Effect of thermal environment on life histories of free living Drosophila melanogaster and D. subobscura

The variability of morphological and lifehistory traits in Drosophila melanogaster and D. subobscura, two sympatric Drosophila species of different climatic origin, were investigated with regard to seasonal and daily temperature and humidity fluctuations to assess thermal sensitivity and the responses of freeliving organisms to oscillating temperatures. Temperature and humidity were measured continuously at the site where Drosophila were observed throughout the day, and thus represent a realistic picture of the temperature environment of these animals. A phenotypic model (gaussian curve) of temperature performance was fitted to daily fecundities and provided estimates of the position of temperature optimum and of the maximum and the breadth of performance for both species. D. melanogaster had a higher optimum temperature than D. subobscura. A trade-off between the maximum and the breadth of performance was detected both within and between the species.     

Ectoparasitic mites and their Drosophila hosts

Only two parasite interactions are known for Drosophila to date: Allantonematid nematodes associated with mycophagous Drosophilids and the ectoparasitic mite Macrocheles subbadius with the Sonoran Desert endemic Drosophila nigrospiracula. Unlike the nematode-Drosophila association, breadth of mite parasitism on Drosophila species is unknown. As M. subbadius is a generalist, parasitism of additional Drosophilids is expected. We determined the extent and distribution of mite parasitism in nature Drosophilids collected in Mexico and southern California. Thirteen additional species of Drosophilids were infested. Interestingly, 10 belong to the repleta species group of the subgenus Drosophila, despite the fact that the majority of flies collected were of the subgenus Sophophora. In all cases but 2, the associated mites were M. subbadius. Drosophila hexastigma was found to have not only M. subbadius, but another Mesostigmatid mite, Paragarmania bakeri, as well. One D. hydei was also found to have a mite from genus Lasioseius attached. In both choice and no-choice experiments, mites were more attracted to repleta group species than to Sophophoran. The extent of mite parasitism clearly is much broader than previously reported and suggests a host bias mediated either by mite preference and/or some mechanism of resistance in particular Drosophilid lineages.     

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.     

Gene activity of polytene chromosomes in Drosophila species of the obscura group

The polytene chromosome puffing patterns of Drosophila guanche were established and compared with those of Drosophila subobscura. A total of 150 loci, active in some of the 17 developmental stages studied, were described and 23 of them were found to form the characteristic puffing pattern of D. guanche. Taking into account the number of puffs as well as the gene activity of each chromosome and the total gene activity, D. guanche seems to be less active than D. subobscura. Although both species show a degree of homology in their puffing patterns lower than that found for sibling species, the degree of homology is stronger than that between species belonging to the same group but to different subgroups. Thus, D. guanche and D. subobscura must be considered as phylogenetically closely related species, belonging to the same subgroup.     

Gypsy homologous sequences inDrosophila subobscura (gypsyDS)

Summary Characterization of sequences homologous to theDrosophila melanogaster gypsy transposable element was carried out inDrosophila subobscura (gypsyDS). They were found to be widely distributed among natural populations of this species. From Southern blot and in situ analyses, these sequences appear to be mobile in this species.GypsyDS sequences are located in both euchromatic and heterochromatic regions. A completegypsyDS sequence was isolated from aD. subobscura genomic library, and a 1.3-kb fragment which aligns with the ORF2 of theD. melanogaster gypsy element was sequenced. Comparisons of this sequence in three species (D. subobscura, D. melanogaster, and D. virilis) indicate that there is greater similarity between theD. subobscura-D. virilis sequences than betweenD. subobscura andD. melanogaster. Molecular divergence ofgypsy sequences betweenD. virilis andD. subobscura is estimated at 16 MY, whereas the most likely divergence time of these two species is more than 60 MY. These data strongly suggest thatgypsy sequences have been horizontally transferred between these species.Offprint requests to: T.M. Alberola     

