Position Effects and Variegation Enhancers in an Autosomal Region of DROSOPHILA MELANOGASTER

A dominant eye color mutation was found associated with a third chromosome inversion broken distally at or near the karmoisin (kar) locus in 87C and proximally within centric heterochromatin. Suppressibility of the mutant phenotype by an extra Y chromosome indicated that this was an example of dominant position-effect variegation. When heterozygous with deficiencies uncovering the kar locus, this inversion chromosome was found to be lethal unless a region in 87EF was also deleted. Extra Y chromosomes rescued inversion/deletion heterozygotes, while removal of the Y chromosome from heterozygous males deficient for the region in 87EF was lethal. Thus, a variegating lethal lies near the breakpoint in 87C, and a wild-type gene that enhances its variegation lies in 87EF. Furthermore, deletion of the region in 87EF was found to strongly suppress white-mottled-4 (w^m4) variegation, while deletion of another region in 87BC suppressed less strongly. These results indicate that essential genes on autosomes are sensitive to position effects, and loci that enhance variegation, as defined by deficiency mapping, are very common.     

Regulation of Ribosomal RNA Gene Multiplicity in DROSOPHILA MELANOGASTER

The ribosomal RNA (rRNA) genes of Drosophila melanogaster can undergo a disproportionate replication of their number. This occurs when the cluster of rRNA genes (rDNA) of one chromosome is maintained with a homologous chromosome that is completely or partially deficient in its rDNA. Under appropriate genetic conditions, it appears that disproportionate rDNA replication can be generated at the level of both somatic and germ line cells. In the latter case, mutants partially deficient for rDNA can increase their rRNA gene number to the wild type level and transmit this new genotype to successive generations.     

A Possible Case of Position Effect on DNA Replication in DROSOPHILA MELANOGASTER

The behavior of T(3;4)10 provides evidence that the centromere of 4 is not in the doublet 101D3,4, and that most of what has been called the left arm of 4 should be called basal 4R. Basal 4R usually does not replicate as much as the tip of 4R when 4 is in its normal position in the chromocenter. Translocated to the tip of 3R, however, basal 4R attains a width equal to that of the rest of the arm-a possible position effect on DNA replication.     

Position Effects at the Hairy Locus in DROSOPHILA MELANOGASTER

Radiation-induced chromosomal rearrangements of h(+) have given rise to several Drosophila stocks that exhibit apparent position-effect inactivation; i.e., flies carrying the rearranged chromosomes heterozygously with h show varying degrees of hairiness. The numbers of hairy chaetae produce a quantifiable index of position effect. Six such "position-allele" stocks are here discussed, both as to their basic expressions and in all possible pair-wise combinations with each other. Such crosses reveal complex interactions between the respective position alleles; little evidence is seen for clear-cut dominance or recessiveness. The stocks appear not to conform unequivocally to classical distinctions between variegated and stable types of position effects, nor to usual dicta relating the degree of inactivity to the proximity to heterochromatin. Indeed, these stocks appear to suggest additional dimensions to several of the principles to which position effects usually subscribe. The evidence additionally suggests that the hairy locus itself is associated with a tissue-specific suppressor effect on an otherwise polygenic system that produces the chaetae associated with the hairy phenotype.     

Maternal and Zygotic Sex-Specific Gene Interactions in DROSOPHILA MELANOGASTER

Sex-lethal (Sxl) is a vital, X-chromosome gene involved in Drosophila sex determination. The most striking aspect of the phenotype of daughterless (da), an autosomal maternal-effect mutation, may be explained by effects on the functioning of the Sxl gene in the zygote. In this paper, new aspects of interactions between various combinations of Sxl and da alleles are explored in order to understand better the complex da phenotype. The study focuses on the relationship between maternal and zygotic da+ gene functions, and on the relationship between aspects of the da phenotype that are sex-specific and aspects that are not. The SxlM#1 allele, which counteracts the female-specific maternal effect of da, is shown to have no effect on two other aspects of the da phenotype (one maternal, one primarily zygotic) that are not sex-specific. The female-lethal da maternal effect is shown to kill daughters even when the progeny are entirely wild-type with respect to da. Recessive mutant alleles of the two genes can interact synergistically when both are heterozygous with their wild-type alleles, disrupting the development of most of the daughters. Surprisingly, even a deficiency of the da+ locus can produce a dominant, temperature-sensitive, female-lethal maternal effect. A new class of subliminal Sxlf alleles is described. These spontaneous mutations can confuse analysis of both da and Sxl if their presence is not appreciated. Finally, conditions are described that facilitate the study of the Enhancer of daughterless mutation.     

Glucose Repression of Amylase Gene Expression in DROSOPHILA MELANOGASTER

We have previously shown that dietary glucose can reduce amylase activity in both adults and larvae of Drosophila; this reduction in enzyme activity reflects a reduction in the quantity of amylase protein, rather than an inhibition of enzyme activity. Here, we report that we have now defined conditions in which the repressive effect of glucose can be greater than 100-fold. Moreover, this repression is partially counteracted by the addition of exogenous cyclic AMP. We also show that there is a direct correlation between changes in amylase activity and changes in the amount of translatable mRNA as assayed in microinjected Xenopus oocytes. This means that the glucose repression is occurring at a pretranslational stage.     

