Relationship between chromosomal polymorphism and wing size in a natural population of Drosophila subobscura

Chromosomal polymorphism and wing size (as a measure of body size) were analysed simultaneously in two samples of Drosophila subobscura from Barcelona, Spain. The very rich chromosomal polymorphism of this species makes it difficult to detect the relationship of this polymorphism with any phenotypical character. However, a positive significant regression of wing size on the percentage of the autosome length with standard arrangement was found. Furthermore, for each polymorphic chromosome, except for the J chromosome, an association between the most frequent arrangements and wing size was observed. This trend, which was the same in the two samples, was that expected according to the latitudinal clines of both characters.     

Continuing studies on chromosomal polymorphism in a Korean natural population of Drosophila melanogaster

Investigations on the chromosomal inversion polymorphism were conducted on a Korean (Taenung) natural population of D. melanogaster during the period 1978 to 1992. A total of 66 different endemic and cosmopolitan inversions were found on both major chromosome pairs II and III. Some of them proved to be rare cosmopolitan types (2LKA, 2LNS, 2LF, 2RCy, 3LM, 3RKI, and 3RK), while others were endemics. The distribution of breakpoints for endemic and rare cosmopolitan inversions are not random along the two autosome arms.With respect to frequency changes, the 15-year survey revealed that five of the cosmopolitan types (2Lt, 2RNS, 3LP, 3RC, and 3RMo) exhibit cyclical frequency changes, whereas gene arrangement 3RP shows relatively stable frequencies. Tests for correlations between gene arrangement frequencies and several climatic variables gave no clear evidence for such relationships. Only one correlation coefficient out of 64 was statistically significant. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of adult experience on oviposition choice and short-distance attraction inDrosophila buzzatii

In a series of experiments, no consistent effect of adult experience, i.e., exposure to the naturally occurring yeasts, Candida sonorensisand Clavispora opuntiae,on oviposition choice or short-distance attraction in inbred lines of Drosophila buzzatiiwas found. The lack of consistent effect on oviposition choice was also found in one experiment in which the flies were starved and in another experiment in which choice was determined on 2 consecutive days.     

Sequential gel electrophoretic analysis of esterase-2 in two populations of Drosophila buzzatii

Sequential electrophoresis, using three different buffer systems on cellulose acetate gels, was used to characterize the allelic variation for esterase-2 in two populations of D. buzzatii in Australia that are separated by 550 km. Twenty-five alleles were detected, of which nine were unique to one population, eight unique to the other, and only eight were common to both populations. Allele frequencies within each population were significantly different between the two major chromosome sequences (standard and j inversion), and for each chromosome sequence allele frequencies were significantly different between populations. Observed allelic frequency distributions were not significantly different from those predicted for selective neutrality using the homozygosity test statistic. However, estimates of the effective sizes of the populations derived from their observed differentiation, together with the history of the species in Australia, provide support for some form of balancing selection affecting at least some of the alleles.     

Differing amounts of genetic polymorphism in testes and male accessory glands ofDrosophila melanogaster andDrosophila simulans

We surveyed genetic polymorphism by two-dimensional gel electrophoresis of male reproductive tract proteins in 20 isofemale lines each ofDrosophila melanogaster andDrosophila simulans. After classifying 244 such proteins ofDrosophila melanogaster and 271 ofDrosophila simulans by their distribution between testes and accessory glands within the reproductive tract, significant correlations were found between genetic polymorphism and tissue distribution. In both species, gland-specific proteins were significantly more polymorphic than testis-specific proteins, as well as those found in both testes and glands. Simultaneously, inDrosophila simulans, proteins found in roughly equivalent relative abundance in both testes and glands were significantly less variable than gland-specific and testis-specific proteins, as well as those with a quantitative difference in relative abundance between testes and glands. These correlations may reflect general differences in variability between extracellular and intracellular proteins and between proteins with broad as opposed to tissue-specific distributions.We thank the Natural Sciences and Engineering Research Council of Canada for financial support (Grant A0235 to R.S.S.).     

