The nonlinearity of offspring-parent regression for total sternopleural bristle number of Drosophila melanogaster

Summary A test of the linearity of the offspring-midparent regression for total sternopleural bristle number showed significant departure from linearity, with a tendency for higher heritability at higher values of the character. Offspring-midparent regression coefficients for each of four subdivisions of the total range of bristle number were not significantly different. The nonlinearity of offspring-midparent regression detected in this study may be due to either chance effects, genotypic asymmetry of genes with large effect, distribution asymmetry, or a combination of these factors. Further work is required to determine the extent and causes of nonlinearity of offspring-parent regression.     

Phenotypic plasticity and reaction norms of abdominal bristle number inDrosophila melanogaster

The phenotypic plasticity of abdominal bristle number (segments 3 and 4 in females) was investigated in 10 isofemale lines from a French population, grown at 7 constant temperatures, ranging from 12‡ to 31‡C. Overall concave reaction norms were obtained with a maximum around 20‡-21‡C. Intraclass correlation (isofemale line heritability) was not affected by temperature. Correlations between segments 3 and 4 strongly contrasted a low within-line phenotypic correlation (r = 0.39 ± 0.04) and a high, between-line genetic correlation (r = 0.89 ± 0.03). A significant decrease of the genetic correlation was observed when comparing more different temperatures. Finally, among 7 other morphometrical traits which were measured on the same set of lines, 3 provided a significant positive genetic correlation with abdominal bristles: thoracic bristles, abdomen pigmentation and thoracic pigmentation.     

High-resolution mapping of quantitative trait loci for sternopleural bristle number in Drosophila melanogaster.

We have mapped quantitative trait loci (QTL) harboring naturally occurring allelic variation for Drosophila bristle number. Lines with high (H) and low (L) sternopleural bristle number were derived by artificial selection from a large base population. Isogenic H and L sublines were extracted from the selection lines, and populations of X and third chromosome H/L recombinant isogenic lines were constructed in the homozygous low line background. The polymorphic cytological locations of roo transposable elements provided a dense molecular marker map with an average intermarker distance of 4.5 cM. Two X chromosome and six chromosome 3 QTL affecting response to selection for sternopleural bristle number and three X chromosome and three chromosome 3 QTL affecting correlated response in abdominal bristle number were detected using a composite interval mapping method. The average effects of bristle number QTL were moderately large, and some had sex-specific effects. Epistasis between QTL affecting sternopleural bristle number was common, and interaction effects were large. Many of the intervals containing bristle number QTL coincided with those mapped in previous studies. However, resolution of bristle number QTL to the level of genetic loci is not trivial, because the genomic regions containing bristle number QTL often did not contain obvious candidate loci, and results of quantitative complementation tests to mutations at candidate loci affecting adult bristle number were ambiguous.     

Effect of stressful and nonstressful growth temperatures on variation of sternopleural bristle number in Drosophila melanogaster

The effect of stressful (31 degrees C) and nonstressful (25 degrees C) growth temperatures on quantitative variation and developmental stability (fluctuating asymmetry) of Drosophila melanogaster was examined in a short-term selection experiment on sternopleural bristle number. Realized heritabilities based on 10 generations of selection in an upward direction did not differ between the two temperature regimes, which indicated that additive genetic variation was not affected by a high, stressful temperature. Phenotypic variability and fluctuating asymmetry of sternopleural bristles were significantly higher under stressful conditions when averaged over generations, although most pairwise comparisons in separate generations showed nonsignificant differences between temperatures.     

A study of canalization and developmental stability in the sternopleural bristle system of Drosophila melanogaster

Among the explanations for minimizing the effects of extraneous variation has been canalization and developmental stability. However, there is little agreement as to whether these two processes reflect a common set of mechanisms. This confusion is inflated due to the lack of consensus as to a precise definition of canalization. In this study, canalization in the sternopleural bristle system is used to investigate the relationships between measures of canalization and developmental stability by comparing how a panel of naturally derived lines responds to both genetic and environmental perturbations. No evidence for a common mechanism between the different measures of canalization was observed. Furthermore, a hypothesis regarding a common mechanism for environmental and genetic canalization was tested, and no evidence was found to support it. However, there is evidence for a relationship between at least one form of canalization and developmental stability.     

Phenotypic plasticity in morphological traits in two populations of Drosophila melanogaster

Isofemale lines of two populations of Drosophila melanogaster, originating from France and Tanzania, were examined over a range of temperatures. Morphological traits showed distinct patterns in phenotypic plasticity; flies of the two populations differed in shape. Genotype-by-Environment (G*E) interactions were frequently found in the Tanzania population, but were hardly present in the France population. If G*E interaction was present over temperature, estimates of additive genetic variance and additive genetic covariance were made to compare theoretical models with our data. The conclusion is that in France Drosophila melanogaster has been selected over a wider range of temperatures, resulting in parallel reaction norms of more optimal slope. In contrast, selection must have taken place over a narrower temperature range in Tanzanian flies, and will have exerted no direct influence on the slope of the reaction norm.     

