Within- and among-population variation in oviposition preference for urea-supplemented food in Drosophila melanogaster

Oviposition preference for ureasupplemented food was assayed by simultaneous choice trials on five pairs of closely related laboratory populations of Drosophila melanogaster.Each pair of populations had been derived from a separate ancestral population about 85 generations prior to this study. One population in each pair had been subjected to selection for larval tolerance to the toxic effects of urea; the other population served as a control. Considerable variation in oviposition preference was seen both within and among populations, with four of the ten populations showing a significant mean preference for ureasupplemented food. The degree of specificity shown by individual females was surprisingly high, leading to a bimodal distribution of oviposition preference in some populations. Overall, selection for larval tolerance to urea did not significantly affect oviposition preference. However, the data indicated that pairwise comparisons between randomly selected populations from the two larval selection regimes would lead to a range of possible outcomes, suggesting, in several cases, that selection for larval urea tolerance had led to significant differentiation of adult oviposition preference for urea in one or the other direction. The results, therefore, highlight the importance of population level replication and caution against the practice, common in ecological studies, of assaying oviposition preference in two populations that utilize different hosts in nature, and then drawing broad evolutionary inferences from the results

Short and long-term effects of environmental urea on fecundity in Drosophila melanogaster

Previous studies have shown that exposure to urea-supplemented food inhibited fecundity in Drosophila females, and that this inhibition was not expressed when females were given a choice between regular and urea-supplemented food as an oviposition substrate. We assayed fecundity, on both regular food and urea-supplemented food, at 5, 15 and 25 days post eclosion on females from ten laboratory populations of Drosophila melanogaster. The females assayed came from one of two treatments; they were maintained as adults on either regular or urea-supplemented food. We found that exposure to urea-supplemented food inhibited fecundity, relative to the levels exhibited on regular food, regardless of whether the urea was present in the assay medium, or in the medium on which the flies were maintained over the course of the experiment, thereby suggesting that urea has both a long-term (possibly physiological) as well as a short-term (possibly behavioural) inhibitory effect on fecundity of Drosophila females. We also tested and ruled out the hypothesis that prior yeasting could ameliorate the inhibitory effect of urea in the assay medium on fecundity, as this was a possible explanation of why flies given a choice between regular and urea-supplemented food did not exhibit a preference for regular food in a previous study

Activation of Steroid and Xenobiotic Receptor (SXR, NR1I2) and Its Orthologs in Laboratory Toxicologic, and Genome Model Species

Background: Nuclear receptor subfamily 1, group I, member 2 (NR1I2), commonly known as steroid xenobiotic receptor (SXR) in humans, is a key ligand-dependent transcription factor responsible for the regulation of xenobiotic, steroid, and bile acid metabolism. The ligand-binding domain is principally responsible for species-specific activation of NR1I2 in response to xenobiotic exposure. Objectives: Our objective in this study was to create a common framework for screening NR1I2 orthologs from a variety of model species against environmentally relevant xenobiotics and to evaluate the results in light of using the species as predictors of xenobiotic disposition and for assessment of environmental health risk. Methods: Sixteen chimeric fusion plasmid vectors expressing the Gal4 DNA-binding domain and species-specific NR1I2 ligand-binding domain were screened for activation against a spectrum of 27 xenobiotic compounds using a standardized cotransfection receptor activation assay. Results: NR1I2 orthologs were activated by various ligands in a dose-dependent manner. Closely related species show broadly similar patterns of activation; however, considerable variation to individual compounds exists, even among species varying in only a few amino acid residues. Conclusions: Interspecies variation in NR1I2 activation by various ligands can be screened through the use of in vitro NR1I2 activation assays and should be taken into account when choosing appropriate animal models for assessing environmental health risk

Molecular evolution of repetitive sequences: The histone genes of Drosophila virilis