Allozyme genotype-environment relationships in natural populations of Drosophila buzzatii

Allozyme frequency data from natural populations of Drosophila buzzatii were analyzed for genotype-environment relationships. Allele frequency and heterozygosity at six loci polymorphic throughout eastern Australia and a number of environmental factors (both means and variabilities) were examined by a variety of multivariate techniques. Significant genotype-environment associations were found for five of the six loci, and after correcting for geographic location significant associations remained for Est-2 and Adh-1 gene frequencies and heterozygosities and for Pgm gene frequencies. The results are discussed in relation to selection and gene flow and provide the basis for laboratory studies to disentangle confounded effects of (1) environmental means and environmental variabilities and (2) allele frequency and heterozygosity, and thus to further test for and determine the nature of any natural selection at particular allozyme loci.This work was supported by a grant from the Australian Research Grants Committee to J. S. F. B.     

Correlated responses to selection for stress resistance and longevity in a laboratory population of Drosophila melanogaster

Laboratory studies on Drosophila have revealed that resistance to one environmental stress often correlates with resistance to other stresses. There is also evidence on genetic correlations between stress resistance, longevity and other fitness-related traits. The present work investigates these associations using artificial selection in Drosophila melanogaster. Adult flies were selected for increased survival after severe cold, heat, desiccation and starvation stresses as well as increased heat-knockdown time and lifespan (CS, HS, DS, SS, KS and LS line sets, respectively). The number of selection generations was 11 for LS, 27 for SS and 21 for other lines, with selection intensity being around 0.80. For each set of lines, the five stress-resistance parameters mentioned above as well as longevity (in a nonstressful environment) were estimated. In addition, preadult developmental time, early age productivity and thorax length were examined in all lines reared under nonstressful conditions. Comparing the selection lines with unselected control revealed clear-cut direct selection responses for the stress-resistance traits. Starvation resistance increased as correlated response in all sets of selection lines, with the exception of HS. Positive correlated responses were also found for survival after cold shock (HS and DS) and heat shock (KS and DS). With regard to values of resistance across different stress assays, the HS and KS lines were most similar. The resistance values of the SS lines were close to those of the LS lines and tended to be the lowest among all selection lines. Developmental time was extended in the SS and KS lines, whereas the LS lines showed a reduction in thorax length. The results indicate a possibility of different multiple-stress-resistance mechanisms for the examined traits and fitness costs associated with stress resistance and longevity.     

Altitudinal patterns for longevity, fecundity and senescence in Drosophila buzzatii

We tested for variation in longevity, senescence rate and early fecundity of Drosophila buzzatii along an elevational transect in Argentina, using laboratory-reared flies in laboratory tests performed to avoid extrinsic mortality. At 25 °C, females from lowland populations lived longer and had a lower demographic rate of senescence than females from highland populations. Minimal instead of maximal temperature at the sites of origin of population best predicted this cline. A very different pattern was found at higher test temperature. At 29.5 °C, longevity of males increased with altitude of origin of population. No clinal trend was apparent for longevity of females at 29.5 °C. There was evidence for a trade-off between early fecundity and longevity at non-stressful temperature (25 °C) along the altitudinal gradient. This trait association is consistent with evolutionary theories of aging. Population-by-temperature and sex-by-temperature interactions indicate that senescence patterns are expressed in environment specific ways.     

Courtship success and multivariate analysis of sexual selection on morphometric traits inDrosophila buzzatii (Diptera: Drosophilidae)

Using wild-reared flies, we examined sexual selection on five phenotypic traits (thorax length, wing length, wing width, head width, and face width) inDrosophila buzzatii, by scoring copulatory status in nine mass mating cages. Only male face width was identified as a direct target of sexual selection in an analysis of selection gradient, while indirect selection was present on all other studied traits, as expected from their correlations with face width. In contrast to males, there was no indication of selection in females. Nor was there evidence of assortative mating. The suggested direct selection on face width seems to take place during licking behavior of the courtship and might be related to courtship feeding. This study suggests that courtship success gives rise to indirect selection on body size.     

