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.     

Evolutionary domestication in Drosophila subobscura

The domestication of plants and animals is historically one of the most important topics in evolutionary biology. The evolutionary genetic changes arising from human cultivation are complex because of the effects of such varied processes as continuing natural selection, artificial selection, deliberate inbreeding, genetic drift and hybridization of different lineages. Despite the interest of domestication as an evolutionary process, few studies of multicellular sexual species have approached this topic using well-replicated experiments. Here we present a comprehensive study in which replicated evolutionary trajectories from several Drosophila subobscura populations provide a detailed view of the evolutionary dynamics of domestication in an outbreeding animal species. Our results show a clear evolutionary response in fecundity traits, but no clear pattern for adult starvation resistance and juvenile traits such as development time and viability. These results supply new perspectives on the confounding of adaptation with other evolutionary mechanisms in the process of domestication.     

The genetics of Drosophila subobscura populations XVIII. Multiple insemination and sperm displacement in Drosophila subobscura

The multiple insemination of Drosophila subobscura females in nature was studied by the analysis of single female offspring using two highly polymorphic enzyme systems. On qualitative grounds it was found that 23% of these females were inseminated at least twice. The power of detection of one enzyme system was estimated using the already known multi-inseminated females detected by the other system. This permitted the true frequency estimation of multi-inseminated females, in nature, which is 0.276 (for 95% confidence limits it ranges from 0.186 to 0.417). In a laboratory experiment it was found that sperm from the second inseminating male displaces the sperm from the first male.     

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.     

Microdistribution and habitat selection in Drosophila subobscura Collin

Microdistribution and habitat selection were investigated in a population of Drosophila subobscura near Leeds, Yorkshire. Microdistribution was examined along a transect running through a deciduous woodland. Two main types of habitat were recognized, those with dry soil and a closed canopy of mature trees (dry and dark) and those with wet soil and an open canopy of mature trees (wet and light). Flies were much more frequent, judged by their attendance at baited traps, in the dry/dark habitat. Flies taken from both habitat types, marked with different coloured fluorescent dust and released at a point mid-way between the original habitats, tended to return to their area of initial capture.     

Drosophila subobscura in Egypt and its probable derivation

In April 1978 a total of 26565 drosophilid flies were collected in Egypt. Of ten species five are new records for Egypt, while two previously reported Drosophila species were not found. Seven of the species belong to the genus Drosophila and are known to be partly, or wholly, domestic.For the first time Drosophila subobscura is reported from Egypt. The species was collected at low frequency in an orchard near Alexandria, and one specimen was trapped in a city park in Cairo. Judged from the gene arrangements of 41 males studied, Egyptian Drosophila subobscura seem likely to be derived from recent immigrants from the south coast of Asia Minor.     

Size and seasonal temperature in free-ranging Drosophila subobscura

Body size of diverse ectotherms is inversely related to developmental temperature in the laboratory. We monitored seasonal variation in wing length of two populations (Oregon, Washington) of D. subobscura, which was introduced in the Pacific Northwest in the late 1970s. Wing length varied seasonally and was shortest in summer. In Washington, however, wing length was longest in spring, not winter. Wing length was inversely and curvilinearly related to mean ambient temperature, as in a few previous studies of drosophilids. Mid-winter D. subobscura might not be the largest either because extremely low temperatures depress size or because flies collected in winter were in fact born the previous autumn, when developmental temperatures were more moderate.     

Adaptation to the laboratory environment in Drosophila subobscura

Adaptation to a novel environment is expected to have a number of features. Among these is a temporal increase in fitness and some or all of its components. It is also expected that additive genetic variances for these fitness characters will fall. Finally, it is expected that at least some additive genetic correlations will decrease, from positive toward negative values. In a study of several life‐history variables in a Drosophila subobscura population sampled from the wild and then cultured in the laboratory, we did not find any such longitudinal trends over the first 29 generations. However, a temporal comparison (over 14 generations) of the later generations of this laboratory‐adapted population with a new population, derived from a more recent wild‐caught sample, indicated clearly that laboratory adaptation was nonetheless occurring. This study suggests the need for extensive replication and control in studies of the features of adaptation to a novel environment.     