Genomic differentiation between temperate and tropical Australian populations of Drosophila melanogaster

Determining the genetic basis of environmental adaptation is a central problem of evolutionary biology. This issue has been fruitfully addressed by examining genetic differentiation between populations that are recently separated and/or experience high rates of gene flow. A good example of this approach is the decades-long investigation of selection acting along latitudinal clines in Drosophila melanogaster. Here we use next-generation genome sequencing to reexamine the well-studied Australian D. melanogaster cline. We find evidence for extensive differentiation between temperate and tropical populations, with regulatory regions and unannotated regions showing particularly high levels of differentiation. Although the physical genomic scale of geographic differentiation is small--on the order of gene sized--we observed several larger highly differentiated regions. The region spanned by the cosmopolitan inversion polymorphism In(3R)P shows higher levels of differentiation, consistent with the major difference in allele frequencies of Standard and In(3R)P karyotypes in temperate vs. tropical Australian populations. Our analysis reveals evidence for spatially varying selection on a number of key biological processes, suggesting fundamental biological differences between flies from these two geographic regions.     

Disruptive selection for sternopleural bristle phenotypes in Drosophila ananassae

Disruptive selection is potentially critical in maintaining variation and initiating speciation and plays an important role in the organization of genetic variability in natural populations. It occurs when extreme phenotypes have a fitness advantage over intermediate phenotypes. Disruptive selection for high and low numbers of sternopleural bristles in Drosophila ananassae was applied for 12 generations to test its effect in induction of behavioural isolation. Pattern of mating between flies of high and low lines was tested in an Elens-Wattiaux mating chamber by using a multiple-choice technique after G(5) and G(12). Data was analyzed by calculating χ(2) under the assumption of random mating to test the difference between homoand heterogamic matings, and sexual isolation was tested by calculating the isolation estimate. The results show that there is no evidence for sexual isolation in G(5) and G(12). The realized heritability, standard error of regression coefficient, and t values suggest that disruptive selection for sternopleural bristle phenotypes was effective, but it does not lead to behavioral isolation in D. ananassae. Rather, it induces differences in mating propensity, which is influenced by sternopleural bristle phenotypes.     

Preference for ethanol in feeding and oviposition in temperate and tropical populations of Drosophila melanogaster

The natural habitat of Drosophila melanogaster Meigen (Diptera: Drosophilidae) is fermenting fruits, which can be rich in ethanol. For unknown reasons, temperate populations of this cosmopolitan species have higher ethanol resistance than tropical populations. To determine whether this difference is accompanied by a parallel difference in preference for ethanol, we compared two European and two tropical African populations in feeding and oviposition preference for ethanol‐supplemented medium. Although females of all populations laid significantly more eggs on medium with ethanol than on control medium, preference of European females for ethanol increased as ethanol concentration increased from 2 to 6%, whereas that of African females decreased. In feeding tests, African females preferred control medium over medium with 4% ethanol, whereas European females showed no preference. Males of all populations strongly preferred control medium. The combination of preference for ethanol in oviposition, and avoidance or neutrality in feeding, gives evidence that adults choose breeding sites with ethanol for the benefit of larvae, rather than for their own benefit. The stronger oviposition preference for ethanol of temperate than tropical females suggests that this benefit may be more important in temperate populations. Two possible benefits of ethanol for which there is some experimental evidence are cryoprotection and protection against natural enemies.     

Developmental thermal plasticity among Drosophila melanogaster populations

Many biotic and abiotic variables influence the dispersal and distribution of organisms. Temperature has a major role in determining these patterns because it changes daily, seasonally and spatially, and these fluctuations have a significant impact on an organism's behaviour and fitness. Most ecologically relevant phenotypes that are adaptive are also complex and thus they are influenced by many underlying loci that interact with the environment. In this study, we quantified the degree of thermal phenotypic plasticity within and among populations by measuring chill‐coma recovery times of lines reared from egg to adult at two different environmental temperatures. We used sixty genotypes from six natural populations of Drosophila melanogaster sampled along a latitudinal gradient in South America. We found significant variation in thermal plasticity both within and among populations. All populations exhibit a cold acclimation response, with flies reared at lower temperatures having increased resistance to cold. We tested a series of environmental parameters against the variation in population mean thermal plasticity and discovered the mean thermal plasticity was significantly correlated with altitude of origin of the population. Pairing our data with previous experiments on viability fitness assays in the same populations in fixed and variable environments suggests an adaptive role of this thermal plasticity in variable laboratory environments. Altogether, these data demonstrate abundant variation in adaptive thermal plasticity within and among populations.     