Perturbation of gene frequencies in a natural population of Drosophila melanogaster: evidence for selection at the Adh locus

The cellar population of Drosophila melanogaster at the Chateau Tahbilk Winery (Victoria, Australia) was perturbed for alcohol dehydrogenase (Adh) gene frequencies. Phenol oxidase (Phox) frequencies were also perturbed and monitored as a control. Subsequent gene frequency changes, together with information on population structure, indicated that selection acted on the chromosome regions of both loci. Adh gene frequencies returned to preperturbation levels in a predictable manner. A model in which the relative fitness of Adh phenotypes was determined by temperature-dependent specific activities of enzymes of Adh genotypes adequately accounts for the rate of gene frequency change at this locus. Thus temperature behaves as a selective agent in modulating Adh gene frequencies in this cellar environment.     

Evolution of a new chromosomal lineage in a laboratory population ofDrosophila through centric fission

Structural rearrangements of chromosomes have played a decisive role in the karyotypic evolution of species. It is also known that inversions, translocations, fusions, fissions, heterochromatin variations and other chromosomal changes occur as transient events in natural populations. Herein we report the occurrence of a rare event of centric fission of a metacentric chromosome in a laboratory population ofDrosophila, called Cytorace 1. This centric fission has been fixed in a sub-population of Cytorace 1, resulting in a new chromosomal lineage called Fissioncytorace-1.     

Efficiency of selection, as measured by single nucleotide polymorphism variation, is dependent on inbreeding rate in Drosophila melanogaster

It is often hypothesized that slow inbreeding causes less inbreeding depression than fast inbreeding at the same absolute level of inbreeding. Possible explanations for this phenomenon include the more efficient purging of deleterious alleles and more efficient selection for heterozygote individuals during slow, when compared with fast, inbreeding. We studied the impact of inbreeding rate on the loss of heterozygosity and on morphological traits in Drosophila melanogaster. We analysed five noninbred control lines, 10 fast inbred lines and 10 slow inbred lines; the inbred lines all had an expected inbreeding coefficient of approximately 0.25. Forty single nucleotide polymorphisms in DNA coding regions were genotyped, and we measured the size and shape of wings and counted the number of sternopleural bristles on the genotyped individuals. We found a significantly higher level of genetic variation in the slow inbred lines than in the fast inbred lines. This higher genetic variation was resulting from a large contribution from a few loci and a smaller effect from several loci. We attributed the increased heterozygosity in the slow inbred lines to the favouring of heterozygous individuals over homozygous individuals by natural selection, either by associative over‐dominance or balancing selection, or a combination of both. Furthermore, we found a significant polynomial correlation between genetic variance and wing size and shape in the fast inbred lines. This was caused by a greater number of homozygous individuals among the fast inbred lines with small, narrow wings, which indicated inbreeding depression. Our results demonstrated that the same amount of inbreeding can have different effects on genetic variance depending on the inbreeding rate, with slow inbreeding leading to higher genetic variance than fast inbreeding. These results increase our understanding of the genetic basis of the common observation that slow inbred lines express less inbreeding depression than fast inbred lines. In addition, this has more general implications for the importance of selection in maintaining genetic variation.     

Natural selection on ecophysiological traits of a fern species in a temperate rainforest

