Variations in morphological and life-history traits under extreme temperatures in Drosophila ananassae

Thermal Stress and Adult Fitness in a Drosophila suzukii Neotropical Propagule

Drosophila suzukii (Matsumura 1931) is a cosmopolitan horticultural pest originally from temperate East Asia; yet, its recent introduction in southeast and central Brazil raises the possibility it might expand into warmer climatic zones. In theoretical terms, the adaptive potential of invasive species can be impaired by the lack of genetic variation, but, on the other hand, phenotypic plasticity might play an important role in the adaptation to the new environment. In this context, we investigated the effects of temperature variation (18°C, 22°C, and 28°C) on fitness traits and size of male reproductive organs (accessory glands and testis) in a natural D. suzukii population recently introduced in the neotropical region. Development time decreased significantly with increasing temperature, but egg-to-adult survival was not affected, attaining rates around 50% for the three temperatures. Development at 28°C affected differentially adult male and female biological performance: males displayed higher mortality and severe and permanent reduction in offspring production, whereas females showed the same mortality as controls and a temporary decrease in offspring production, followed of a clear recovery. Finally, reproductive organs size in immature and mature males was affected by developmental temperature variation in the following ways. Testis length decreased with body size (i.e., at higher temperatures) and increased with maturation time after adult hatching, whereas for accessory glands there was no significant difference between different temperatures, resulting in proportionally larger glands for smaller body sizes. These results show differences in developmental dynamics of reproductive tract structures due to temperature variation.

No evidence for direct thermal carryover effects on starvation tolerance in the obligate blood-feeder, Glossina morsitans morsitans

Thermal stress during development can prime animals to cope better with similar conditions in later life. Alternatively, negative effects of thermal stress can persist across life stages and result in poorer quality adults (negative carryover effects). As mean temperatures increase due to climate change, evidence for such effects across diverse taxa is required. Using Glossina morsitans morsitans, a species of tsetse fly and vector of trypanosomiasis, we asked whether (i) adaptive developmental plasticity allows flies to survive for longer under food deprivation when pupal and adult temperatures are matched; or (ii) temperature stress during development persists into adulthood, resulting in a greater risk of death. We did not find any advantage of matched pupal and adult temperature in terms of improved starvation tolerance, and no direct negative carryover effects were observed. There was some evidence for indirect carryover effects-high pupal temperature produced flies of lower body mass, which, in turn, resulted in greater starvation risk. However, adult temperature had the largest impact on starvation tolerance by far: flies died 60% faster at 31°C than those experiencing 25°C, consequently reducing survival time from a median of 8 (interquartile range (IQR) 7-9) to 5 (IQR 5-5.25) days. This highlights differences in temperature sensitivity between life stages, as there was no direct effect of pupal temperature on starvation tolerance. Therefore, for some regions of sub-Saharan Africa, climate change may result in a higher mortality rate in emerging tsetse while they search for their first blood meal. This study reinforces existing evidence that responses to temperature are life stage specific and that plasticity may have limited capacity to buffer the effects of climate change.

Additive genetic variance for traits least related to fitness increases with environmental stress in the desert locust, Schistocerca gregaria

Under environmental stress, previously hidden additive genetic variation can be unmasked and exposed to selection. The amount of hidden variation is expected to be higher for life history traits, which strongly correlate to individual fitness, than for morphological traits, in which fitness effects are more ambiguous. However, no consensual pattern has been recovered yet, and this idea is still debated in the literature. Here, we hypothesize that the classical categorization of traits (i.e., life history and morphology) may fail to capture their proximity to fitness. In the desert locust, Schistocerca gregaria, a model organism for the study of insect polyphenism, we quantified changes in additive genetic variation elicited by lifetime thermal stress for ten traits, in which evolutionary significance is known. Irrespective of their category, traits under strong stabilizing selection showed genetic invariance with environmental stress, while traits more loosely associated with fitness showed a marked increase in additive genetic variation in the stressful environment. Furthermore, traits involved in adaptive phenotypic plasticity (growth compensation) showed either no change in additive genetic variance or a change of moderate magnitude across thermal environments. We interpret this mitigated response of plastic traits in the context of integrated evolution to adjust the entire phenotype in heterogeneous environments (i.e., adaptiveness of initial plasticity, compromise of phenotypic compensation with stress, and shared developmental pathway). Altogether, our results indicate, in agreement with theoretical expectations, that environmental stress can increase available additive genetic variance in some desert locust traits, but those closely linked to fitness are largely unaffected. Our study also highlights the importance of assessing the proximity to fitness of a trait on a case-by-case basis and in an ecologically relevant context, as well as considering the processes of canalization and plasticity, involved in the control of phenotypic variation.

