Patterns of puffing activity in the salivary gland chromosomes of Drosophila

Salivary gland X chromosome puffing patterns are described for the Oregon stock of Drosophila melanogaster and for the Berkeley stock of D. simulans. In D. melanogaster regular phase specific puffing was recorded at 21 loci in the third larval instar and subsequent prepupal stage. A comparison of the X chromosome puffing patterns of male and female larvae failed to show any qualitative differences although in the males a group of puffs were active for a longer time during development than in females. The X chromosome puffing patterns of D. simulans are similar to those described for D. melanogaster although two puffs (4F 1–4 and 7B 1–3) were active in D. simulans but not in D. melanogaster. The sex differences in puffing observed in D. melanogaster were also observed in D. simulans.     

Comparative population genomics of latitudinal variation in Drosophila simulans and Drosophila melanogaster

Examples of clinal variation in phenotypes and genotypes across latitudinal transects have served as important models for understanding how spatially varying selection and demographic forces shape variation within species. Here, we examine the selective and demographic contributions to latitudinal variation through the largest comparative genomic study to date of Drosophila simulans and Drosophila melanogaster, with genomic sequence data from 382 individual fruit flies, collected across a spatial transect of 19 degrees latitude and at multiple time points over 2 years. Consistent with phenotypic studies, we find less clinal variation in D. simulans than D. melanogaster, particularly for the autosomes. Moreover, we find that clinally varying loci in D. simulans are less stable over multiple years than comparable clines in D. melanogaster. D. simulans shows a significantly weaker pattern of isolation by distance than D. melanogaster and we find evidence for a stronger contribution of migration to D. simulans population genetic structure. While population bottlenecks and migration can plausibly explain the differences in stability of clinal variation between the two species, we also observe a significant enrichment of shared clinal genes, suggesting that the selective forces associated with climate are acting on the same genes and phenotypes in D. simulans and D. melanogaster.     

The effects of acclimation and rearing conditions on the response of tropical and temperate populations ofDrosophila melanogaster andD. simulans to a temperature gradient (Diptera: Drosophilidae)

The effects of rearing and acclimation on the response of adultDrosophila to temperature were investigated in a gradient.D. melanogaster flies preferred a higher mean temperature and were distributed over a wider range of temperatures thanD. simulans flies. Acclimating adults at different temperatures for a week did not influence the response of either species. Adults reared at 28°C as immatures had a lower mean preference than those reared at cooler temperatures, suggesting that flies compensated for the effects of rearing conditions. Adults from tropical and temperate populations ofD. melanogaster andD. simulans did not differ in the mean temperature they preferred in a gradient, suggesting little genetic divergence for this trait within species. The species differences and environmental responses may be related to changes in optimal physiological conditions for the flies.     

The generation of transplasmic Drosophila simulans by cytoplasmic injection: effects of segregation and selection on the perpetuation of mitochondrial DNA heteroplasmy

Summary Experimental transplasmic Drosophila simulans were obtained through cytoplasm microinjection between eggs carrying different mitochondrial genomes. These genomes (siII and siIII) show a 1.5% difference in their sequences. They produced a large number of heteroplasmic flies in their F₁ progeny and several flies were still heteroplasmic at the eighth generation. The distribution of frequencies of mitochondrial genotypes in the offspring of heteroplasmic females suggests that the stochastic processes involved in the evolution of experimental heteroplasmy of multiple nucleotide sites are very similar to those previously described for spontaneous length heteroplasmy. In addition, the siII genome has a noticeable advantage over the siIII genome in both directions of injection. This advantage is estimated at 58% per fly generation and 5% per cell generation.     

Transposable elements in individual genotypes of Drosophila simulans

Transposable elements are abundant, dynamic components of the genome that affect organismal phenotypes and fitness. In Drosophila melanogaster, they have increased in abundance as the species spread out of Africa, and different populations differ in their transposable element content. However, very little is currently known about how transposable elements differ between individual genotypes, and how that relates to the population dynamics of transposable elements overall. The sister species of D. melanogaster, D. simulans, has also recently become cosmopolitan, and panels of inbred genotypes exist from cosmopolitan and African flies. Therefore, we can determine whether the differences in colonizing populations are repeated in D. simulans, what the dynamics of transposable elements are in individual genotypes, and how that compares to wild flies. After estimating copy number in cosmopolitan and African D. simulans, I find that transposable element load is higher in flies from cosmopolitan populations. In addition, transposable element load varies considerably between populations, between genotypes, but not overall between wild and inbred lines. Certain genotypes either contain active transposable elements or are more permissive of transposition and accumulate copies of particular transposable elements. Overall, it is important to quantify genotype‐specific transposable element dynamics as well as population averages to understand the dynamics of transposable element accumulation over time.     

