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.     

WOLBACHIA AND THE EVOLUTION OF REPRODUCTIVE ISOLATION BETWEEN DROSOPHILA RECENS AND DROSOPHILA SUBQUINARIA

Endosymbiotic bacteria of the genus Wolbachia are widespread among insects and in many cases cause cytoplasmic incompatibility in crosses between infected males and uninfected females. Such findings have been used to argue that Wolbachia have played an important role in insect speciation. Theoretical models, however, indicate that Wolbachia alone are unlikely to lead to stable reproductive isolation between two formerly conspecific populations. Here we analyze the components of reproductive isolation between Drosophila recens, which is infected with Wolbachia, and its uninfected sister species Drosophila subquinaria. Laboratory pairings demonstrated that gene flow via matings between D. recens females and D. subquinaria males is hindered by behavioral isolation. Matings readily occurred in the reciprocal cross (D. quinaria females × D. recens males), but very few viable progeny were produced. The production of viable hybrids via this route was restored by antibiotic curing of D. recens of their Wolbachia symbionts, indicating that hybrid offspring production is greatly reduced by cytoplasmic incompatibility in the crosses involving infected D. recens males. Thus, behavioral isolation and Wolbachia-induced cytoplasmic incompatibility act as complementary asymmetrical isolating mechanisms between these two species. In accordance with Haldane's rule, hybrid females were fertile, whereas hybrid males invariably were sterile. Levels of mtDNA variation in D. recens are much lower than in either D. subquinaria or D. falleni, neither of which is infected with Wolbachia. The low haplotype diversity in D. recens is likely due to an mtDNA sweep associated with the spread of Wolbachia. Nevertheless, the existence of several mtDNA haplotypes in this species indicates that Wolbachia have been present as a potential isolating mechanism for a substantial period of evolutionary time. Finally, we argue that although Wolbachia by themselves are unlikely to bring about speciation, they can increase the rate of speciation in insects.     

GENETICS OF SEXUAL ISOLATION AND COURTSHIP DYSFUNCTION IN MALE HYBRIDS OF DROSOPHILA PSEUDOOBSCURA AND DROSOPHILA PERSIMILIS

Despite the importance of sexual isolation to speciation, few studies have analyzed the genetic basis of interspecific mating discrimination, particularly using hybrid males. In this study, I investigated the genetic basis of sexual isolation using male hybrids of Drosophila pseudoobscura and D. persimilis. Hybrid male mating success was caused by interactions between the X-chromosome and autosomes (or Y-chromosome), and different arms of the X-chromosome contributed to mating success with females of each species. Further, although there was an X-chromosome component to mating success, its magnitude was not disproportionately large when compared with the proportion of the genome contained on this chromosome. Some hybrid males courted with an anomalously low intensity, so I simultaneously mapped the genetic basis of this “courtship dysfunction.” The courtship dysfunction was caused by an interaction between the left arm of the X-chromosome in D. persimilis with the autosomes or Y-chromosome from D. pseudoobscura. Anomalous courtship behavior in interspecific hybrids can obscure the conclusions of studies of the genetics of sexual isolation, so courtship intensity should be evaluated in all such investigations.     

NATURAL HYBRIDIZATION BETWEEN THE SYMPATRIC HAWAIIAN SPECIES DROSOPHILA SILVESTRIS AND DROSOPHILA HETERONEURA

Two newly formed, morphologically distinct species of Drosophila from the island of Hawaii have been found to form fertile hybrids in two areas of sympatry. Both F₁ and backcross hybrids have been recognized in nature; in one case, the hybridization events extended over three years. Original hybridizations involved one or more D. silvestris females mating with D. heteroneura males. Female F₁ hybrids from this cross have participated in backcrosses to D. silvestris. In any one locality, less than 2% hybrids have been found in nature. A hybrid swarm was not formed; selection appears to favor a strict maintenence of morphologies characteristic of the separate species. This result is attributed to pervasive sexual selection, which serves to preserve the syndromes of sexual characteristics that arose during past allopatric divergence. Populations of D. silvestris both within and outside the present range of D. heteroneura often display heritable variation in color patterns involving the abdomen, pleurae, legs, and wings. Genes effecting variation in these characters may be derived from genes involved in a past introgression from D. heteroneura. Independent evidence for past hybridization between these species comes from study of mitochondrial DNA. Although the inferred direction of the cross is the opposite of that observed in the recent case described here, both reciprocal crosses have been obtained experimentally in the laboratory. Accordingly, we suggest that these species may have been open to hybridization since their first sympatic encounters following their inception in allopatry. That they remain as strictly recognizable morphological entities is due both to their current partial allopatry and to the action of sexual selection in maintaining two separate major modes of efficient reproduction. There is no reason to invoke specific reinforcing selection that has imposed reproductive isolation.     

COMPARATIVE PHYLOGEOGRAPHY OF A SIBLING PAIR OF RAINFOREST DROSOPHILA SPECIES (DROSOPHILA SERRATA AND D. BIRCHII)

Drosophila serrata and D. birchii are presumed sibling species; the former is a widespread generalist and the latter is restricted to rainforests. Comparison of the mtDNA sequence phylogeographies revealed two highly divergent, geographically distinct lineages of D. serrata that are as distinct from each other as either is from D. birchii. However, diversity in D. birchii is low and unstructured. The low diversity in D. birchii corresponds with a late-Pleistocene-Holocene contraction of lowland rainforests. We suggest that future studies of speciation and adaptation should compare the two lineages of D. serrata to each other as well as to D. birchii.     

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.