Variation in genetic architecture of olfactory behaviour among wild-derived populations of Drosophila melanogaster

Odour-guided behaviour is a quantitative trait determined by many genes that are sensitive to gene-environment interactions. Different natural populations are likely to experience different selection pressures on the genetic underpinnings of chemosensory behaviour. However, few studies have reported comparisons of the quantitative genetic basis of olfactory behaviour in geographically distinct populations. We generated isofemale lines of Drosophila melanogaster from six populations in Argentina and measured larval and adult responses to benzaldehyde. There was significant variation within populations for both larval and adult olfactory behaviour and a significant genotype x sex interaction (GSI) for adult olfactory behaviour. However, there is substantial variation in the contribution of GSI to the total phenotypic variance among populations. Estimates of evolvability are orders of magnitude higher for larvae than for adults. Our results suggest that the potential for evolutionary adaptation to the chemosensory environment is greater at the larval feeding stage than at the adult reproductive stage.     

Quantitative genetic analysis of larval life history traits in two alpine populations of Rana temporaria

We estimated genetic and maternal variance components of larval life history characters in alpine populations of Rana temporaria (the common frog) using a full-sib/half-sib breeding design. We studied trait plasticity by raising tadpoles at 14 or 20°C in the laboratory. Larval period and metamorphic mass were greater at 14°C. Larval period did not differ between populations, but high elevation metamorphs were larger than low elevation metamorphs. Significant additive variation for larval period was detected in the low altitude population. No significant additive variation was detected for mass at metamorphosis (MM), which instead displayed significant maternal effects. Plasticity in metamorphic mass of froglets was greater in the high altitude population. The plastic response of larval period to temperature did not differ between the populations. Evolution of metamorphic mass is likely constrained by lack of additive genetic variation. In contrast, significant heritability for larval period suggests this trait may evolve in response to environmental change. These results differ from other studies on R. temporaria, suggesting that populations of this broadly distributed species present substantial geographic variation in the genetic architecture and plasticity of tadpole life history traits.     

Phenotypic plasticity and the evolution of trade-offs: the quantitative genetics of resource allocation in the wing dimorphic cricket, Gryllus firmus

In the wing dimorphic sand cricket, Gryllus firmus, there is a pronounced trade-off between flight capability and fecundity. This trade-off is found both between morphs and within the macropterous morph, in which fecundity is negatively correlated with the mass of the principle flight muscles, the dorso-longitudinal muscles (DLM). In this paper, we examine how this trade-off is affected by a reduction in food and its genetic basis. We find that the relative fitness of the two wing morphs is not changed although both fecundity and DLM mass are decreased. A quantitative genetic analysis shows that the trade-off function is genetically variable but that most of the variation occurs in the intercept rather than the slope of the function. Analysis further indicates a very high genetic correlation between environments (food ration) supporting the hypothesis of a strong functional constraint between reproduction and flight capability.     

Substantial changes in the genetic basis of tadpole morphology of Rana lessonae in the presence of predators

Predator-induced morphological plasticity is a model system for investigating phenotypic plasticity in an ecological context. We investigated the genetic basis of the predator-induced plasticity in Rana lessonae by determining the pattern of genetic covariation of three morphological traits that were found to be induced in a predatory environment. Body size decreased and tail dimensions increased when reared in the presence of preying dragonfly larvae. Genetic variance in body size increased by almost an order of magnitude in the predator environment, and the first genetic principal component was found to be highly significantly different between the two environments. The across environment genetic correlation for body size was significantly below 1 indicating that different genes contributed to this trait in the two environments. Body size may therefore be able to respond to selection independently in the two environments to some extent.     

Predicting the pathway to wind pollination: heritabilities and genetic correlations of inflorescence traits associated with wind pollination in Schiedea salicaria (Caryophyllaceae)

The transition from biotic to abiotic pollination was investigated using Schiedea, a genus exhibiting a remarkable diversity of inflorescence architecture associated with pollination biology. Heritabilities and genetic correlations of inflorescence traits were estimated in gynodioecious Schiedea salicaria (Caryophyllaceae), a species that has likely undergone a recent transition to wind-pollination. Using a partial diallel crossing design, significant narrow-sense heritabilities were detected for inflorescence condensation (h2 = 0.56 to 0.68 in the two sexes) and other traits related to the extent of wind pollination in Schiedea species. Heritabilities were generally higher in hermaphrodites than in females. Strong genetic correlations may constrain the evolution of some inflorescence traits that facilitate wind pollination, such as simultaneous shortening of inflorescence length and elongation of the subtending internode. The presence of significant narrow-sense heritabilities for traits associated with wind pollination suggests, however, that selection for more effective wind pollination in the windy, pollinator-limited environments where S. salicaria grows could lead to the evolution of the highly condensed inflorescences characteristic of other wind-pollinated species of Schiedea.     