Genetics and the environment in interspecific competition: a study using the sibling species Drosophila melanogaster and Drosophila simulans

The outcome of interspecific competition of two closely related species may depend upon genetic variation in the two species and the environment in which the experiment is carried out. Interspecific competition in the two sibling species, Drosophila melanogaster and D. simulans, is usually investigated using longterm laboratory stocks that often have mutant markers that distinguish them. To examine competition in flies that genetically more closely resemble flies in nature, we utilized freshly caught wildtype isofemale lines of the two species collected at the same site in San Carlos, Mexico. Under ordinary laboratory conditions, D. melanogaster always won in competition. However, in hotter and drier conditions, D. simulans competed much more effectively. In these environmental conditions, there were genetic differences in competitive ability among lines with the outcome of competition primarily dependent upon the line of D. melanogaster used but in some cases also influenced by the line of D. simulans used. Differences in the measures of productivity and developmental time did not explain the differences in competitive ability among lines. This suggests that the outcome of competition was not due to differences in major fitness components among the isofemale lines but to some other attribute(s) that influenced competitive ability. When lines of flies were combined, the outcome of competition was generally consistent with competitive outcomes between pairs of lines. In several cases, the combination of lines performed better than the best of the constituent lines, suggesting that competitive ability was combined heterotically and that the total amount of genetic variation was important in the outcome of interspecific competition.     

Evolution of the LINE-like I element in the Drosophila melanogaster species subgroup

LINE-like retrotransposons, the so-called I elements, control the system of I-R (inducer-reactive) hybrid dysgenesis in Drosophila melanogaster. I elements are present in many Drosophila species. It has been suggested that active, complete I elements, located at different sites on the chromosomes, invaded natural populations of D. melanogaster recently (1920–1970). But old strains lacking active I elements have only defective I elements located in the chromocenter. We have cloned I elements from D. melanogaster and the melanogaster subgroup. In D. melanogaster, the nucleotide sequences of chromocentral I elements differed from those on chromosome arms by as much as 7%. All the I elements of D. mauritiana and D. sechellia are more closely related to the chromosomal I elements of D. melanogaster than to the chromocentral I elements in any species. No sequence difference was observed in the surveyed region between two chromosomal I elements isolated from D. melanogaster and one from D. simulans. These findings strongly support the idea that the defective chromocentral I elements of D. melanogaster originated before the species diverged and the chromosomal I elements were eliminated. The chromosomal I elements reinvaded natural populations of D. melanogaster recently, and were possibly introduced from D. simulans by horizontal transmission.     

Differential responses to artificial selection on oviposition site preferences in Drosophila melanogaster and D. simulans

The preference–performance relationship in plant–insect interactions is a central theme in evolutionary ecology. Among many insects, eggs are vulnerable and larvae have limited mobility, making the choice of an appropriate oviposition site one of the most important decisions for a female. We investigated the evolution of oviposition preferences in Drosophila melanogaster Meigen and Drosophila simulans Sturtevant by artificially selecting for the preference for 2 natural resources, grape and quince. The main finding of our study is the differential responses of D. melanogaster and D. simulans. Although preferences evolved in the experimental populations of D. melanogaster, responses were not consistent with the selection regimes applied. In contrast, responses in D. simulans were consistent with expectations, demonstrating that this species has selectable genetic variation for the trait. Furthermore, crosses between D. simulans divergent lines showed that the genetic factors involved in grape preference appear to be largely recessive. In summary, our artificial selection study suggests that D. melanogaster and D. simulans possess different genetic architectures for this trait.     

