INCIPIENT SPECIATION BY SEXUAL ISOLATION IN DROSOPHILA MELANOGASTER: VARIATION IN MATING PREFERENCE AND CORRELATION BETWEEN SEXES

Genetic divergence for characters pertaining to reproductive isolation is of considerable interest in evolutionary biology. Since most studies concentrate on sibling species (for recent reviews, see Wu et al. 1996), we would like to know how much genetic variation exists between populations that are at an incipient stage of speciation. To answer this question, we have begun measuring variations in mating preference among natural isolates of Drosophila melanogaster, represented by the cosmopolitan and Zimbabwe sexual races. We quantify the variation in mating preference and success in both sexes by using a multiple-choice design and an index that is suited to cases of strong asymmetry in mate choice. Different designs and indices for measuring sexual isolation are also discussed. These sexual traits are entirely genetically determined. Surveying four populations in southern Africa and additional cosmopolitan lines, we observe extensive genetic variation in sexual characters as well as strong correlation between sexes. The populations are highly differentiated and represent various stages of evolution between the African and the cosmopolitan type of sexual behaviors. The genetic variation and correlation for these sexual characters coupled with their geographical pattern have interesting implications for models of speciation by sexual selection.     

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.     

THE GENETIC BASIS OF SEXUAL ISOLATION BETWEEN DROSOPHILA MELANOGASTER AND D. SIMULANS

The genetic analysis of sexual isolation between the closely-related species Drosophila melanogaster and Drosophila simulans involved two experiments with no-choice tests. The efficiency of sexual isolation was measured by the frequency of courtship initiation and interspecific mating. We first surveyed the variation in sexual isolation between D. melanogaster strains and D. simulans strains of different geographic origin. Then, to investigate variation in sexual isolation within strains, we made F₁ diallel sets of reciprocal crosses within strains of D. melanogaster and D. simulans. The F₁ diallel progeny of one sex were paired with the opposite sex of the other species. The first experiment showed significant differences in the frequency of interspecific mating between geographic strains. There were more matings between D. simulans females and D. melanogaster males than between D. melanogaster females and D. simulans males. The second experiment uncovered that the male genotypes in the D. melanogaster diallel significantly differed in interspecific mating frequency, but not in courtship initiation frequency. The female genotypes in the D. simulans diallel were not significantly different in courtship initiation and interspecific mating frequency. Genetic analysis reveals that in D. melanogaster males sexual isolation was not affected by either maternal cytoplasmic effects, sex-linked effects, or epistatic interaction. The main genetic components were directional dominance and overdominance. The F₁ males achieved more matings with D. simulans females than the inbred males. The genetic architecture of sexual isolation in D. melanogaster males argues for a history of weak or no selection for lower interspecific mating propensity. The behavioral causes of variation in sexual isolation between the two species are discussed.     

STABILIZING SELECTION DETECTED FOR BRISTLE NUMBER IN DROSOPHILA MELANOGASTER

Stabilizing selection, which favors intermediate phenotypes, is frequently invoked as the selective force maintaining a population's status quo. Two main alternative reasons for stabilizing selection on a quantitative trait are possible: (1) intermediate trait values can be favored through the causal effect of the trait on fitness (direct stabilizing selection); or (2) through a pleiotropic, deleterious side effect on fitness of mutants affecting the trait (apparent stabilizing selection). Up to now, these alternatives have never been experimentally disentangled. Here we measure fitness as a function of the number of abdominal bristles within four Drosophila melanogaster lines, one with high, one with low, and two with intermediate average bristle number. The four were inbred nonsegregating lines, so that apparent selection due to pleiotropy is not possible. Individual fitness significantly increased (decreased) with bristles number in the low (high) line. No significant fitness-trait association was detected within each intermediate line. These results reveal substantial direct stabilizing selection on the trait.     

DIRECTIONAL AND STABILIZING DENSITY-DEPENDENT NATURAL SELECTION FOR PUPATION HEIGHT IN DROSOPHILA MELANOGASTER

Six populations of Drosophila melanogaster have been kept at extreme population densities, three high and three low, for 175 generations. Larvae from the high density populations pupate 50%-100% higher than larvae from the low density populations. At high larval test densities there is both a directional and a stabilizing component to selection, with viabilities ranging from 0.14 to 0.992, depending on the choice of pupation site. The directional component is stronger on the populations which have evolved at low densities, while the stabilizing component is stronger on the populations which have evolved at high densities. There is no indication that the evolution of this trait, in response to density, has altered its phenotypic plasticity.