Does fluctuating asymmetry of wing traits capture relative environmental stress in a lepidopteran?

Fluctuating asymmetry (FA) is hypothesized to be a useful predictor of population canalization, especially for organisms at risk from environmental change.Identification of traits that meet statistical criteria as FA measures remains a challenge.Here, a laboratory experiment subjected immature butterflies (Vanessa cardui) to diet and temperature conditions of varying stress levels. Variation in dietary macronutrient ratio (protein: carbohydrate) and rearing temperature (optimal: 25°C; elevated: 32°C) was introduced as stressors. Temperature and nutrition are key variables influencing ectotherm growth and fitness and so are likely to be important stressors that influence FA.Individuals subjected to stressful conditions were predicted to show elevated FA of three wing size traits, as well as increased mortality and decreased adult body size.Trait FA did not vary across treatments. Instead, treatment levels impacted viability: The combined incidence of pupal death and expression of significant wing malformations increased in treatment levels designated as stressful. Variation in adult dry mass also reflected predicted stress levels. Results suggest that individuals predicted to display increased FA either died or displayed gross developmental aberrations.This experiment illustrates important constraints on the investigation of FA, including selection of appropriate traits and identification of appropriate levels of stressors to avoid elevated mortality. The latter concern brings into question the utility of FA as an indicator of stress in vulnerable, natural populations, where stress levels cannot be controlled, and mortality and fitness effects are often not quantifiable.

Phenotypic plasticity, global change, and the speed of adaptive evolution

The role phenotypic plasticity might play in adaptation to the ongoing climate changes is unclear. Plasticity allows for the production of a diversity of intra-generational responses, whose inter-generational evolutionary consequences are difficult to predict. In this article, we review theory and empirical studies addressing this question in insects by considering three scenarios. The first scenario corresponds to adaptive plasticity that should lead to slow or no evolution. The second scenario is the case of non-adaptive phenotypic plasticity to new environmental conditions that should lead either to extinction or, on the contrary, to rapid evolutionary change. The third scenario deals with how plasticity alters the variance selection acts upon. These scenarios are then discussed by highlighting examples of empirical studies on insects. We conclude that more studies are needed to better understand the relationship between phenotypic plasticity and evolutionary processes in insects.

Microorganism-Based Larval Diets Affect Mosquito Development, Size and Nutritional Reserves in the Yellow Fever Mosquito Aedes aegypti (Diptera: Culicidae)

Background

Mosquito larvae feed on organic detritus from the environment, particularly microorganisms comprising bacteria, protozoa, and algae as well as crustaceans, plant debris, and insect exuviae. Little attention has been paid to nutritional studies in Aedes aegypti larvae.

Objectives

We investigated the effects of yeast, bacteria and microalgae diets on larval development, pupation time, adult size, emergence, survivorship, lifespan, and wing morphology.

Materials and Methods

Microorganisms (or Tetramin^® as control) were offered as the only source of food to recently hatched first instar larvae and their development was followed until the adult stage. Protein, carbohydrate, glycogen, and lipid were analyzed in single larvae to correlate energetic reserve accumulation by larva with the developmental rates and nutritional content observed. FITC-labeled microorganisms were offered to fourth instar larvae, and its ingestion was recorded by fluorescence microscopy and quantitation.

