Reversibility of predator-induced plasticity and its effect at a life-history switch point

Toxicity of POEA-containing glyphosate-based herbicides to amphibians is mainly due to the surfactant, not to the active ingredient

Current international legislation regarding agrochemicals requires thorough toxicological testing mainly of the active ingredients. In a 96-h acute toxicity test we exposed Rana dalmatina and Bufo bufo tadpoles to either one of three concentrations of glyphosate, three concentrations of the surfactant (POEA), three concentrations of the two components together, or to non-contaminated water (control), and subsequently assessed mortality and body mass. To investigate whether simultaneous exposure to another stress factor influences effects of the contaminants, we performed tests both in the presence or absence of predator chemical cues. We found that the surfactant had significant harmful effects on tadpoles; survival was lowered by the highest concentration of the surfactant in case of R. dalmatina, while in B. bufo tadpoles it reduced survival already at medium concentrations. Body mass was significantly influenced by medium and high surfactant concentrations in both species. The presence of glyphosate did not have a significant effect by itself, but it slightly increased mortality in tadpoles exposed to medium concentrations of the surfactant in both species. The presence of chemical cues did not have an effect on the examined variables. Our study confirms that the toxicity of glyphosate-based herbicides is mainly due to the examined surfactant. Nonetheless, we found that glyphosate can enhance the harmful effect of the surfactant. These results stress that during the authorization process of new pesticide formulations, not only the active ingredients would need to be examined but the excipients should also be taken into account in an obligatory and systematic manner.

Ineffective integration of multiple anti-predator defenses in a rotifer: a low-cost insurance?

To maximize survival, prey often integrates multiple anti-predator defenses. How the defenses interact to reduce predation risk is, however, poorly known. We used the rotifer Brachionus calyciflorus to investigate how morphological (spines) and behavioral (floating) defenses are integrated against a common predatory rotifer, Asplanchna brightwellii, and if their combined use improves survival. To this end, we assessed the cost of the behavioral defense and the efficiency of both defenses, individually and combined, as well as their mutual dependency. The results show that the behavioral defense is costly in reducing foraging activity, and that the two defenses are used simultaneously, with the presence of the morphological defense enhancing the use of the behavioral defense, as does the pre-exposure to predator cues. However, while the morphological defense reduces predation risk, the behavioral defense does not, thus, adding the costly behavioral defense to the morphological defense does not improve survival. It is likely that the cost of the behavioral defense is low given its reversibility—compared to the cost of misidentifying the predator species—and that this has promoted the adoption of both defenses, as general low-cost insurance rather than as a tailored strategy toward specific predators. Thus, the optimal strategy in the rotifer appears to be to express both morphological and behavioral defenses when confronted with the cues of a potential predator.

The role of inter-individual intolerance in group cohesion and the transition to sociality in spiders

Conspecific tolerance is key for maintaining group cohesion in animals. Understanding shifts from conspecific tolerance to intolerance is therefore important for understanding transitions to sociality. Subsocial species disperse to a solitary lifestyle after a gregarious juvenile phase and display conspecific intolerance as adults as a mechanism to maintain a solitary living. The development of intolerance towards group members is hypothesized to play a role in dispersal decisions in subsocial species. One hypothesis posits that dispersal is triggered by factors such as food competition with the subsequent development of conspecific intolerance, rather than conspecific intolerance developing prior to and potentially driving dispersal. Consistent with this hypothesis, we show that intolerance (inferred by inter-individual distance) developed post-dispersal in the subsocial spider Stegodyphus lineatus. The development of conspecific intolerance was delayed when maintaining spiders in groups showing plasticity in this trait, which is advantageous when trade-offs are not fixed over time. However, major evolutionary transitions, such as the transition to sociality, can permanently modify trade-offs and cause derived adaptations by the evolution of new or modified traits or evolutionary loss of traits that become redundant. Sociality in spiders has evolved repeatedly from subsocial ancestors, and social life in family groups combined with a lack of interaction with competing groups suggests relaxed selection for the development of conspecific intolerance. In the social Stegodyphus sarasinorum we found no evidence for the development of conspecific intolerance, consistent with the loss of this trait. Instead, we found evidence for conspecific attraction, which is likely to govern group cohesion.

Effects of conspecific density on tadpole risk assessment and defensive behaviour

Prey species assess predation risk by using either direct and indirect cues and both may contribute to a proper evaluation of the actual risk. As postulated by the risk assessment hypothesis, conspecific density may also provide useful information for tuning defensive responses. We tested this hypothesis using a combination of five density levels (1, 2, 4, 8 and 16 individuals) of Italian agile frog Rana latastei tadpoles and three treatments (control, predatory cues of common backswimmer Notonecta glauca and a waterjet of tap water as mechanical disturbance). Tadpole activity decreased in response to all stimuli but, as expected, backswimmer cues induced a stronger and lasting response. However, tadpole activity level did not vary with group size, thus providing no support for the risk assessment hypothesis and confirming that conspecific density might have less consistent effects on short-term behavioural responses than morphological and life history traits.

