Quantitative genetics of functional characters inDrosophila melanogaster populations subjected to laboratory selection

Phenotypic stasis with genetic divergence

Whether or not genetic divergence in the short-term of tens to hundreds of generations is compatible with phenotypic stasis remains a relatively unexplored problem. We evolved predominantly outcrossing, genetically diverse populations of the nematode Caenorhabditis elegans under a constant and homogeneous environment for 240 generations and followed individual locomotion behavior. Although founders of lab populations show highly diverse locomotion behavior, during lab evolution, the component traits of locomotion behavior - defined as the transition rates in activity and direction - did not show divergence from the ancestral population. In contrast, transition rates' genetic (co)variance structure showed a marked divergence from the ancestral state and differentiation among replicate populations during the final 100 generations and after most adaptation had been achieved. We observe that genetic differentiation is a transient pattern during the loss of genetic variance along phenotypic dimensions under drift during the last 100 generations of lab evolution. These results suggest that short-term stasis of locomotion behavior is maintained because of stabilizing selection, while the genetic structuring of component traits is contingent upon drift history.

Idiosyncratic evolution of maternal effects in response to juvenile malnutrition in Drosophila

Maternal effects often affect fitness traits, but there is little experimental evidence pertaining to their contribution to response to selection imposed by novel environments. We studied the evolution of maternal effects in Drosophila populations selected for tolerance to chronic larval malnutrition. To this end, we performed pairwise reciprocal F1 crosses between six selected (malnutrition tolerant) populations and six unselected control populations and assessed the effect of cross direction on larval growth and developmental rate, adult weight and egg-to-adult viability expressed under the malnutrition regime. Each pair of reciprocal crosses revealed large maternal effects (possibly including cytoplasmic genetic effects) on at least one trait, but the magnitude, sign and which traits were affected varied among populations. Thus, maternal effects contributed significantly to the response to selection imposed by the malnutrition regime, but these changes were idiosyncratic, suggesting a rugged adaptive landscape. Furthermore, although the selected populations evolved both faster growth and higher viability, the maternal effects on growth rate and viability were negatively correlated across populations. Thus, genes mediating maternal effects can evolve to partially counteract the response to selection mediated by the effects of alleles on their own carriers' phenotype, and maternal effects may contribute to evolutionary trade-offs between components of offspring fitness.

Epistasis and maternal effects in experimental adaptation to chronic nutritional stress in Drosophila

Based on ecological and metabolic arguments, some authors predict that adaptation to novel, harsh environments should involve alleles showing negative (diminishing return) epistasis and/or that it should be mediated in part by evolution of maternal effects. Although the first prediction has been supported in microbes, there has been little experimental support for either prediction in multicellular eukaryotes. Here we use a line-cross design to study the genetic architecture of adaptation to chronic larval malnutrition in a population of Drosophila melanogaster that evolved on an extremely nutrient-poor larval food for 84 generations. We assayed three fitness-related traits (developmental rate, adult female weight and egg-to-adult viability) under the malnutrition conditions in 14 crosses between this selected population and a nonadapted control population originally derived from the same base population. All traits showed a pattern of negative epistasis between alleles improving performance under malnutrition. Furthermore, evolutionary changes in maternal traits accounted for half of the 68% increase in viability and for the whole of 8% reduction in adult female body weight in the selected population (relative to unselected controls). These results thus support both of the above predictions and point to the importance of nonadditive effects in adaptive microevolution.