Results and Discussion

Immature stages developed in all diets, however, larvae fed with bacteria and microalgae showed a severe delay in development rates, pupation time, adult emergence and low survivorship. Adult males emerged earlier as expected and had longer survival than females. Diets with better nutritional quality resulted in adults with bigger wings. Asaia sp. and Escherichia coli resulted in better nutrition and developmental parameters and seemed to be the best bacterial candidates to future studies using symbiont-based control. The diet quality was measured and presented different protein and carbohydrate amounts. Bacteria had the lowest protein and carbohydrate rates, yeasts had the highest carbohydrate amount and microalgae showed the highest protein content. Larvae fed with microalgae seem not to be able to process and store these diets properly. Larvae were shown to be able to process yeast cells and store their energetic components efficiently.

Conclusion

Together, our results point that Ae. aegypti larvae show high plasticity to feed, being able to develop under different microorganism-based diets. The important role of Ae. aegypti in the spread of infectious diseases requires further biological studies in order to understand the vector physiology and thus to manage the larval natural breeding sites aiming a better mosquito control.

Impact of heat stress on development and fertility of Drosophila suzukii Matsumura (Diptera: Drosophilidae)

Drosophila suzukii is a globally invasive fruit pest that costs millions in yield losses and increased pest management costs. Management practices for D. suzukii currently rely heavily on calendar-based applications of broad-spectrum insecticides, but decision-based applications are theoretically possible with refined population modeling and monitoring. Temperature conditions are strongly deterministic of insect growth rates, fecundity, fertility, and resulting population densities. Therefore, information about the effects of temperature can be incorporated into population modeling to accurately predict D. suzukii population densities in the field which is crucial to maximize pesticide application efficiency and improve sustainability. Here, we investigated the effects of chronic heat stress during all of juvenile development on egg-to-adult viability and fertility. We also investigated egg-to-adult viability under heat stress after heat shock of the maternal parent. We found that heat stress during development results in lower egg-to-adult viability, and reduced lifespan and fertility for surviving adults. However, heat-shock treatment of females prior to egg laying increased the egg-to-adult viability of their eggs under heat stress. Female flies that developed at 30 °C had smaller ovaries than the untreated group and male flies had less sperm in their testes, and no sperm in their seminal vesicles. We conclude that heat stress during development is likely to have negative effect on D. suzukii population dynamics in the field. However, the intensity of such negative impact will depend on the phenotypic state of their maternal parents.

Trait variation in extreme thermal environments under constant and fluctuating temperatures

Climate change is increasingly exposing populations to rare and novel environmental conditions. Theory suggests that extreme conditions will expose cryptic phenotypes, with a concomitant increase in trait variation. Although some empirical support for this exists, it is also well established that physiological mechanisms (e.g. heat shock protein expression) change when organisms are exposed to constant versus fluctuating temperatures. To determine the effect of common, rare and novel temperatures on the release of hidden variation, we exposed fathead minnows, Pimephales promelas, to five fluctuating and four constant temperature regimes (constant treatments: 23.5, 25, 28.5 and 31°C; all fluctuating treatments shared a minimum temperature of 22°C at 00.00 and a maximum of 25, 28, 31, 34 or 37°C at 12.00). We measured each individual's length weekly over 60 days, critical thermal maximum (CTmax), five morphometric traits (eye anterior-posterior distance, pelvic fin length, pectoral fin length, pelvic fin ray count and pectoral fin ray count) and fluctuating asymmetry (FA, absolute difference between left and right morphometric measurements; FA is typically associated with stress). Length-at-age in both constant and fluctuating conditions decreased with temperature, and this trait's variance decreased with temperature under fluctuating conditions but increased and then decreased in constant temperatures. CTmax in both treatments increased with increasing water temperature, while its variance decreased in warmer waters. No consistent pattern in mean or variance was found across morphometric traits or FA. Our results suggest that, for fathead minnows, variance can decrease in important traits (e.g. length-at-age and CTmax) as the environment becomes more stressful, so it may be difficult to establish comprehensive rules for the effects of rarer or stressful environments on trait variation. This article is part of the theme issue 'The role of plasticity in phenotypic adaptation to rapid environmental change'.

