Microspatial structure of Drosophila melanogaster populations in Brazzaville: evidence of natural selection acting on morphometrical traits

Lack of alignment across yeast-dependent life-history traits may limit Drosophila melanogaster dietary specialization

Heterogeneity in food resources is a major driver of local adaptation and speciation. Dietary specialization typically involves multiple life‐history traits and may thus be limited by the extent to which these traits adapt in concert. Here, we use Drosophila melanogaster, representing an intermediate state in the generalist‐specialist continuum, to explore the scope for dietary specialization. D. melanogaster has a close association with yeast, an essential but heterogeneous food resource. We quantify how different D. melanogaster strains from around the globe respond to different yeast species, across multiple yeast‐dependent life‐history traits including feeding, mating, egg‐laying, egg development and survival. We find that D. melanogaster strains respond to different yeast species in different ways, indicating distinct fly strain–yeast interactions. However, we detect no evidence for trade‐offs: fly performance tends to be positively rather than negatively correlated across yeast species. We also find that the responses to different yeast species are not aligned across traits: different life‐history traits are maximized on different yeast species. Finally, we confirm that D. melanogaster is a resource generalist: it can grow, reproduce and survive on all the yeast species we tested. Together, these findings provide a possible explanation for the limited extent of dietary specialization in D. melanogaster.

Seven Questions on the Chemical Ecology and Neurogenetics of Resource-Mediated Speciation

Adaptation to different environments can result in reproductive isolation between populations and the formation of new species. Food resources are among the most important environmental factors shaping local adaptation. The chemosensory system, the most ubiquitous sensory channel in the animal kingdom, not only detects food resources and their chemical composition, but also mediates sexual communication and reproductive isolation in many taxa. Chemosensory divergence may thus play a crucial role in resource-mediated adaptation and speciation. Understanding how the chemosensory system can facilitate resource-mediated ecological speciation requires integrating mechanistic studies of the chemosensory system with ecological studies, to link the genetics and physiology of chemosensory properties to divergent adaptation. In this review, we use examples of insect research to present seven key questions that can be used to understand how the chemosensory system can facilitate resource-mediated ecological speciation in consumer populations.

Natural selection reduces energy metabolism in the garden snail, helix aspersa (cornu aspersum)

Phenotypic selection is widely recognized as the primary cause of adaptive evolution in natural populations, a fact that has been documented frequently over the last few decades, mainly in morphological and life-history traits. The energetic definition of fitness predicts that natural selection will maximize the residual energy available for growth and reproduction, suggesting that energy metabolism could be a target of selection. To address this problem, we chose the garden snail, Helix aspersa (Cornu aspersum). We performed a seminatural experiment for measuring phenotypic selection on standard metabolic rate (SMR), the minimum cost of maintenance in ectotherm organisms. To discount selection on correlated traits, we included two additional whole-organism performance traits (mean speed and maximum force of dislodgement). We found a combination of linear (negative directional selection, beta=-0.106 +/- 0.06; P= 0.001) and quadratic (stabilizing selection, gamma=-0.012 +/- 0.033; P= 0.061) selection on SMR. Correlational selection was not significant for any possible pair of traits. This suggests that individuals with average-to-reduced SMRs were promoted by selection. To the best of our knowledge, this is the first study showing significant directional selection on the obligatory cost of maintenance in an animal, providing support for the energetic definition of fitness.

African Drosophila melanogaster and D. simulans populations have similar levels of sequence variability, suggesting comparable effective population sizes

Drosophila melanogaster and D. simulans are two closely related species with a similar distribution range. Many studies suggested that D. melanogaster has a smaller effective population size than D. simulans. As most evidence was derived from non-African populations, we readdressed this question by sequencing 10 X-linked loci in five African D. simulans and six African D. melanogaster populations. Contrary to previous results, we found no evidence for higher variability, and thus larger effective population size, in D. simulans. Our observation of similar levels of variability of both species will have important implications for the interpretation of patterns of molecular evolution.