A review on the DNA methyltransferase family of insects: Aspect and prospects

The DNA methyltransferase family contains a conserved set of DNA-modifying enzymatic proteins. They are responsible for epigenetic gene modulation, such as transcriptional silencing, transcription activation, and post-transcriptional modulation. Recent research has revealed that the canonical DNA methyltransferases (DNMTs) biological roles go beyond their traditional functions of establishing and maintaining DNA methylation patterns. Although a complete DNA methylation toolkit is absent in most insect orders, recent evidence indicates the de novo DNA methylation and maintenance function remain conserved. Studies using various molecular approaches provided evidence that DNMTs are multi-functional proteins. However, still in-depth studies on their biological role lack due to the least studied area in insects. Here, we review the DNA methylation toolkit of insects, focusing on recent research on various insect orders, which exhibit DNA methylation at different levels, and for which DNMTs functional studies have become available in recent years. We survey research on the potential roles of DNMTs in the regulation of gene transcription in insect species. DNMTs participate in different physiological processes by interacting with other epigenetic factors. Future studies on insect's DNMTs will benefit to understand developmental processes, responses to various stimuli, and adaptability of insects to different environmental conditions.

Bayesian updating during development predicts genotypic differences in plasticity

Interactions between genotypes and environments are central to evolutionary genetics, but such interactions are typically described, rather than predicted from theory. Recent Bayesian models of development generate specific predictions about genotypic differences in developmental plasticity (changes in the value of a given trait as a result of a given experience) based on genotypic differences in the value of the trait that is expressed by naïve subjects. We used these models to make a priori predictions about the effects of an aversive olfactory conditioning regime on the response of Drosophila melanogaster larvae to the odor of ethyl acetate. As predicted, across 116 genotypes initial trait values were related to plasticity. Genotypes most strongly attracted to the odor of ethyl acetate when naïve reduced their attraction scores more as a result of the aversive training regime than those less attracted to the same odor when naïve. Thus, as predicted, the variance across genotypes in attraction scores was higher before than after the shared experience. These results support predictions generated by Bayesian models of development and indicate that such models can be successfully used to investigate how variation across genotypes in information derived from ancestors combines with personal experience to differentially affect developmental plasticity in response to specific types of experience.

Interacting effects of predation risk and resource level on escape speed of amphibian larvae along a latitudinal gradient

Fast-growing genotypes living in time-constrained environments are often more prone to predation, suggesting that growth-predation risk trade-offs are important factors maintaining variation in growth along climatic gradients. However, the mechanisms underlying how fast growth increases predation-mediated mortality are not well understood. Here, we investigated if slow-growing, low-latitude individuals have faster escape swimming speed than fast-growing high-latitude individuals using common frog (Rana temporaria) tadpoles from eight populations collected along a 1500 km latitudinal gradient. We measured escape speed in terms of burst and endurance speeds in tadpoles raised in the laboratory at two food levels and in the presence and absence of a predator (Aeshna dragonfly larvae). We did not find any latitudinal trend in escape speed performance. In low food treatments, burst speed was higher in tadpoles reared with predators but did not differ between high-food treatments. Endurance speed, on the contrary, was lower in high-food tadpoles reared with predators and did not differ between treatments at low food levels. Tadpoles reared with predators showed inducible morphology (increased relative body size and tail depth), which had positive effects on speed endurance at low but not at high food levels. Burst speed was positively affected by tail length and tail muscle size in the absence of predators. Our results suggest that escape speed does not trade-off with fast growth along the latitudinal gradient in R. temporaria tadpoles. Instead, escape speed is a plastic trait and strongly influenced by the interaction between resource level and predation risk.

Does corticosterone mediate predator-induced responses of larval Hylarana indica?

Prey-predator interactions have been studied extensively in terms of morphological and behavioural responses of prey to predation risk using diverse model systems. However, the underlying physiological changes associated with morphological, behavioural or life historical responses have been rarely investigated. Herein, we studied the effect of chronic predation risk on larval growth and metamorphosis of Hylarana indica and the underlying physiological changes in prey tadpoles. In the first experiment, tadpoles were exposed to a caged predator from Gosner stage 25-42 to record growth and metamorphosis. Further, whole body corticosterone (CORT) was measured to determine the physiological changes underlying morphological and life historical responses of these prey tadpoles. Surprisingly, tadpoles experiencing continuous predation risk grew and developed faster and metamorphosed at a larger size. Interestingly, these tadpoles had significantly lower CORT levels. In the second experiment, tadpoles were exposed to predation risk (PR) or PR+CORT from stage 25-42 to determine the role of CORT in mediating predator-induced responses of H. indica. Tadpoles facing continuous predation risk grew and developed faster and metamorphosed at a larger size reinforcing the results of the first experiment. However, when CORT was administered along with predation risk, tadpoles grew and developed slowly leading to delayed metamorphosis. Interestingly, growth and metamorphic traits of tadpoles exposed to PR+CORT were comparable to those of the control group indicating that exogenous CORT nullified the positive effect of predation risk. Apparently, CORT mediates predator-induced morphological responses of H. indica tadpoles by regulating their physiology.