A comparison of stable and fluctuating resources with respect to evolutionary adaptation and life-history traits using individual-based modeling and machine learning

There are three non-mutually-exclusive key strategies evolved by gene pools to cope with fluctuating food resource availability, including evolutionary adaptation, phenotypic plasticity, and migration. We focus primarily on evolutionary adaptation and behavioral plasticity, which is a type of phenotypic plasticity, resulting in life-history changes as ways of dealing with fluctuations in food resource availability. Using EcoSim, a predator-prey individual-based model, we compare individuals with stable food resources with those in environments where there are fluctuating food resources in terms of adaptation through behavioral plasticity and evolution. The purpose of our study is to determine whether evolution and behavioral plasticity truly play a role in adapting to an environment with fluctuating food resources, as well as to determine whether there are specific gene divergences between gene pools in fluctuating food resource environments versus gene pools where food resources are relatively stable. An important result of our study is that individuals in environments that are unstable with respect to food resource availability exhibited significant differences in behaviors versus those in environments with stable food resources. Although behavioral plasticity facilitates a rapid response to unstable food conditions, our study revealed the evolution of perceptual traits such as vision range in reaction to fluctuating food resources, indicating the importance of evolution in adapting to unstable resource environments in the long run. Moreover, using decision trees, we found that there were significant behavioral gene divergences between individuals in environments with fluctuating food resources as opposed to individuals in environments with stable food resources.

Effects of scarcity and excess of larval food on life history traits of Aedes aegypti (Diptera: Culicidae)

Few studies have assessed the effects of food scarcity or excess on the life history traits of Aedes aegypti (L.) (Diptera: Culicidae) independently from larval density. We assessed immature survival, development time, and adult size in relation to food availability. We reared cohorts of 30 Ae. aegypti larvae from newly hatched to adult emergence with different food availability. Food conditions were kept constant by transferring larvae each day to a new food solution. Immature development was completed by some individuals in all treatments. The shortest development time, the largest adults, and the highest survival were observed at intermediate food levels. The most important effects of food scarcity were an extension in development time, a decrease in the size of adults, and a slight decrease in survival, while the most important effects of food excess were an important decrease in survival and a slight decrease in the size of adults. The variability in development time and adult size within sex and treatment increased at decreasing food availability. The results suggest that although the studied population has adapted to a wide range of food availabilities, both scarcity and excess of food have important negative impacts on fitness.

Revealing hidden evolutionary capacity to cope with global change

The extent to which global change will impact the long-term persistence of species depends on their evolutionary potential to adapt to future conditions. While the number of studies that estimate the standing levels of adaptive genetic variation in populations under predicted global change scenarios is growing all the time, few studies have considered multiple environments simultaneously and even fewer have considered evolutionary potential in multivariate context. Because conditions will not be constant, adaptation to climate change is fundamentally a multivariate process so viewing genetic variances and covariances over multivariate space will always be more informative than relying on bivariate genetic correlations between traits. A multivariate approach to understanding the evolutionary capacity to cope with global change is necessary to avoid misestimating adaptive genetic variation in the dimensions in which selection will act. We assessed the evolutionary capacity of the larval stage of the marine polychaete Galeolaria caespitosa to adapt to warmer water temperatures. Galeolaria is an important habitat-forming species in Australia, and its earlier life-history stages tend to be more susceptible to stress. We used a powerful quantitative genetics design that assessed the impacts of three temperatures on subsequent survival across over 30 000 embryos across 204 unique families. We found adaptive genetic variation in the two cooler temperatures in our study, but none in the warmest temperature. Based on these results, we would have concluded that this species has very little capacity to evolve to the warmest temperature. However, when we explored genetic variation in multivariate space, we found evidence that larval survival has the potential to evolve even in the warmest temperatures via correlated responses to selection across thermal environments. Future studies should take a multivariate approach to estimating evolutionary capacity to cope with global change lest they misestimate a species' true adaptive potential.