Genetic architecture underlying morning and evening circadian phenotypes in fruit flies Drosophila melanogaster

Circadian rhythms are perhaps among the genetically best characterized behaviours. Several mutations with drastic effects on circadian processes have been identified and models developed to explain how clock genes and their products generate self-sustained oscillations. Although natural variations in circadian phenotypes have been studied extensively, the genetic basis of such adaptive variations remains largely unknown. Here we report the results of a preliminary genetic analysis of adaptive divergence of circadian phenotypes in populations of fruit flies Drosophila melanogaster. Two sets of populations, 'early' and 'late', were created in a long-term laboratory selection for morning and evening emergence, with four independent replicates each. Over the course of ∼55 generations, the early flies evolved increased morning emergence and a shorter circadian period, whereas late flies evolved increased evening emergence and longer period. To examine the genetic basis of circadian phenotypes, we set up crosses between early and late flies, and monitored emergence and activity/rest rhythms in the F1, backcrossed and F2 progeny. Our analysis suggests that the genetic basis of divergent circadian phenotypes in early and late stocks is primarily autosomal. Line-cross analysis revealed that additive and non-additive genetic effects contribute to the divergence of circadian phenotypes in early and late flies.

Genetic architecture underlying convergent evolution of egg-laying behavior in a seed-feeding beetle

Independent populations subjected to similar environments often exhibit convergent evolution. An unresolved question is the frequency with which such convergence reflects parallel genetic mechanisms. We examined the convergent evolution of egg-laying behavior in the seed-feeding beetle Callosobruchus maculatus. Females avoid ovipositing on seeds bearing conspecific eggs, but the degree of host discrimination varies among geographic populations. In a previous experiment, replicate lines switched from a small host to a large one evolved reduced discrimination after 40 generations. We used line crosses to determine the genetic architecture underlying this rapid response. The most parsimonious genetic models included dominance and/or epistasis for all crosses. The genetic architecture underlying reduced discrimination in two lines was not significantly different from the architecture underlying differences between geographic populations, but the architecture underlying the divergence of a third line differed from all others. We conclude that convergence of this complex trait may in some cases involve parallel genetic mechanisms.

Evolutionary biology of starvation resistance: what we have learned fromDrosophila

Most animals face periods of food shortage and are thus expected to evolve adaptations enhancing starvation resistance (SR). Most of our knowledge of the genetic and physiological bases of those adaptations, their evolutionary correlates and trade-offs, and patterns of within- and among-population variation, comes from studies on Drosophila. In this review, we attempt to synthesize the various facets of evolutionary biology of SR emerging from those studies. Heritable variation for SR is ubiquitous in Drosophila populations, allowing for large responses to experimental selection. Individual flies can also inducibly increase their SR in response to mild nutritional stress (dietary restriction). Both the evolutionary change and the physiological plasticity involve increased accumulation of lipids, changes in carbohydrate and lipid metabolism and reduction in reproduction. They are also typically associated with greater resistance to desiccation and oxidative stress, and with prolonged development and lifespan. These responses are increasingly seen as facets of a shift of the physiology towards a 'survival mode', which helps the animal to survive hard times. The last decade has seen a great progress in revealing the molecular bases of induced responses to starvation, and the first genes contributing to genetic variation in SR have been identified. In contrast, little progress has been made in understanding the ecological significance of SR in Drosophila; in particular it remains unclear to what extent geographical variation in SR reflect differences in natural selection acting on this trait rather than correlated responses to selection on other traits. Drosophila offers a unique opportunity for an integrated study of the manifold aspects of adaptation to nutritional stress. Given that at least some major molecular mechanisms of response to nutritional stress seem common to animals, the insights from Drosophila are likely to apply more generally than just to dipterans or insects.

Quantitative genetics of speciation: additive and non-additive genetic differentiation between Drosophila madeirensis and Drosophila subobscura

The role of dominance and epistasis in population divergence has been an issue of much debate ever since the neoDarwinian synthesis. One of the best ways to dissect the several genetic components affecting the genetic architecture of populations is line cross analysis. Here we present a study comparing generation means of several life history-traits in two closely related Drosophila species: Drosophila subobscura, D. madeirensis as well as their F1 and F2 hybrids. This study aims to determine the relative contributions of additive and non-additive genetic parameters to the differentiation of life-history traits between these two species. The results indicate that both negative dominance and epistatic effects are very important in the differentiation of most traits. We end with considerations about the relevance of these findings for the understanding of the role of non-additive effects in speciation.