THE DETECTION OF GAMETIC DISEQUILIBRIUM BETWEEN ALLOZYME LOCI IN NATURAL POPULATIONS OF DROSOPHILA

The capacity of the usual tests (chi-square and related tests) to detect gametic disequilibrium between allozyme loci in natural populations of Drosophila has been investigated. We analyzed a large collection of previously reported gametic samples from natural populations involving a variety of loosely linked allozyme loci located along the O chromosome of Drosophila subobscura and the second chromosome of D. melanogaster. It is found that the statistical power of the individual tests to detect the sample disequilibria between allozyme loci is remarkably low, being the average (over pairs of loci) of power estimates close to 0.20 in both species. Moreover, the average minimum disequilibrium (D‘_min) that would be required to reject (90% probability) the hypothesis of gametic equilibrium is higher than 0.50 given the observed degree of polymorphism and sample sizes used. This means that statistically significant associations between allozyme loci would rarely be detected by single-sample tests even when much disequilibrium is present in natural populations of Drosophila. However, an alternative approach based on the analysis of disquilibrium for large sets of gametic samples, combining probabilities from single independent tests and assessing significance by a bootstrap procedure, reveals that most of the locus pairs within segment I and II of the O chromosome of D. subobscura and left arm of the second chromosome of D. melanogaster present significant nonrandom associations. Within these chromosomal sections, the observed average absolute value of disquilibrium (D‘) between loci is around 0.25 (under the more conservative estimation). Also, a positive relationship between the magnitude of disequilibrium and linkage was detected. These findings suggest that weak or moderate values of disequilibrium between loosely linked allozyme loci are more frequent in natural populations of Drosophila than is currently believed.     

Sex‐ratio in natural and experimental populations of Drosophila subobscura from North America

The sex‐ratio (SR), defined as the proportion of males, has been studied in three North American colonizing populations of Drosophila subobscura (Eureka, Davis and Gilroy). The proportion of sexes under laboratory conditions was studied using the one‐generation serial transfer technique in one‐ and two‐species populations, to infer whether biased SR affects the outcome when competing with Drosophila pseudoobscura, another member of the same group now in sympatry with D. subobscura in North America. The wild samples of D. subobscura yielded a significantly higher number of males than females during those months where the species is more abundant. However, there was no significant deviation in the 1 : 1 proportion of sexes in the descendants of D. subobscura at any of the experimental conditions. On the contrary, D. pseudoobscura produced a higher proportion of females which could be responsible for the exclusion of D. subobscura in laboratory competition experiments with overlapping generations. Thus, if sexes are equal at birth and survival is similar, the preponderance of males of D. subobscura in our wild collections could indicate greater activity and probably greater chance of dispersal of males versus females especially under favourable conditions.     

Multilocus analysis of nucleotide variation in Drosophila madeirensis, an endemic species of the Laurisilva forest in Madeira

Drosophila madeirensis is an endemic species of Madeira that inhabits the island Laurisilva forest. Nucleotide variation in D. madeirensis is analysed in six genomic regions and compared to that previously reported for the same regions in Drosophila subobscura, an abundant species in the Palearctic region that is closely related to D. madeirensis. The gene regions analysed are distributed along the O₃ inversion. The O₃ arrangement is monomorphic in D. madeirensis, and it was present in ancestral populations of D. subobscura but went extinct in this species after the origin of the derived O_ST and O₃₊₄ arrangements. Levels of nucleotide polymorphism in D. madeirensis are similar to those present in the O_ST and O₃₊₄ arrangements of D. subobscura, and the frequency spectrum is skewed towards rare variants. Purifying selection against deleterious nonsynonymous mutations is less effective in D. madeirensis. Although D. madeirensis and D. subobscura coexist at present in Madeira, no clear evidence of introgression was detected in the studied regions.     

INTERSPECIFIC LABORATORY COMPETITION OF THE RECENTLY SYMPATRIC SPECIES DROSOPHILA SUBOBSCURA AND DROSOPHILA PSEUDOOBSCURA

Abstract.—Drosophila subobscura and D. pseudoobscura are closely related species coexisting on the West Coast of North America, which was recently colonized by D. subobscura. In competition experiments with overlapping generations, D. subobscura is eliminated by D. pseudoobscura in a few generations at all four temperatures and two initial frequencies tested. Yet in one-species cultures, D. subobscura thrives at all experimental conditions. Single-generation competition experiments reveal lower survivorship and productivity of D. subobscura at all temperatures and frequencies. Productivity per female is dependent on the initial frequencies: greater for D. subobscura as its initial frequency becomes higher, but lower for D. pseudoobscura as its frequency becomes higher. Strains of D. subobscura from three disparate geographic origins yield similar results.     

Monitoring chromosomal polymorphism in Drosophila subobscura over 40 years

The inversion chromosomal polymorphism of Drosophila subobscura is considered to be adaptive as a result of its responses at different time scales to temperature changes. This work reports the longest‐term study of chromosomal polymorphism for a single population of D. subobscura with climatic data from the collecting site itself. The chromosomal analysis of D. subobscura samples collected six times over a 40‐year period at the same location and in the same seasonal interval has revealed the continuous presence of 16 common and six moderately rare chromosomal arrangements through the period. This analysis also corroborates the previously detected negative relationship between the frequencies of the standard (cold‐climate) arrangement on each of its five chromosomes and temperature, as well as between a comprehensive measure of cold adaptation (the total autosomal proportion of standard arrangement) and temperature. These and previous results would support that species harboring cold‐ and warm‐adapted polymorphic chromosomal arrangements, like D. subobscura, can rapidly respond to environmental changes.     