Genetic variability and phenotypic plasticity of metric thoracic traits in an invasive drosophilid in America

Thermal phenotypic plasticity of 5 metric thoracic traits (3 related to size and 2 to pigmentation) was investigated in Zaprionus indianus with an isofemale line design. Three of these traits are investigated for the first time in a drosophilid, i.e. thorax width and width of pigmented longitudinal white and black stripes. The reaction norms of white and black stripes were completely different: white stripes were insensitive to growth temperature while the black stripes exhibited a strong linear decrease with increasing temperatures. Thorax width exhibited a concave reaction norm, analogous but not identical to those of wing length and thorax length: the temperatures of maximum value were different, the highest being for thorax width. All traits exhibited a significant heritable variability and a low evolvability. Sexual dimorphism was very variable among traits, being nil for white stripes and thorax width, and around 1.13 for black stripes. The ratio thorax length to thorax width (an elongation index) was always >1, showing that males have a more rounded thorax at all temperatures. Black stripes revealed a significant increase of sexual dimorphism with increasing temperature. Shape indices, i.e. ratios between size traits all exhibited a linear decrease with temperature, the least sensitive being the elongation index. All these results illustrate the complexity of developmental processes but also the analytical strength of biometrical plasticity studies in an eco-devo perspective.

Phenotypic diversity, population structure and stress protein-based capacitoring in populations of Xeropicta derbentina, a heat-tolerant land snail species

The shell colour of many pulmonate land snail species is highly diverse. Besides a genetic basis, environmentally triggered epigenetic mechanisms including stress proteins as evolutionary capacitors are thought to influence such phenotypic diversity. In this study, we investigated the relationship of stress protein (Hsp70) levels with temperature stress tolerance, population structure and phenotypic diversity within and among different populations of a xerophilic Mediterranean snail species (Xeropicta derbentina). Hsp70 levels varied considerably among populations, and were significantly associated with shell colour diversity: individuals in populations exhibiting low diversity expressed higher Hsp70 levels both constitutively and under heat stress than those of phenotypically diverse populations. In contrast, population structure (cytochrome c oxidase subunit I gene) did not correlate with phenotypic diversity. However, genetic parameters (both within and among population differences) were able to explain variation in Hsp70 induction at elevated but non-pathologic temperatures. Our observation that (1) population structure had a high explanatory potential for Hsp70 induction and that (2) Hsp70 levels, in turn, correlated with phenotypic diversity while (3) population structure and phenotypic diversity failed to correlate provides empirical evidence for Hsp70 to act as a mediator between genotypic variation and phenotype and thus for chaperone-driven evolutionary capacitance in natural populations.

Analysis of the effects of early nutritional environment on inbreeding depression in Drosophila melanogaster

The impact of nutritional deficiencies early in life in determining life-history variation in organisms is well recognized. The negative effects of inbreeding on fitness are also well known. Contrary to studies on vertebrates, studies on invertebrates are not consistent with the observation that inbreeding compromises resistance to parasites and pathogens. In this study, we investigated the effect of early nutrition on the magnitude of inbreeding depression in development time, adult body size and adult resistance to the bacterium Serratia marcescens in Drosophila melanogaster. We found that early nutritional environment had no effect on the magnitude of inbreeding depression in development time or adult body size but may have played a small role in adult resistance to the bacterial infection. Estimates of heritabilities for development time under the poor nutritional environment were larger than those measured under the standard nutritional conditions.

Resistance to environmental stress in Drosophila ananassae: latitudinal variation and adaptation among populations

Geographical variation in traits related to fitness is often the result of adaptive evolution. Stress resistance traits in Drosophila often show clinal variation, suggesting that selection affects resistance traits either directly or indirectly. Multiple stress resistance traits were investigated in 45 natural populations of Drosophila ananassae collected from all over India. There was significant positive correlation between starvation resistance and lipid content. Significant negative correlations between desiccation and lipid content and between desiccation and heat resistance were also found. Flies from lower latitudes had higher starvation resistance, heat resistance and lipid content but the pattern was reversed for desiccation resistance. These results suggest that flies from different localities varied in their susceptibility to starvation because of difference in their propensity to store body lipid. Multiple regression analysis provided evidence of climatic selection driven by latitudinal variation in the seasonal amplitude of temperature and humidity changes within the Indian. Finally, our results suggest a high degree of variation in stress resistance at the population level in D. ananassae.