EVOLUTION OF GENERALISTS AND SPECIALISTS IN SPATIALLY HETEROGENEOUS ENVIRONMENTS

Quantitative genetic models are used to investigate the evolution of generalists and specialists in a coarse-grained environment with two habitat types when there are costs attached to being a generalist. The outcomes for soft and hard selection models are qualitatively different. Under soft selection (e.g., for juvenile or male-reproductive traits) the population evolves towards the single peak in the adaptive landscape. At equilibrium, the population mean phenotype is a compromise between the reaction that would be optimal in both habitats and the reaction with the lowest cost. Furthermore, the equilibrium is closer to the optimal phenotype in the most frequent habitat, or the habitat in which selection on the focal trait is stronger. A specialist genotype always has a lower fitness than a generalist, even when the costs are high. In contrast, under hard selection (e.g., for adult or female-reproductive traits) the adaptive landscape can have one, two, or three peaks; a peak represents a population specialized to one habitat, equally adapted to both habitats, or an intermediate. One peak is always found when the reaction with the lowest cost is not much different from the optimal reaction, and this situation is similar to the soft selection case. However, multiple peaks are present when the costs become higher, and the course of evolution is then determined by initial conditions, and the region of attraction of each peak. This implies that the evolution of specialization and phenotypic plasticity may not only depend on selection regimes within habitats, but also on contingent, historical events (migration, mutation). Furthermore, the evolutionary dynamics in changing environments can be widely different for populations under hard and soft selection. Approaches to measure costs in natural and experimental populations are discussed.     

The heritability of inducible defenses in tadpoles

The evolution of plastic traits requires phenotypic trade-offs and heritable traits, yet the latter requirement has received little attention, especially for predator-induced traits. Using a half-sib design, I examined the narrow-sense heritability of predator-induced behaviour, morphology, and life history in larval wood frogs (Rana sylvatica). Many of the traits had significant additive genetic variation in predator (caged Anax longipes) and no-predator environments. Whereas most traits had moderate to high heritability across environments, tail depth exhibited high heritability with predators but low heritability without predators. In addition, several traits had significant heritability for plasticity, suggesting a potential for selection to act on plasticity per se. Genetic correlations confirmed known phenotypic relationships across environments and identified novel relationships within each environment. This appears to be the first investigation of narrow-sense heritabilities for predator-induced traits and confirms that inducible traits previously shown to be under selection also have a genetic basis and should be capable of exhibiting evolutionary responses.     

Do hermaphrodites of gynodioecious Phacelia linearis (Hydrophyllaceae) trade off seed production to attract pollinators?

Sexual selection theory predicts that hermaphroditic plants might trade off seed production to attract pollinators. This paper reports a test of this prediction in gynodioecious Phacelia linearis (Hydrophyllaceae), a species in which attractiveness to pollinators increases with corolla diameter. The relationship between corolla diameter and seed production was determined in three natural and three experimental populations. Phenotypic selection analysis was used, with lifetime seed production as a surrogate for fitness. Negative directional selection was expected on hermaphrodite corolla diameter. No directional selection (but possibly stabilizing selection) was expected on corolla diameter in females, which have smaller corollas than hermaphrodites. Shoot biomass and flowering time were included in selection analyses so that the effects of corolla size could be assessed independendy of these correlated characters. A parent-offspring study of the quantitative genetics of these characters also was performed. High seed production was associated with large shoot biomass (especially in natural populations) and early flowering (especially in experimental populations). The effect of hermaphrodite corolla size on seed production was not as predicted. Directional selection on hermaphrodite corolla diameter was generally positive. Directional selection on female corolla diameter was relatively weak and variable; stabilizing selection was found in one population. Most of the phenotypic selection on corolla diameter was caused by its (environmental) correlation with shoot biomass. Although these findings do not support the hypothesis that hermaphrodites trade off seed production to attract pollinators, a cautious interpretation is warranted. In one natural population, insect herbivory generated positive covariance between hermaphrodite corolla diameter and seed number, leading to a spurious estimate of positive directional selection for corolla diameter. Other, undetected sources of positive covariance between corolla size and seed production also might have obscured the expected tradeoff. Corolla diameter was found to be heritable, but shoot biomass and flowering time were not. The genetic correlation between hermaphrodite corolla diameter and female corolla diameter was positive and significant, but it was significantly less than 1, so corolla size could evolve independently in the two genders.