Gene Flow and Selection in a Natural Population of DROSOPHILA MELANOGASTER

A marked genetic differentiation to the presence of alcohol in the environment has been shown to occur between inside cellar and adjacent outside sections of a vineyard population of D. melanogaster ( McKenzie and Parsons 1974). Estimates of migration during the vintage period suggest considerable movement occurs from outside into the cellar and that the most tolerant genotypes are the most successful migrants. A quantitative model of this system suggests that the selection intensity may not be a limiting factor in maintaining the differentiation. It also suggests that gene flow must be restricted between sections of the population at all but vintage periods if this differentiation is to persist.     

Defective Histone Transition during Spermiogenesis in Heterozygous SEGREGATION DISTORTER Males of DROSOPHILA MELANOGASTER

Males of Drosophila melanogaster that are heterozygous for the segregation distorter (SD) chromosome produce a gross excess of SD-bearing offspring because most of the non-SD-bearing sperm are dysfunctional. These dysfunctional sperm exhibit abnormalities in chromatin condensation and compaction during spermiogenesis. Use of the fluorescent dye sulfoflavine, which is specific for basic proteins, has now revealed that the dysfunctional sperm are also defective in the normal transition from somatic to spermatid-specific histones.     

The Question of the Total Gene Number in DROSOPHILA MELANOGASTER

A statistical analysis has been carried out on the distribution and allelism of nearly 500 sex-linked, X-ray-induced, cytologically normal and rearranged lethal mutations in Drosophila melanogaster that were obtained by G. Lefevre. The mutations were induced in four different regions of the X chromosome: (1) 1A1-3E8, (2) 6D1-8A5, (3) 9E1-11A7 and (4) 19A1-20F4, which together comprise more than one-third of the entire chromosome.--The analysis shows that the number of alleles found at different loci does not fit a Poisson distribution, even when the proper procedures are taken to accommodate the truncated nature of the data. However, the allele distribution fits a truncated negative binomial distribution quite well, with cytologically normal mutations fitting better than rearrangement mutations. This indicates that genes are not equimutable, as required for the data to fit a Poisson distribution.--Using the negative binomial parameters to estimate the number of genes that did not produce a detectable lethal mutation in our experiment (n0) gave a larger number than that derived from the use of the Poisson parameter. Unfortunately, we cannot estimate the total numbers of nonvital loci, loci with undetectable phenotypes and loci having extremely low mutabilities. In any event, our estimate of the total vital gene number was far short of the total number of bands in the analyzed regions; yet, in several short intervals, we have found more vital genes than bands; in other intervals, fewer. We conclude that the one-band, one-gene hypothesis, in its literal sense, is not true; furthermore, it is difficult to support, even approximately.--The question of the total gene number in Drosophila will, not doubt, eventually be solved by molecular analyses, not by statistical analysis of mutation data or saturation studies.     

Dosage of the Maternal Effect Gene Associated with the Tumorous-Head Abnormality in DROSOPHILA MELANOGASTER

The penetrance of tuh-3 observed in the progeny of triploids with two doses of tuh-1 was not statistically different from that occurring in the progeny of their diploid sisters with two doses. A higher penetrance was observed in the progeny of triploids with three doses of tuh-1 than in the progeny of their diploid sisters with two doses. These observations suggest that the maternal effect responsible for increasing the penetrance of tuh-3, the gene causing the tumorous-head abnormality, is caused by a specific gene product of tuh-1, the maternal-effect allele. In addition there is probably a maternal-effect threshold, lying between amounts produced by one and two doses of tuh-1, below which no increased penetrance of tuh-3 is observed.     

Position Effect Variegation: Role of the Local Chromatin Context in Gene Expression Regulation

Molecular Biology - Position effect variegation (PEV) is a phenomenon wherein the expression level of a gene strongly depends on its genomic position. PEV can be observed when a gene is moved via a...     

Cytogenetic Localization of the Acid Phosphatase-1 Gene in DROSOPHILA MELANOGASTER

A translocation in which a segment of chromosome 3 is inserted into the Y chromosome was found to contain the acid phosphatase-1 gene (Acph-1). In flies hyperploid for that gene, acid phosphatase-1 levels are proportional to the dose of the gene. The locus is placed within the salivary chromosome subdivisions 99D and 99E on the basis of its inclusion in the translocated segment and on the previous placement of the claret locus. Several chromosomal rearrangements involving heterochromatic breakpoints and euchromatic breakpoints adjacent to 99D-99E were tested for possible postiion-effect variegation of acid phosphatase-1. No decrease in the synthesis of the electorphoretic subunit encoded by the relocated gene was observed within any of the rearrangements.