Mitochondrial DNA effects on fitness in Drosophila subobscura

We tested different fitness components on a series of conspecific mtDNA haplotypes, detected by RFLPs in Drosophila subobscura. Additionally, haplotype VIII, endemic to the Canary Islands, was tested upon its own native nuclear DNA background and upon that of the rest of mtDNAs tested herein. We found that both nuclear and mitochondrial DNA can have a significant effect upon their hosts' fitness, and that negative selection is one of the mechanisms that can intervene in this species' mtDNA haplotype pattern. We discuss the importance of this mechanism in relation to genetic drift, in the form of periodic population bottlenecks, and how the latter can enhance the former. We also detected a significant positive effect of haplotype VIII upon fitness that could explain in part the dominance of this endemic haplotype on some of the Canary Islands, and a mitochondrial heterosis involving this haplotype when on a foreign nuclear DNA background.     

The genetic structure of Balkan populations of Drosophila subobscura

Although Drosophila subobscura has been a model organism for European and American population geneticists, little information is available on the genetic structure of its natural populations. In this paper we report the estimates of some population parameters. We have used data from lethal allelism tests in four Balkan populations (Kamariste, Djerdap and Petnica in Serbia and Zanjic in Montenegro). In all populations, lethal genes were found to have a deleterious effect on heterozygotes. The N(e) values varied greatly from 370 (Petnica) to 19413 (Kamariste), depending on the habitat conditions and some environmental factors. Finally, gene flow between the Balkan populations was detected by the estimates of N(m) (from 4.68 in Petnica to 106.2 in Kamariste) and m (from 0.0041 in Djerdap to 0.0126 in Petnica). These results agree with those obtained in a previous study where the frequencies of allelism between populations were greater than predicted by independently arising lethal genes.     

Seasonal cycles of allozyme-by-chromosomal-inversion gametic disequilibrium in Drosophila subobscura

Allozyme loci are frequently found non randomly associated to the chromosomal inversions in which they are included in Drosophila. Two opposite views compete to explain strong allozyme-by-inversion gametic disequilibria: they result from natural selection or, conversely, merely represent remnants of associations accidentally established at the origin of inversions. Empirical efforts aimed at deciding between adaptive and historical scenarios have focused on the spatial distribution of disequilibria. Yet, the evolutionary significance of these associations remains uncertain. I report here the results of a time-series analysis of the seasonal variation of alleles at six allozyme loci (Acph, Lap, Pept-1, Ao, Mpi, and Xdh) in connection with the O chromosomal polymorphisms of D. subobscura. The findings were: (1) in the segment I of the O chromosome, Lap and Pept-1 allozymes changed seasonally in a cyclical fashion within the ST gene arrangement, but they changed erratically within the 3 + 4 gene configuration; (2) the frequencies of Lap1.11 and Pept-1(0.40) within ST dropped to their lowest values in early and late summer, respectively, when the seasonal level of the ST arrangement is lowest. Furthermore, Lap1.11 and Pept-1(0.40) covary with ST only within these seasons, yet in a fashion inconsistent with these alleles having a major influence on the dynamics of the inversion; (3) seasonal cycling of alleles within inversions were not detected at Acph, Ao, Mpi, and Xdh, yet these loci are nearly monomorphic at the study population, and/or their sampled series were shorter than those for Lap and Pept-1; and (4) simply monitoring allozyme frequencies separately for each inversion proved to be superior, for evidencing the seasonal cycles of the disequilibria, to the use of the D' coefficient of association. Observed seasonal cycles of allozymes within inversions likely reflect natural selection.     