Unlike other species of the genus Blechnum, the fern Blechnum chilense occurs in a wide range of habitats in Chilean temperate rainforest, from shaded forest understories to abandoned clearings and large gaps. We asked if contrasting light environments can exert differential selection on ecophysiological traits of B. chilense. We measured phenotypic selection on functional traits related to carbon gain: photosynthetic capacity (A _max), dark respiration rate (R _d), water use efficiency (WUE), leaf size and leaf thickness in populations growing in gaps and understorey environments. We assessed survival until reproductive stage and fecundity (sporangia production) as fitness components. In order to determine the potential evolutionary response of traits under selection, we estimated the genetic variation of these traits from clonally propagated individuals in common garden experiments. In gaps, survival of B. chilense was positively correlated with WUE and negatively correlated with leaf size. In contrast, survival in shaded understories was positively correlated with leaf size. We found positive directional fecundity selection on WUE in gaps population. In understories, ferns of lower R _d and greater leaf size showed greater fecundity. Thus, whereas control of water loss was optimized in gaps, light capture and net carbon balance were optimized in shaded understories. We found a significant genetic component of variation in WUE, R _d and leaf size. This study shows the potential for evolutionary responses to heterogeneous light environments in functional traits of B. chilense, a unique fern species able to occupy a broad successional niche in Chilean temperate rainforest.     

Effects on ADH activity and distribution,following selection for tolerance to ethanol in Drosophila melanogaster

Strains of Drosophila melanogaster homozygous for either the Adh^F or the Adh^S allele were kept on food supplemented with ethanol for 20 generations. These strains (FE and SE) were tested for tolerance to ethanol and compared with control strains (FN and SN). The E strains showed increased tolerance to ethanol both in the adult and in the juvenile life stages. In adults the increase in tolerance was not accompanied by an increase in overall ADH activity. However, there were changes in the distribution of ADH over the body parts. Flies of the FE strain possessed significantly more ADH in the abdomen, compared with FN. Another set of FN and SN populations were started both on standard food and on ethanol food with reduced yeast concentrations. After 9 months ADH activities were determined in flies from these populations which had been placed on three different media: the food the populations had been kept on, regular food and regular food supplemented with ethanol. The phenotypic effects of yeast reduction on ADH activity were considerably, but longterm genetic effects were limited.     

The contribution of ancestry,chance, and past and ongoing selection to adaptive evolution

The relative contributions of ancestry, chance, and past and ongoing election to variation in one adaptive (larval feeding rate) and one seemingly nonadaptive (pupation height) trait were determined in populations ofDrosophila melanogaster adapting to either low or high larval densities in the laboratory. Larval feeding rates increased rapidly in response to high density, and the effects of ancestry, past selection and chance were ameliorated by ongoing selection within 15–20 generations. Similarly, in populations previously kept at high larval density, and then switched to low larval density, the decline of larval feeding rate to ancestral levels was rapid (15-20 generations) and complete, providing support for a previously stated hypothesis regarding the costs of faster feeding inDrosophila larvae. Variation among individuals was the major contributor to variation in pupation height, a trait that would superficially appear to be nonadaptive in the environmental context of the populations used in this study because it did not diverge between sets of populations kept at low versus high larval density for many generations. However, the degree of divergence among populations (FST) for pupation height was significantly less than expected for a selectively neutral trait, and we integrate results from previous studies to suggest that the variation for pupation height among populations is constrained by stabilizing selection, with a flat, plateau-like fitness function that, consequently, allows for substantial phenotypic variation within populations. Our results support the view that the genetic imprints of history (ancestry and past selection) in outbreeding sexual populations are typically likely to be transient in the face of ongoing selection and recombination. The results also illustrate the heuristic point that different forms of selection-for example directional versus stabilizing selection—acting on a trait in different populations may often not be due to differently shaped fitness functions, but rather due to differences in how the fitness function maps onto the actual distribution of phenotypes in a given population. We discuss these results in the light of previous work on reverse evolution, and the role of ancestry, chance, and past and ongoing selection in adaptive evolution.     

Dynamics of mating success in experimental groups ofDrosophila melanogaster (Diptera: Drosophilidae)

Determinants of male courtship success in Drosophila melanogasterwere examined in groups of five males sequentially presented with five individual females. Thirty-three percent of males never mated, while approximately half of the males mated two or three times. Rapid courtship initiation was associated with male success in early matings only. Male size was important for courtship outcome, but the size distributions of mating and nonmating males and their progeny numbers indicate balancing rather than directional selection on size- dependent courtship success.     