Reproductive strategy in Drosophila melanogaster: Significance of a genetic divergence between temperate and tropical populations

Summary Reproductive capacities of tropical and temperate populations of D. melanogaster were compared using three complementary techniques: (1) measure of egg production by females grown in the laboratory under uncrowded conditions and provided as adults with abundant food; (2) study of egg production of flies of unknown ages, collected in nature and then kept in similar conditions; and (3) analysis of ovarian activity of wild females dissected just after their capture.Tropical populations showed a lower fecundity in the laboratory and this was also observed in laboratory reared adults. On the average, flies also appeared to be older in the tropics than in temperate countries. These data, together with ecological observations showing that tropical populations live in a more predictable and stable environment, suggest that temperature populations are r-selected, while tropical ones are K-selected. The study of ovarian activity of wild females failed however to confirm this expectation. Tropical flies, which have a lower genetic fecundity, generally appeared to produce more propagules than did temperate flies. Such a contradiction shows how the ideas of r- and K-selection are difficult to apply to natural populations of Drosophila. Population density and interindividual competition are probably not the main selective forces in nature. Attention must also be paid to the necessity of exploring the environment to find resources, to the role of predation and parasitism, and to the occurrence in temperate countries of seasonal fluctuations with different selective pressures on successive generations.     

Selection for increased abdominal bristle number in Drosophila melanogaster with concurrent irradiation

Summary Replicate lines, each initially with one hundred pairs of parents selected at 50% intensity, were derived from the Canberra strain. In later generations population size was reduced and selection intensity increased. Three lines were selected without irradiation and five with 1000 r X-rays per generation for thirty generations. Selection was continued until generation 66. Long-term responses were similar in unirradiated and irradiated lines, and there was evidence that genes with large effects influenced response patterns in both groups of lines.This work was carried out while B.H. was on leave from the New South Wales Department of Agriculture, and held a Commonwealth Research Studentship. Robin Hall and Tricia Brown provided much appreciated technical assistance, and we are indebted to the Radio-Therapy Unit, Royal Prince Alfred Hospital, for the use of their equipment.     

The isolation of polygenic factors controlling bristle score in Drosophila melanogaster. I. Allocation of third chromosome sternopleural bristle effects to chromosome sections

A single third chromosome C, with a high sternopleural bristle score, had been extracted from an artificial selection line. C was divided into five chromosomal sections by recombination with a multiply marked third chromosome ruseca, which had a low sternopleural bristle score. A nonuniform distribution of sternopleural bristle effect with physical length of chromosome was observed. The second section (26-44 cM) of C carried the most sternopleural bristle effect (10 bristles when homozygous), the first (0-26 cM) and third (44-62 cM) also carried significant sternopleural bristle effects (six and five bristles, respectively). The fourth section (62-71 cM) carried a small but significant effect (less than one bristle) while the fifth section (71-101 cM) carried little effect when alone (less than one bristle), though it did carry effects which had an epistatic interaction with those of the first and second sections.     

Selection for increased abdominal bristle number in Drosophila melanogaster with concurrent irradiation

Summary Replicate lines, each, with one hundred pairs of parents selected at 50% intensity, were derived from an inbred line. For twenty generations three lines were selected without irradiation and five with 1000 r X-rays per generation given to both females and males. After adjustment for level of crowding in the cultures, the final mean was 1.3 bristles higher in females and 1.0 bristles higher in males in the irradiated lines than in the unirradiated lines. In terms of phenotypic standard deviations in each sex in the base population, these total responses were 0.74 and 0.59 respectively. Radiation can induce mutations useful in increasing responses in selection programmes, but the average response attributable to radiation is small, and heterogeneity between replicate lines is to be expected.This work was carried out while B. H. was on leave from the New South Wales Department of Agriculture, and held a Commonwealth Research Studentship. Our thanks to Robin Hall and Tricia Brown for technical assistance, to Dr. B. L. Sheldon, C.S.I.R.O. Division of Animal Genetics, for providing the inbred line, and to the Radio-Therapy Unit, Royal Prince Alfred Hospital, for the use of their equipment.     

Variance component analysis of bristle characters in local populations of Drosophila melanogaster.