High density of long dinucleotide microsatellites in Drosophila subobscura

We isolated 96 dinucleotide repeats with five or more tandemly repeated units from a subgenomic Drosophila subobscura library. The mean repeat unit length of microsatellite clones in D. subobscura is 15, higher than that observed in other Drosophila species. Population variation was assayed in 32-40 chromosomes from Barcelona, Spain, using 18 randomly chosen microsatellite loci. Positive correlation between measures of variation and perfect repeat length measures (mean size, most common, and longest allele) is consistent with a higher mutation rate in loci with longer repeat units. Levels of microsatellite variation measured as variance in repeat number and heterozygosity in D. subobscura were similar to those of Drosophila pseudoobscura and higher than those of Drosophila melanogaster and Drosophila simulans. Our data suggest that higher levels of microsatellite variation, and possibly density, in D. subobscura compared with D. melanogaster are due to both a higher average effective population and a higher intrinsic slippage rate in the former species.     

The ontogeny and cost of migration in Drosophila subobscura Collin

The ontogeny of migratory potential of male and female Drosophila subobscura has been quantified using tethered flight in the laboratory. In both sexes there is an increase in the few days immediately after eclosion, and a decline before the flies cease reproductive activity. There is an intervening peak in the potential of males, which may result from an increased rejection-rate by inseminated females. The migratory potential generally appears greater than that usually utilized in the field. Such extended migration extracts a significant cost from female flies in terms of reduced fecundity.     

Patterns of puffing activity and chromosomal polymorphism in Drosophila subobscura

The puffing patterns in polytene E chromosomes of Drosophila subobscura were followed in third-instar larvae and throughout the prepupa period. Two gene arrangements, E_st and E_1·2+9–12 were studied. A majority of puffs exhibit a similar pattern, but the puffs 61AC and 67AB behave differently in the two chromosomal arrangements, both in homozygotes and in heterozygotes. These two puffs are located at the end of the E₁₂ inversion. This position effect is an interesting phenomenon that probably is not due to a heterochromatinization effect.     

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     

Microsatellite variation in colonizing and palearctic populations of Drosophila subobscura

The recent colonization of North America by Drosophila subobscura has provided a great opportunity to analyze a colonization process from the beginning. A comparative study using 10 microsatellite loci was conducted for five European and two North American populations. No genetic differentiation between European populations was detected, indicating that gene flow is high among them and that the microsatellites used in the present work represent neutral markers not subject to differentiation due to selection. Extensive reduction in the number of alleles and a significant decrease in heterozygosity in colonizing populations were detected that could be explained by the founder effect and a subsequent quick but not infinite expansion. Assuming that all alleles present in the colonized area were carried by the sample of colonizers, we estimated that most probably 4-11 individuals expanded in the new area. F(ST) and the chord distance measures reflect the colonization process more accurately, since drift has been the major force in differentiating the Old and New World populations, and thus other measures considering allele size differences, such as Rho(ST) and deltamu2, are less reliable for studying nonequilibrium populations. Finally, our results were consistent with the two-phase microsatellite mutational model, indicating that most alleles are generated by gain or loss of a repeat unit, while some alleles originate by more complex mutations.     

Open ground individuals and population structure in Drosophila subobscura Collin

The distribution of Drosophila subobscura adults within, and their movement through, open ground habitats has been investigated, along with the status of such individuals within populations as a whole. Movement occurs between open ground and woodland, to some extent as a result of passive transport by air currents, but flies can remain in open ground for at least several days at a time. There are no detectable differences in either the reproductive age-structure or the body-size distributions of flies collected from woodland and open ground habitats, although a significantly higher proportion of females are found in open ground than in woodland. The large number of gravid females in open ground provide the potential for breeding there, but circumstantial evidence suggests that this is unlikely to occur. Rather, their presence appears to reflect the foraging strategy adopted by females in response to their high nutritional requirements. The flux of flies through open ground between woodlands suggests that previous estimates of the genetically-important parameter, effective population size, have been seriously underestimated for natural D. subobscura populations.