A large fraction of the eukaryotic genome consists of repetitive sequences. Many times repetitive sequences undergo an evolutionary process known as concerted evolution. Concerted evolution is a non-independent evolution that results in greater similarity between paralogs as compared to orthologs of even closely related species. For example, the histone repeats of Drosophila virilis are a concertedly evolving gene family. This study elucidates the molecular evolution of the D. virilis historic repeats. The D. virilis histone repeats consist of quintets (consists of divergently transcribed H2A:H2B and divergently transcribed H3:H4 histone genes as well as an H1 histone gene) and quartets (same histone gene geometry as quintets minus H1). Both quintets and quartets exhibit extensive length variations, allowing for easy identification. Also, these repeats are in tandem at two non-allelic positions, the major locus and the minor locus. The major locus consists of approximately 30 repeats and the minor locus has 6 repeats. Using the advantages of the D. virilis histone gene family we show the following: First, low resolution sequence analysis of repeats from both loci look very similar. However, high resolution analysis determined that the major and minor loci are evolving on different trajectories. Second, there is a hypothesis of an inverse relationship between the size of an array and the amount of variation. We show this is not the case with this gene family. Third, the minor locus does not show a correlation between repeat distance and variation. That is, neighboring repeats are not more alike than distant repeats. Fourth, the minor locus is the ancestral locus. Fifth, the evolution of two loci and length variations are not a recent event specific to D. virilis, but common to at least five other species of the virilis group.

Short- and long-term effects of environmental urea on fecundity inDrosophila melanogaster

Previous studies have shown that exposure to urea-supplemented food inhibited fecundity in Drosophila females, and that this inhibition was not expressed when females were given a choice between regular and urea- supplemented food as an oviposition substrate. We assayed fecundity, on both regular food and urea-supplemented food, at 5, 15 and 25 days post eclosion on females from ten laboratory populations of Drosophila melanogaster. The females assayed came from one of two treatments; they were maintained as adults on either regular or urea-supplemented food. We found that exposure to urea-supplemented food inhibited fecundity, relative to the levels exhibited on regular food, regardless of whether the urea was present in the assay medium, or in the medium on which the flies were maintained over the course of the experiment, thereby suggesting that urea has both a long-term (possibly physiological) as well as a short-term (possibly behavioural) inhibitory effect on fecundity of Drosophila females. We also tested and ruled out the hypothesis that prior yeasting could ameliorate the inhibitory effect of urea in the assay medium on fecundity, as this was a possible explanation of wily flies given a choice between regular and urea- supplemented food did not exhibit a preference for regular food in a previous study

Activation of Steroid and Xenobiotic Receptor (SXR, NR1I2) and Its Orthologs in Laboratory, Toxicologic, and Genome Model Species

BACKGROUND: Nuclear receptor subfamily 1, group I, member 2 (NR1I2), commonly known as steroid and xenobiotic receptor (SXR) in humans, is a key ligand-dependent transcription factor responsible for the regulation of xenobiotic, steroid, and bile acid metabolism. The ligand-binding domain is principally responsible for species-specific activation of NR1I2 in response to xenobiotic exposure. OBJECTIVES: Our objective in this study was to create a common framework for screening NR1I2 orthologs from a variety of model species against environmentally relevant xenobiotics and to evaluate the results in light of using these species as predictors of xenobiotic disposition and for assessment of environmental health risk. METHODS: Sixteen chimeric fusion plasmid vectors expressing the Gal4 DNA-binding domain and species-specific NR1I2 ligand-binding domain were screened for activation against a spectrum of 27 xenobiotic compounds using a standardized cotransfection receptor activation assay. RESULTS: NR1I2 orthologs were activated by various ligands in a dose-dependent manner. Closely related species show broadly similar patterns of activation; however, considerable variation to individual compounds exists, even among species varying in only a few amino acid residues. CONCLUSIONS: Interspecies variation in NR1I2 activation by various ligands can be screened through the use of in vitro NR1I2 activation assays and should be taken into account when choosing appropriate animal models for assessing environmental health risk