Preadult competition between Drosophila subobscura and Drosophila pseudoobscura

The Palaearctic species Drosophila subobscura has recently colonized a large area of North America where it coexists with Drosophila pseudoobscura. The viability and developmental rate of these species were studied at 13 d?C, 18d?C and 23 d?C and at densities of 10, 50, 100 and 200 eggs per vial. The two species were differently affected by density and temperature in the ranges studied. Both intra- and interspecific cultures showed that D. pseudoobscura was best adapted to 23 d?C, where it was clearly the dominant species. On the other hand, at 18 d?C and especially at 13 d?C, although D. subobscura was less viable than D. pseudoobscura, its developmental time was shorter, which may give advantage to this species. Results reported here agree with the observed distribution of these species in North America.     

Recombination and the frequency spectrum in Drosophila melanogaster and Drosophila simulans

Most "tests of neutrality" assess whether particular data sets depart from the predictions of a standard neutral model with no recombination. For Drosophila, where nuclear polymorphism data routinely show evidence of genetic exchange, the assumption of no recombination is often unrealistic. In addition, while conservative, this assumption is made at the cost of a great loss in power. Perhaps as a result, tests of the frequency spectrum based on zero recombination suggest an adequate fit of Drosophila polymorphism data to the predictions of the standard neutral model. Here, we analyze the frequency spectrum of a large number of loci in Drosophila melanogaster and D. simulans using two summary statistics. We use an estimate of the population recombination rate based on a laboratory estimate of the rate of crossing over per physical length and an estimate of the species' effective population size. In contrast to previous studies, we find that roughly half of the loci depart from the predictions of the standard neutral model. The extent of the departure depends on the exact recombination rate, but the global pattern that emerges is robust. Interestingly, these departures from neutral expectations are not unidirectional. The large variance in outcomes may be due to a complex demographic history and inconsistent sampling, or to the pervasive action of natural selection.     

Phylogenetic relationship between Drosophila takahashii Sturtevant and Drosophila pseudotakahashii Mather

The phylogenetic relationship between two Drosophila species of the takahashii subgroup, D. takahashii and D. pseudotakahashii, was determined through the study of male genitalia, metaphase and salivary chromosomes, mating behaviour and interspecific hybridization. From all lines of evidence it is concluded that the species are very closely related, and now constitute a pair of allopatric sibling species.     

Clonal analysis in hybrids between Drosophila melanogaster and Drosophila simulans

We have analysed the viability of cellular clones induced by mitotic recombination in Drosophila melanogaster/D. simulans hybrid females during larval growth. These clones contain a portion of either melanogaster or simulans genomes in homozygosity. Analysis has been carried out for the X and the second chromosomes, as well as for the 3L chromosome arm. Clones were not found in certain structures, and in others they appeared in a very low frequency. Only in abdominal tergites was a significant number of clones observed, although their frequency was lower than in melanogaster abdomens. The bigger the portion of the genome that is homozygous, the less viable is the recombinant melano-gaster/simulans hybrid clone. The few clones that appeared may represent cases in which mitotic recombination took place in distal chromosome intervals, so that the clones contained a small portion of either melanogaster or simulans chromosomes in homozygosity. Moreover, Lhr, a gene of D. simulans that suppresses the lethality of male and female melanogaster/simulans hybrids, does not suppress the lethality of the recombinant melanogaster/simulans clones. Thus, it appears that there is not just a single gene, but at least one per tested chromosome arm (and maybe more) that cause hybrid lethality. Therefore, the two species, D. melanogaster and D. simulans, have diverged to such a degree that the absence of part of the genome of one species cannot be substituted by the corresponding part of the genome of the other, probably due to the formation of co-adapted gene complexes in both species following their divergent evolution after speciation. The disruption of those coadapted gene complexes would cause the lethality of the recombinant hybrid clones.     

Purification and enzyme stability of alcohol dehydrogenase from Drosophila simulans, Drosophila virilis and Drosophila melanogaster adhS.

Three alcohol dehydrogenases from Drosophila simulans, Drosophila virillis and Drosophila melanogaster adhS (which possesses an alloenzyme with slow electrophoretic mobility) were purified essentially to homogeneity. The purification procedure involves a new step of affinity chromatography, which efficiently lowers the amount of contaminants in the final preparation, producing a very stable enzyme. The purification procedure developed consists of a salmine sulphate precipitation, two CM-Sepharose CL-6B colume-chromatography steps, an affinity-chromatography step and a Sephacryl gel filtration. A minimum of 30-fold purification is obtained and the yield is not less than 34%. The isoelectric points and molar absorption coefficients were determined.     