Molecular Phylogeny of the <Emphasis Type="Italic">Drosophila melanogaster</Emphasis> Species Subgroup

Although molecular and phenotypic evolution have been studied extensively in Drosophila melanogaster and its close relatives, phylogenetic relationships within the D. melanogaster species subgroup remain unresolved. In particular, recent molecular studies have not converged on the branching orders of the D. yakuba–D. teissieri and D. erecta–D. orena species pairs relative to the D. melanogaster–D. simulans–D. mauritiana–D. sechellia species complex. Here, we reconstruct the phylogeny of the melanogaster species subgroup using DNA sequence data from four nuclear genes. We have employed vectorette PCR to obtain sequence data for orthologous regions of the Alcohol dehydrogenase (Adh), Alcohol dehydrogenase related (Adhr), Glucose dehydrogenase (Gld), and rosy (ry) genes (totaling 7164 bp) from six melanogaster subgroup species (D. melanogaster, D. simulans, D. teissieri, D. yakuba, D. erecta, and D. orena) and three species from subgroups outside the melanogaster species subgroup [D. eugracilis (eugracilis subgroup), D. mimetica (suzukii subgroup), and D. lutescens (takahashii subgroup)]. Relationships within the D. simulans complex are not addressed. Phylogenetic analyses employing maximum parsimony, neighbor-joining, and maximum likelihood methods strongly support a D. yakuba–D. teissieri and D. erecta–D. orena clade within the melanogaster species subgroup. D. eugracilis is grouped closer to the melanogaster subgroup than a D. mimetica–D. lutescens clade. This tree topology is supported by reconstructions employing simple (single parameter) and more complex (nonreversible) substitution models. Present address (Ryan M. David): University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78284, USA     

Concerted evolution of sequence repeats inDrosophila mitochondrial DNA

Summary In the eightDrosophila species of themelanogaster subgroup, the mitochondrial DNA (mtDNA) contains an A+T-rich region in which replication originates. The length of this region, in contrast with that of the coding part of the genome, varies extensively among these species. The A+T-rich region ranges from about 1kbp inD. yakuba, D. teissieri, D. erecta, andD. orena to 5 kbp inD. melanogaster, D. simulans, D. mauritiana, andD. sechellia. The difference in size is due in part to the amplification, in the species with long genomes, of a 470-bp sequence that is present only once in each of the four species with short genomes.Usually three to six repeats of this sequence occur in direct tandem repetition in the species with long genomes. The sequence is characterized by the relative positions of the Hpa I and Acc I cleavage sites. Comparative study of the genomes found in the species with long mtDNA molecules reveals relative homogeneity of the repeat units within a given genome, which contrasts with the variability found among the repeats of different genomes. This result is suggestive of a process of a concerted evolution.The examination of heteroplasmic flies of three species (D. simulans, D. mauritiana, andD. sechellia) has shed light on this process. In most cases the molecular types of mtDNA present in a heteroplasmic individual differ by one repeat unit. Addition or deletion of this sequence appears to be the original mutational event generating transient heteroplasmy. Cycles of addition or deletion may consequently maintain the intragenomic homogeneity of the repeats.Finally, we have analyzed an exceptional isofemale line in which three molecular lengths of mtDNA are found (molecules with four, five, and six repeats, respectively). Individual offspring of this line carry from one to three of the molecular types, in all combinations. This indicates that the remodeling of the mitochondrial genome occurs through a mechanism that is at present unknown, but that is site specific and rather frequent.Presented at the FEBS Symposium on Genome Organization and Evolution, held in Crete, Greece, September 1–5, 1986     

Insertion sites of the transposable elementmariner are fixed in the genome ofDrosophila sechellia

Summary The abundance of the transposable elementmariner differs dramatically in the genomes of the closely related speciesDrosophila simulans, D. mauritiana, D. sechellia, andD. melanogaster. Natural populations ofD. simulans andD. mauritiana have 1–10 and 20–30 copies per diploid genome, respectively, and the insertion sites are polymorphic. The element has not been found inD. melanogaster. In this paper we show thatD. sechellia, a species endemic to the Seychelles Islands, contains only twomariner elements that are at fixed sites in the genome. One element, inserted in chromosome 2R at 51A1–2, contains three deletions and is missing much of the 3 end. The other element, inserted in chromosome 3L at 64A10–11, is the full length of 1286 bp. Although the sequence of the full-length element is polymorphic in populations ofD. sechellia, at least some of the sequences are closely related to elements fromD. simulans andD. mauritiana that are known to be active. However, judging from the progeny of crosses betweenD. sechellia andD. simulans, the biological activity of the full-lengthD. sechellia element appears to be low, either because of the nucleotide sequence of the element or because of its position in the genome, or both.     

