch by allowing the locus to become sensitive to chromosomal position effects emanating from distal sequences. In the studies reported here, we show that a DNA fragment spanning the fa(swb) region can insulate reporter transgenes against chromosomal position effects and can block enhancer-promoter interactions. Moreover, we find that insulating activity is dependent on sequences deleted in fa(swb). These results provide evidence that the element defined by the fa(swb) mutation corresponds to an insulator.     

Estimation of microsatellite mutation rates in Drosophila melanogaster

Microsatellite mutations were studied in a set of 175 mutation accumulation lines, all of them independently derived from a completely homozygous population of Drosophila melanogaster and maintained under strong inbreeding during 80 generations. We assayed 28 microsatellites and detected two mutations. One mutation consisted of a single addition of a dinucleotide repeat and the other was a deletion of five trinucleotide repeats. The average mutation rate was 5.1 x 10(-6), in full agreement with previous estimates from two different sets of mutation accumulation lines.     

The effects of spontaneous mutation on competitive fitness in Drosophila melanogaster

Abstract We have analysed the effect of 288 generations of mutation accumulation (MA) on chromosome II competitive fitness in 21 full‐sib lines of Drosophila melanogaster and in a large control population, all derived from the same isogenic base. The rate of mean log‐fitness decline and that of increase of the between‐line variance were consistent with a low rate (λ ≈ 0.03 per gamete and generation), and moderate average fitness effect [E(s) ≈ 0.1] of deleterious mutation. Subsequently, crosses were made between pairs of MA lines, and these were maintained with effective size on the order of a few tens. In these crosses, MA recombinant chromosomes quickly recovered to about the average fitness level of control chromosomes. Thus, deleterious mutations responsible for the fitness decline were efficiently selected against in relatively small populations, confirming that their effects were larger than a few percent.     

Intergenic suppression of the black mutation of Drosophila melanogaster

Summary Five X-linked, recessive alleles were isolated which suppress both the pupal and adult coloration phenotypes of the black mutation. Electron micrographs of shed puparia revealed that the aberrant sclerotization of the black cuticle is also suppressed. Amino acid analysis indicated that suppression is associated with an increased concentration of -alanine, an amino acid known to be deficient in black. The suppressor locus mapped at the tip of the X, to the right of scute, and intragenic complementation was observed among the alleles.