Microsatellite variation in populations of Drosophila pseudoobscura and Drosophila persimilis

We have isolated, characterized and mapped 33 dinucleotide, three trinucleotide and one tetranucleotide repeat loci from the four major chromosomes of Drosophila pseudoobscura. Average inferred repeat unit length of the dinucleotide repeats is 12 repeat units, similar to D. melanogaster. Assays of D. pseudoobscura and populations of its sibling species, D. persimilis, using 10 of these loci show extremely high levels of variation compared with similar studies of dinucleotide repeat variation in D. melanogaster populations. The high levels of variation are consistent with an average mutation rate of approximately 10(-6) per locus per generation and an effective population size of D. pseudoobscura approximately four times larger than that of D. melanogaster. Consistent with allozymes and nucleotide sequence polymorphism, the dinucleotide repeat loci reveal minimal structure across four populations of D. pseudoobscura. Finally, our preliminary recombinational mapping of 24 of these microsatellites suggests that the total recombinational genome size may be larger than previously inferred using morphological mutant markers.     

New Drosophila P-like elements and reclassification of Drosophila P-elements subfamilies

Genomic searches for P-like transposable elements were performed (1) in silico in the 12 available Drosophila genomes and (2) by PCR using degenerate primers in 21 Neotropical Drosophila species. In silico searches revealed P-like sequences only in Drosophila persimilis and Drosophila willistoni. Sixteen new P-like elements were obtained by PCR. These sequences were added to sequences of previously described P-like elements, and a phylogenetic analysis was performed. The subfamilies of P-elements described in the literature (Canonical, M, O, T, and K) were included in the reconstructed tree, and all were monophyletic. However, we suggest that some subfamilies can be enlarged, other subdivided, and some new subfamilies may be proposed, totalizing eleven subfamilies, most of which contain new P-like sequences. Our analyses support the monophyly of P-like elements in Drosophilidae. We suggest that, once these elements need host-specific factors to be mobilizable, the horizontal transfer (HT) of P-like elements may be inhibited among more distant taxa. Nevertheless, HT among Drosophilidae species appears to be a common phenomenon.     

Mitochondrial DNA in Drosophila. An analysis of genome organization and transcription in Drosophila melanogaster and Drosophila virilis

Transfer RNAs of Escherichia coli were separated by two-dimensional polyacrylamide gel electrophoresis, and the relative abundance of each of the 26 known tRNAs thus separated was measured on the basis of molecular numbers in cells. Based on this relative abundance, the distributions of cognate codons in E. coli genes (lacI, rpA, asnA, recA, lpp and four ribosomal protein genes) and in coliphage (MS2, φX174 and λ) genes were examined. A strong positive correlation between the tRNA abundance and the choice of codons, among both synonymous codons and those corresponding to different amino acids, was found for all E. coli protein genes that had been sequenced completely. However, the correlation was less significant for the phage genes. The relationship between tRNA abundance and its usage (namely anticodon usage) was examined by regression analysis. The degree of the relationship found for individual E. coli genes differed from gene to gene: those of r-protein genes and recA were higher than those of trpA, lacI and asnA. The dependent relationship of tRNA usage on its content for the first two genes seems to be greater than that expected from the proportional relationship between the two variables; i.e. these genes selectively use codons corresponding to major tRNAs but nearly avoid using those of minor tRNAs.     

Divergence of the Regulation of alpha-Amylase Activity in Drosophila melanogaster, Drosophila funebris, and Drosophila saltans

The regulation of amylase activity in threeDrosophila species, D. melanogaster,D. funebris and D. saltans, wasanalyzed by measuring the specific activity levels infour dietary environments, cornmeal, glucose, 5% starch, and 10% starch, at threedevelopmental stages, i.e., the third-instar larval,pupal, and 2-day-old adult stages. The developmentalprofiles of amylase activity for the threeDrosophila species showed that the level of activity washigh at the larval and adult stages but substantiallylow at the pupal stage, suggesting thatDrosophila does not utilize starch at the pupalstage. Divergence in the regulation of amylase was observed amongthe three Drosophila species on the followingpoints. (1) The order of amylase specific activity wasD. melanogaster > D. funebris >D. saltans. (2) The response pattern to the dietary environment varied amongthe species and changed during development. (3) Thetiming of the switch in the response pattern to thedietary environment during development was before pupation in D. funebris and D.saltans but after pupation in D.melanogaster. The significance of the divergence inthe regulation of amylase activity for adaptation to astarch environment in Drosophila is discussed.     

Drosophila Nonsense Suppressors: Functional Analysis in Saccharomyces Cerevisiae, Drosophila Tissue Culture Cells and Drosophila Melanogaster

Amber (UAG) and opal (UGA) nonsense suppressors were constructed by oligonucleotide site-directed mutagenesis of two Drosophila melanogaster leucine-tRNA genes and tested in yeast, Drosophila tissue culture cells and transformed flies. Suppression of a variety of amber and opal alleles occurs in yeast. In Drosophila tissue culture cells, the mutant tRNAs suppress hsp70:Adh (alcohol dehydrogenase) amber and opal alleles as well as an hsp70:β-gal (β-galactosidase) amber allele. The mutant tRNAs were also introduced into the Drosophila genome by P element-mediated transformation. No measurable suppression was seen in histochemical assays for Adh(n4) (amber), Adh(nB) (opal), or an amber allele of β-galactosidase. Low levels of suppression (approximately 0.1-0.5% of wild type) were detected using an hsp70:cat (chloramphenicol acetyltransferase) amber mutation. Dominant male sterility was consistently associated with the presence of the amber suppressors.