Induction of mitochondrial DNA heteroplasmy by intra- and interspecific transplantation of germ plasm in Drosophila

A new experimental system for inducing mitochondrial DNA heteroplasmy in Drosophila was developed. By transplanting the germ plasm of Drosophila melanogaster and Drosophila mauritiana into the posterior pole of the recipient eggs of D. melanogaster, it was possible to introduce foreign mitochondria into the recipient female germline. Heteroplasmic individuals containing both donor and recipient mtDNA were obtained in intra- and interspecific combinations at similar frequencies. The proportion of donor-derived mtDNA in the heteroplasmic individuals varied considerably from individual to individual irrespective of the donor species used. No significant decrease in or elimination of donor mtDNA was observed, and the heteroplasmic state in female germlines persisted for several generations. The present system should serve very much to promote the study and clarification of the transmission genetics of mtDNA in insects.     

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.     

OVIPOSITION SITE SELECTION BY DROSOPHILA MELANOGASTER AND DROSOPHILA SIMULANS

The effects of texture and larval residues in the medium on oviposition site selection (OSS) by Drosophila melanogaster and Drosophila simulans were studied. Drosophila melanogaster laid over 95% of its eggs in sieved medium (vs. unsieved medium); D. simulans laid all of its eggs in sieved medium. Surgical removal of antennal segments, and of fore-, mid-, or hindtarsi did not affect this result, indicating that sense organs involved in discriminating between sieved and unsieved medium are not confined to only one of the tested structures. In a “multiple choice” experiment, females were allowed to lay eggs in sieved medium of three types: unconditioned (fresh) medium, medium conditioned by D. melanogaster larvae (i.e., medium containing larval residues of D. melanogaster), and medium conditioned by D. simulans larvae. This choice experiment was performed with D. melanogaster and with D. simulans, using three densities of females (10, 20, and 40 per experimental unit). Both species laid more eggs in unconditioned medium than in either of the conditioned media, and density had no effect. D. melanogaster laid more eggs near the edges of food patches than in the center, whereas D. simulans showed no preference for edge or center. Under crowded conditions, both species survived at a higher rate in conditioned media (egg-to-adult survival) than in unconditioned medium, leading to the anomalous conclusion that females of these species seem not to maximize the survival of their offspring. This anomaly was partially resolved by the finding that medium already containing larvae gave lower survival rates than unoccupied medium.     

ADAPTATION AND SPECIALIZATION IN A TWO-RESOURCE ENVIRONMENT IN DROSOPHILA SPECIES

We assayed two components of performance (development time and survivorship), on food medium with and without ethanol, in laboratory populations of Drosophila simulans and D. melanogaster for which ethanol-medium was a novel food resource. These assays were done before and after 12 generations of rearing in either one-(regular medium only) or two-resource (regular medium and ethanol medium) environments. Initially, D. simulans was highly susceptible to ethanol, whereas D. melanogaster was relatively unaffected. After 12 generations in the two-resource environment, D. simulans showed significantly improved mean performance on ethanol medium; mean performance of D. melanogaster did not significantly change. Variation among families for both traits was higher on ethanol medium in D. simulans. Variation in D. melanogaster was not significantly affected by ethanol level, suggesting that resource quality was more important than novelty per se. In both species, the least variation was seen in populations after 12 generations in the two-resource environment. For development time in D. simulans, the decrease in variation was largely due to reduced variation within families, suggesting the evolution of canalization. Development time on the two media was not negatively correlated. In D. simulans, correlations measured before and after the experiment were not heterogeneous, suggesting that trade-offs in performance did not block diet expansion. In D. melanogaster, correlations became significantly less positive after 12 generations in the two-resource environment, supporting the view that correlations between performance on different resources may become less positive over time through selection.     

Structural and genetic studies of the proliferation disrupter genes of Drosophila simulans and D. melanogaster

To examine whether structural and functional differences exist in the proliferation disrupter (prod) genes between Drosophila simulans and D. melanogaster, we analyzed and compared both genes. The exon–intron structure of the genes was found to be the same – three exons were interrupted by two introns, although a previous report suggested that only one intron existed in D. melanogaster. The prod genes of D. simulans and D. melanogaster both turn out to encode 346 amino acids, not 301 as previously reported for D. melanogaster. The numbers of nucleotide substitutions in the prod genes was 0.0747 ±  per synonymous site and 0.0116 ± 0.0039 per replacement site, both comparable to those previously known for homologous genes between D. simulans and D. melanogaster. Genetic analysis demonstrated that D. simulans PROD can compensate for a deficiency of D. melanogaster PROD in hybrids. The PRODs of D. simulans and D. melanogaster presumably share the same function and a conserved working mechanism. The prod gene showed no significant interaction with the lethality of the male hybrid between these species. This revised version was published online in July 2006 with corrections to the Cover Date.     