Structural and numerical chromosomal polymorphism in natural populations ofAlopecurus (Poaceae)

Considerable structural and numerical chromosomal variation has been found in natural populations ofAlopecurus. Interchange heterozygotes, identified by multivalent formation during meiosis, have been recovered in four out of six species studied and are reported for the first time in the diploidsA. bulbosus, andA. myosuroides, and the tetraploidA. pratensis. B chromosomes have been found in two species,A. pratensis andA. myosuroides and also in inter-specific hybrids betweenA. pratensis andA. arundinaceus. The characteristics, distribution and meiotic behaviour of both interchange heterozygotes and B chromosomes are described.     

Factors affecting breeding habitat selection in a cliff-nesting peregrine Falco peregrinus population

We studied factors affecting breeding habitat selection in a population of cliff-nesting peregrines Falco peregrinus across multiple spatial levels (cliff site, cliff context and land-use of the surrounding landscape), over a 2,100 km² study area in the Alps (Italy and Switzerland). We detected 30 breeding pairs (density: 1.43 territorial pairs/100 km²), whose territories were uniformly distributed over the study area. We compared 15 habitat features characterising occupied cliffs and 30 randomly selected unoccupied cliffs by means of stepwise forward logistic regression and hierarchical partitioning. The logistic regression analysis showed that occupied cliffs were longer (horizontal length), steeper, and had a greater extension of urban areas in the surrounding landscape compared to non-occupied cliffs. The model had a discrimination ability of 0.95. Hierarchical partitioning indicated that the logistic regression model was appropriate. Neither the distance of cliffs to the nearest site occupied by a potential competitor and predator, the eagle owl Bubo bubo, which occurred at a low density (0.67 pairs/100 km²), nor the distance to the nearest site occupied by peregrines had any effect on the cliff suitability model. Therefore, habitat selection in cliff-nesting peregrines was mainly influenced by cliff site features and surrounding landscape characteristics, whereas the proximity to conspecifics and to an intraguild predator had no apparent effects on peregrine settlement in our study population.     

Electrophoretic and heat-stability polymorphism at the phosphoglucomutase (Pgm) locus in natural populations of Drosophila melanogaster

Genetic polymorphism for electrophoretic and heat-sensitive alleles is known at the phosphoglucomutase (Pgm) locus in Drosophila melanogaster. Analysis of the distribution of electrophoretic and thermosensitive (ts) alleles was carried out in natural populations from Canada and West Africa and compared with already known data on Italian populations [Trippa, G., Loverre, A., and Catamo, A. (1976). Nature 260:42]. The data show the existence of five common alleles, Pgm ^1.00,tr, Pgm ^1,00,ts, Pgm ^0.70,ts, Pgm ^1.20,ts, and Pgm ^1.50,tr, and two rare alleles, Pgm ^0.55,ts and Pgm ^1.20,tr. The most frequent allele is always Pgm ^1.00,tr; the second most common allele is always of the ts type. The cumulated frequencies of ts alleles in the populations varies between 11 and 32%. The heat stability polymorphism is present in all populations examined and shows again the uniform geographic pattern that has been found for electrophoretic variation at this locus.This research was partially supported by an operating grant (to G.R.C.) from the Canadian National Science and Engineering Research Council (NSERC).     

Patterns of puffing activity and chromosomal polymorphism in Drosophila subobscura

Summary The effect of inbreeding on the puffing patterns of polytene chromosomes of Drosophila subobscura was analysed. Puffing activity was studied in two strains of D. subobscura: one which had been subject to inbreeding for 288 generations, and in the hybrids from a cross between them. A strong overall decrease in puffing activity was found in the inbred line. In general, hybrids behaved in a similar way to the inbred line or showed activity intermediate between the two lines. The fertility and viability of the two homozygous lines and of the hybrids were also determined. These parameters of fitness are very low in the inbred line. Hybrids displayed intermediate behaviour.Publication No. 109, Departamento de Genética, Universidad de Valencia, Spain