The genetic variabilities of sternopleural and abdominal bristle numbers existing in local natural populations were assessed. Using second chromosome lines of Drosophila melanogaster extracted from three natural populations in Japan (the Ishigakijima, Ogasawara and Aomori populations), experiments were conducted to estimate the components of genetic variances, additive and dominance variances. The following results were obtained: For both sternopleural and abdominal bristle numbers, the additive genetic variances (sigma 2A) were much larger than the dominance variances (sigma 2D) especially in the southern populations. For example, in the Ishigakijima population, for females sternopleural bristle numbers of the inversion-free chromosome group, the additive and dominance variances were estimated to be 1.255 +/- 0.2034 and 0.0552 +/- 0.0180, respectively. The magnitudes of the estimates of additive genetic variances were nearly equal from north to south. By comparing the additive genetic variances of the inversion-free chromosome group with those of the In(2L)t-carrying chromosome group, it was inferred that sufficient number of generations to achieve the equilibrium state has not passed since the introduction of a single or a small number of the In(2L)t-carrying chromosomes to the Ishigakijima population.     

Polygenic mutation in Drosophila melanogaster: Mapping spontaneous mutations affecting sensory bristle number

Our ability to predict long-term responses to artificial and natural selection, and understand the mechanisms by which naturally occurring variation for quantitative traits is maintained, depends on detailed knowledge of the properties of spontaneous polygenic mutations, including the quantitative trait loci (QTL) at which mutations occur, mutation rates, and mutational effects. These parameters can be estimated by mapping QTL that cause divergence between mutation-accumulation lines that have been established from an inbred base population and selected for high and low trait values. Here, we have utilized quantitative complementation to deficiencies to map QTL at which spontaneous mutations affecting Drosophila abdominal and sternopleural bristle number have occurred in 11 replicate lines during 206 generations of divergent selection. Estimates of the numbers of mutations were consistent with diploid per-character mutation rates for bristle traits of 0.03. The ratio of the per-character mutation rate to total mutation rate (0.023) implies that >2% of the genome could affect just one bristle trait and that there must be extensive pleiotropy for quantitative phenotypes. The estimated mutational effects were not, however, additive and exhibited dependency on genetic background consistent with diminishing epistasis. However, these inferences must be tempered by the potential for epistatic interactions between spontaneous mutations and QTL affecting bristle number on the deficiency-bearing chromosomes, which could lead to overestimates in numbers of QTL and inaccurate inference of gene action.     

Sexual dimorphism of body size and sternopleural bristle number: a comparison of geographic populations of an invasive cosmopolitan drosophilid

Zaprionus indianus is a cosmopolitan drosophilid, of Afrotropical origin, which has recently colonized South America. The sexual dimorphism (SD) of body size is low, males being almost as big as females. We investigated 10 natural populations, 5 from America and 5 from Africa, using the isofemale line technique. Three traits were measured on each fly: wing and thorax length and sternopleural bristle number. Two indices of SD were compared, and found to be highly correlated (r > 0.99). For the sake of simplicity, only the female/male (F/M) ratio was further considered. A significant genetic variability of SD was found in all cases, although with a low heritability (intra-class correlation of 0.13), about half the value found for the traits themselves. For size SD, we did not find any variation among continents or any latitudinal trend, and average values were 1.02 for wing length and 1.01 for thorax length. Bristle number SD was much greater (1.07). Among mass laboratory strains, SD was genetically much more variable than in recently collected populations, a likely consequence of laboratory drift. Altogether, SD, although genetically variable and prone to laboratory drift, is independent of size variations and presumably submitted to a stabilizing selection in nature.     

The tumorous-head-1 locus affects bristle number of the Drosophila melanogaster cuticle.

The tuh-1 maternal effect locus contains two naturally occurring isoalleles, tuh-1h and tuh-1g. Until recently there has been no possibility to distinguish between the tuh-lh and the tuh-1g maternal effects other than evaluating their effect on the Bithorax-Complex (BXC) Abdominal B (Abd-B) mutant tuh-3. However, in this report we identify a bristle phenotype associated with the tuh-1 locus that has very interesting evolutionary implications. Females homozygous for tuh-1h always produce adult offspring with more bristles than females homozygous or heterozygous for tuh-1g. The effect is global. Increased bristle number occurs in the head, the thorax, and the anterior and posterior abdomen. Females totally deficient for the tuh-1 gene produce offspring with high bristle number. Thus, the bristle phenotype results from the absence of the maternally contributed tuh-1g factor. Genetic evidence shows that the bristle phenotype is caused by the tuh-1 locus and that tuh-1h is completely recessive to tuh-1g. The tuh-1 locus is located at the euchromatin-beta-heterochromatin junction near the centromere of the X chromosome and deficiency analysis places the locus between the lethal genes extra organs (eo) and lethal B20 (lB20). The variance in bristle number attributable to the tuh-1 locus in nature is approximately 10.1%, an indication that the bristle phenotype is most likely a neutral, pleiotrophic side effect of tuh-1.