EXPERIMENTAL EVIDENCE FOR MITOCHONDRIAL DNA INTROGRESSION BETWEEN DROSOPHILA SPECIES

Differential introgression of mitochondrial genomes has been used to explain the occurrence in some species of individuals bearing mtDNA from a related species. This situation has been observed for Drosophila mauritiana (endemic to Mauritius) where a high proportion of individuals (88%) carries an mtDNA also found in D. simulans populations from Madagascar and Réunion. Using these two species, experiments were carried out to test for differential mtDNA introgression. A single virgin female from one species (initial frequency 0.03) was introduced into a population of the other. D. simulans mtDNA can, within three generations, almost entirely displace (frequency up to 0.80) D. mauritiana mtDNA. Hybrid male sterility probably curtails to a large degree parallel introgression of nuclear genes. The progress of cytoplasmic introgression is dependent on the degree of inbreeding of the recipient D. mauritiana strains. In reciprocal experiments, introgression was much less likely: few D. mauritiana migrant females are inseminated and their mtDNA frequency always remains very low. The results of these experiments support the hypothesis that a selective advantage of hybrids (probably at the nuclear level) has promoted mtDNA transfer from D. simulans Madagascar or Reunion populations into D. mauritiana through introgressive hybridization.     

Male fruit flies learn to avoid interspecific courtship

Experimental data suggest, and theoretical models typicallyassume, that males of many fruit flies (Drosophila spp) areat least partially indiscriminate while searching for mates,and that it is mostly the females who exert selective mate choice,which can lead to incipient speciation. Evidence on learningby male D. melanogaster in the context of courtship, however,raises the possibility that the initially indiscriminate malesbecome more selective with experience. I tested this possibilityby comparing the courtship behavior of male D. melanogasterexperienced at courting females of the closely related species,D. simulans, and inexperienced males. I found that comparedwith the inexperienced males, the males experienced with courtingD. simulans females showed significantly lower courtship towardfemale D. simulans. Both male treatments, however, showed virtuallyidentical courtship durations with female D. melanogaster. Theseresults indicate that male fruit flies adaptively refine theircourtship behavior with experience and suggest that the malescontribute more to assortative mating and incipient speciationthan is commonly assumed.     

Molecular analysis of P element behavior in Drosophila simulans transformants

Summary In this report we describe the successful transformation of Drosophila simulans with an autonomous P element from Drosophila melanogaster without the use of a selectable marker. This result demonstrates that there is no species barrier for P element transposition. Utilizing gel blotting and in situ hybridization techniques, we have monitored the behavior of newly-introduced P elements in several D. simulans transformed lines over twelve generations. In most instances, an overall increase in the number of P elements was observed. An examination of the frequency of P-element-bearing individuals in one line revealed the rapid spread of P elements through the population. Analysis of well-characterized sublines confirmed that P elements increase in number by transposition to new genomic sites. The formation of degenerate elements occurred in at least one case. These observations suggest that P elements may behave similarly in D. melanogaster and D. simulans.     

Variations of male cuticular hydrocarbons with geoclimatic variables: an adaptative mechanism in Drosophila melanogaster?

7-tricosene (7T) and 7-pentacosene (7P) are the major components of cuticular hydrocarbons in Drosophila simulans and D. melanogaster males. A chemical study of 16 isofemale lines of D. melanogaster sampled at the first and eighth generations in laboratory conditions showed the stability of chromatographical profiles. Then a large scale study of male 7T/7P polymorphism was performed with 85 populations of D. melanogaster and 29 of D. simulans collected all over the world. There were significant correlations of the values of the balanced ratio (7T − 7P)/(7T + 7P) with geo-climatic parameters, such as latitude, longitude, mean temperature, temperature range and vapour pressure. Parallel variations were also reported for the homologous linear alkanes (23 and 25 Carbon atoms) but not for the longer branched alkanes (27 and 29 Carbon atoms). No correlation was significant for the D. simulans populations studied. In this species a similar polymorphism of 7T/7P was found but restricted to a few populations from West Equatorial Africa. This revised version was published online in July 2006 with corrections to the Cover Date.