DNA variation at the rp49 gene region in Drosophila madeirensis and D. subobscura from Madeira: inferences about the origin of an insular endemic species

An ~1.6 ‐ kb fragment spanning the rp49 gene was sequenced in 16 lines of Drosophila subobscura from Madeira and in 22 lines of the endemic species D. madeirensis. Nucleotide diversity in D. subobscura from Madeira (π=0.0081) was similar to that in lines from Spain carrying the O₃₊₄ chromosomal arrangement (π=0.0080). No significant genetic differentiation was detected between insular and continental O₃₊₄ lines of D. subobscura. These results are compatible both with a rather recent and massive colonization, and with multiple colonization events from the continent. Nucleotide diversity in D. madeirensis (π=0.0076) was similar to that in D. subobscura, which deviates from the expectation, under strict neutrality, of a lower level of variation in an insular species with a small population size. The observed numbers of shared polymorphisms and of fixed differences between D. madeirensis and D. subobscura are compatible with the isolation model of speciation, where shared polymorphisms are due to common ancestry.     

Lack of seasonal changes in mitochondrial DNA variability of a Drosophila subobscura population

Restriction site analysis of mtDNA of 550 isofemale lines corresponding to different seasonal samples of a single geographic population of Drosophila subobscura was carried out. The distribution pattern of haplotypes was similar to that observed for the entire range of the species on the European continent: two haplotypes were equally and highly frequent, and a set of sporadic haplotypes were almost never present in more than one seasonal sampling. No statistically significant evidence was found for between-population heterogeneity across time, and the mean within-population variation was similar to other mtDNA restriction site analyses previously reported for D. subobscura populations. These observations could be interpreted in terms of the neutral mutation hypothesis where the entire population has not yet reached an equilibrium. The causes of this non-equilibrium are most likely periodic winter bottlenecks, phenomena that particularly affect the distribution of mtDNA haplotypes across time.     

Studies of the species barrier between Drosophila subobscura and D. madeirensis IV. A genetic dissection of the X chromosome for speciation genes

The genetics of four traits contributing to the isolation mechanism between the two closely related species of Drosophila belonging to the obscura group, D. subobscura and D. madeirensis, have been investigated, especially regarding the influence exerted by the X chromosome. This chromosome has been roughly dissected genetically by the use of four markers. It was found that factors affecting viability of backcross males are spread from the centromeric end of the chromosome up the region marked by Bx. Three sections were responsible for male sterility/fertility. The abnormal head shape of the backcross males was affected by factor(s) on the madeirensis and the subobscura sex chromosome located at the region of MAD1 inversion. Finally, an abnormal trait in these males (presence of extra sex combs) was found to be controlled by four sections, two on the madeirensis X chromosome and two on the subobscura one.     

Drosophila reared from the stinkhorn, Phallus impudicus, near Zurich, Switzerland

Drosophila species were reared from, baited with and netted over sporophores of the fungal species Phallus impudicus. The presence and proportions of species varied between years, areas and methods. In the first year, 110 sporophores were used for breeding, and in the second year, 16 for baiting and subsequent breeding. 6417 specimens of 10 Drosophila species were obtained. Drosophila suhohscura was reported from England to breed in stinkhorn, but did not breed in or was even attracted to it in Switzerland, suggesting a remarkable geographical variation in breeding substrate utilization.     

The distribution and abundance of the British fungal-breeding Drosophila

Abstract.

• 1 The distribution and abundance of five Drosophila species breeding in fungi are examined by attracting adult flies to baited traps and collecting fungal fruit bodies containing the larval stages.
• 2 Changes in species frequency follow a temporal (seasonal and yearly), rather than a spatial (between and within woods) pattern.
• 3 There is a regular diurnal pattern of activity in D.subobscura but not in D.phalerata. Temperature, light intensity, humidity and wind speed have no effect upon the daily activity of D.phalerata. Only temperature affects the activity of D.subobscura.
• 4 Out of 125 species of fungi collected, only forty-eight species produced Drosophila. However, the flies are even more selective, since only five species of fungi (Phallus impudicus, Polyporus squamosus, Amanita rubescens, Pluteus cervinus and Lactarius quietus) produce 80% of all Drosophila reared.
• 5 The commonest species, D.phalerata, has two or three generations per year with a winter diapause. An outline of the yearly life cycle is given, with the major breeding sites for each generation indicated.
• 6 Measures of niche overlap calculated from the data on individual fruit bodies are very low, suggesting that the five species are effectively isolated during the larval period. This ecological isolation is achieved by a summation of several niche dimensions, woodlands, seasons, fungal species, and individual fruiting body differences.