A Drosophila male pheromone affects female sexual receptivity

Sex pheromones are chemical signals frequently required for mate choice, but their reciprocal role on mate preference has rarely been shown in both sexes. In Drosophila melanogaster flies, the predominant cuticular hydrocarbons (CHs) are sexually dimorphic: only females produce 7,11-dienes, whereas 7-tricosene (7-T) is the principal male CH. Males generally prefer females with 7,11-dienes, but the role of 7-T on female behaviour remains unclear. With perfumed males, control females mated faster and more often with males carrying increased levels of 7-T showing that this CH acts as a chemical stimulant for D. melanogaster females. Control females-but not antenna-less females-could detect small variation of 7-T. Finally, our finding that desat1 mutant female showed altered response towards 7-T provides an additional role for this gene which affects the production and the perception of pheromones involved in mate choice, in both sexes.

Pre-reproductive isolation as a consequence of allopatric differentiation between populations of Drosophila melanogaster

While pre-reproductive isolations are more and more frequently described between closely related species or within species, very little is known about their conditions of emergence. In Brazzaville, two populations (Kronenbourg and Loua) of Drosophila melanogaster show a premating isolation. Two hypotheses were proposed to explain such a situation: a local sympatric differentiation or an allopatric divergence followed by a secondary contact. A microsatellite analysis, using markers on all chromosomes, strongly suggests that the Kronenbourg population has a European origin. Therefore, the allopatric divergence between Kronenbourg and Loua populations is probably responsible for the sexual isolation observed today in sympatry, after a recent introduction of a European propagule in Brazzaville.

Isofemale lines in Drosophila: an empirical approach to quantitative trait analysis in natural populations

Founding isofemale lines from wild collected females is a basic tool for investigating the genetic architecture of Drosophila natural populations. The method permits the analysis of quantitative traits under laboratory conditions, with a much broader scope than the mere evidence of a significant genetic heterogeneity among lines. Genetic variability is generally demonstrated by a significant coefficient of intraclass correlation, but several experimental precautions are needed and explained here. The relationship between classical (additive) heritability and intraclass correlation is not straightforward, presumably because the genetic bottlenecks due to the initiation of the lines unravel a significant, nonadditive genetic variance due to dominance and epistatic effects. It is thus suggested to consider intraclass correlation as a specific genetic parameter that enables comparisons between different traits, different populations or different environments. The use of isofemale lines is, however, not restricted to the calculation of an intraclass correlation. It can be used to estimate genetic correlations among traits or environments. The method is also convenient for the analysis of phenotypic plasticity in relation to an environmental gradient. A precise description of the response curves (the reaction norms) is possible, distinguishing trait parameters and plasticity parameters. A fairly general conclusion is that, for a given trait, mean value and plasticity are genetically independent. It is also possible to analyze traits, which, like sexual dimorphism, must be measured on different individuals, and even to demonstrate their genetic variability. In many cases, further empirical and theoretical analyses are possible and needed. It is argued that, in the future, isofemale lines will have an increasing significance among the various techniques appropriate to the analysis of quantitative evolutionary genetics in a diversity of species.

Microclinal variation for ovariole number and body size in Drosophila melanogaster in ?Evolution Canyon?

Sites that display strong environmental contrasts in close proximity, such as 'Evolution Canyon' on Mt. Carmel, Israel, are natural theatres for investigating adaptive evolution in action. We reared Drosophila melanogaster from collection sites along altitudinal transects on the north- and south-facing canyon slopes in each of three temperature environments, and assessed genetic variation in ovariole number and body size between and within collection sites, and temperature plasticity. Both traits exhibited significant genetic variation within collection sites and phenotypic plasticity in response to temperature, but not genetic variation for plasticity. Between-site genetic variation in ovariole number was negatively correlated with altitude on both slopes of the canyon, and collections from the north- and south-facing slopes were genetically differentiated for male, but not female, body size. Genetic variation between sites within easy dispersal range is consistent with the action of strong natural selection, although neither the selective agent(s) nor the direct targets of selection are known.