     

Global warming and chromosomal inversion adaptation in isolated islands: Drosophila subobscura populations from Madeira

Global warming is an environmental phenomenon to which species must adapt to survive. Drosophila subobscura presents an adaptive capacity due to its chromosomal inversion polymorphism. Until now, the impact of global warming on this polymorphism has been studied in D. subobscura populations located either on a continental mainland or on islands not far from a continent. In this context, gene flow could be a relevant mechanism allowing the movement of thermally adapted inversions between populations. Our aim was to sample and study the chromosomal polymorphism on Madeira, a small isolated island in the Atlantic Ocean. We compared our findings with those reported in the same location approximately four and five decades ago. Moreover, we studied whether global warming has occurred on this island by analyzing mean, maximum and minimum temperatures over a 55‐year period. All atmospheric parameters have increased significantly, consistent with climate change expectations. Frequencies and chromosomal thermal index values of thermal adapted inversions remained quite stable over years. Furthermore, J, U and O chromosomes are almost fixed for “warm” adapted inversions. Thus, if there is little genetic variability remaining and temperatures continue increasing, island populations of D. subobscura might be on the threshold of endangerment. However, apart from selection, genetic drift and inbreeding, other processes, such as phenotypic plasticity or thermoregulatory behavior, could be involved in the survival of the species’ populations. Finally, although in danger, D. subobscura is a generalist that lives in humanized environments, and this fact could favor its persistence on Madeira Island.     

Quantitative genetics of speciation: additive and non-additive genetic differentiation between <Emphasis Type="Italic">Drosophila madeirensis</Emphasis> and <Emphasis Type="Italic">Drosophila subobscura</Emphasis>

The role of dominance and epistasis in population divergence has been an issue of much debate ever since the neoDarwinian synthesis. One of the best ways to dissect the several genetic components affecting the genetic architecture of populations is line cross analysis. Here we present a study comparing generation means of several life history-traits in two closely related Drosophila species: Drosophila subobscura, D. madeirensis as well as their F ₁ and F ₂ hybrids. This study aims to determine the relative contributions of additive and non-additive genetic parameters to the differentiation of life-history traits between these two species. The results indicate that both negative dominance and epistatic effects are very important in the differentiation of most traits. We end with considerations about the relevance of these findings for the understanding of the role of non-additive effects in speciation.     

Nucleotide diversity of a ND5 fragment confirms that population expansion is the most suitable explanation for the mtDNA haplotype polymorphism of <Emphasis Type="Italic">Drosophila subobscura</Emphasis>

Results from mitochondria (mt) DNA restriction site analyses (RSAs) have revealed that wild populations of Drosophila subobscura are formed by two common (I and II) and some rare, often endemic, low-frequency haplotypes. In the study reported here, we analysed nucleotide diversity in a 942-bp fragment of the mtDNA ND5 gene in 48 D. subobscura individuals captured from three populations that showed haplotypes I, II or the less common ones, as well as in one additional individual belonging to D. guanche that was taken as an outgroup. RSAs and sequencing results were compared. The two approaches yielded similar nucleotide variability parameters, suggesting a consistency in the results obtained from mtDNA dynamics in natural populations of D. subobscura. Patterns of polymorphism at ND5 are most consistent with the hypothesis of population expansion after a bottleneck that may have occurred since the last glaciation or which may occur seasonally after the summer and winter. However, we cannot rule out that selection has a role in maintaining the two major haplotypes at intermediate frequencies in worldwide populations of D. subobscura.     

Stress response in Drosophila subobscura

The pattern of puffing and protein synthesis was determined in individuals of Drosophila subobscura subjected to heat shock. Depending on the extent of the heat treatment, the response at the puffing level varied. Some puffs were expressed at 31°–34°C, and others at 37° C. Considering the response as a whole the depression of gene activity after shock at 31°–34° C in individuals raised at 19° C was greater than with the other treatments. Six major heat shock proteins (Hsps) were found in this species. The properties of the high molecular weight proteins are conserved their electrophoretic characteristics and the range of temperatures over which they are synthesized are close to those in other Drosophila species. Remarkable heterogeneity was found in the small Hsps. In addition, an M_r=41000 Hsp was clearly identified in this species. A low level of variability in the patterns of protein synthesis compared with those of puffing activity was detected.