Acute changes in whole body corticosterone in response to perceived predation risk: A mechanism for anti-predator behavior in anurans?

Anuran larvae exhibit behavioral and morphological plasticity in response to perceived predation risk, although response type and magnitude varies through ontogeny. Increased baseline corticosterone is related to morphological response to predation risk, whereas the mechanism behind behavioral plasticity remains enigmatic. Since tadpoles alter behavioral responses to risk immediately upon exposure to predator cues, we characterized changes in whole body corticosterone at an acute (<1h post-exposure) timescale. Tadpoles (Lithobates sylvaticus) at Gosner stage (GS) 25 (free-swimming, feeding larvae) increased corticosterone levels to a peak at 10-20min post-exposure to predator cues, paralleling the acute stress response observed among other taxa. Tadpoles reared for 3weeks (mean GS29) with predation risk (caged, fed Aeshnid dragonfly nymph) had lower corticosterone levels at 10-20min post-exposure to dragonfly cues than predator-naïve controls, suggesting habituation, although the magnitude of increase was markedly diminished when compared to younger tadpoles (GS25). These experiments represent the first assessment of tadpole hormonal responses to predation risk at the acute timescale. Further research is required to establish causality between hormonal responses and behavioral changes, and to examine how and why responsiveness changes over ontogeny and with chronic exposure to risk.

Here today, gone tomorrow: Short-term retention of pesticide-induced tolerance in amphibians

Pesticide use has led to the ubiquitous contamination of natural habitats, which has inadvertently increased pesticide tolerance in target and nontarget species. Historically, increased pesticide tolerance has been attributed to natural selection for tolerance among individuals of affected populations. Recent research, however, has discovered that pesticide tolerance can be increased through phenotypic plasticity. Although induced pesticide tolerance may benefit organisms experiencing contaminated systems, little is known about its occurrence in vertebrates, its retention through ontogeny, or potential life history tradeoffs. Using time-to-death assays at 2 distinct developmental windows, the authors discovered that gray treefrog (Hyla versicolor) tadpoles exposed to sublethal concentrations (0 mg a.i./L, 0.5 mg a.i./L, and 1.0 mg a.i./L) of the insecticide Sevin® (carbaryl) early in life increased their pesticide tolerance to a lethal carbaryl concentration 5 d after sublethal exposure. However, this increased tolerance was not retained later in ontogeny (23 d post-sublethal exposure). Moreover, no indication was found of pesticide-induced treefrogs experiencing life-history tradeoffs in terms of survival to metamorphosis, mass, or snout-vent length. Gray treefrogs are only the second vertebrate species and the second amphibian family to exhibit pesticide-induced tolerance after sublethal exposure. The authors' data suggest that the ability to induce increased pesticide tolerance may play a critical role in amphibian survival in contaminated ecosystems. However, future work is needed to test the occurrence of inducible pesticide tolerance among numerous amphibian populations worldwide.

Rapid evolution of constitutive and inducible defenses against an invasive predator

Invasive alien predators can impose strong selection on native prey populations and induce rapid evolutionary change in the invaded communities. However, studies on evolutionary responses to invasive predators are often complicated by the lack of replicate populations differing in coexistence time with the predator, which would allow the determination of how prey traits change during the invasion. The red swamp crayfish Procambarus clarkii has invaded many freshwater areas worldwide, with negative impacts for native fauna. Here, we examined how coexistence time shapes antipredator responses of the Iberian waterfrog (Pelophylax perezi) to the invasive crayfish by raising tadpoles from five populations differing in historical exposure to P. clarkii (30 years, 20 years, or no coexistence). Tadpoles from non-invaded populations responded to the presence of P. clarkii with behavioral plasticity (reduced activity), whereas long-term invaded populations showed canalized antipredator behavior (constant low activity level). Tadpoles from one of the long-term invaded populations responded to the crayfish with inducible morphological defenses (deeper tails), reflecting the use of both constitutive and inducible antipredator defenses against the exotic predator by this population. Our results suggest that, while naive P. perezi populations responded behaviorally to P. clarkii, the strong predation pressure imposed by the crayfish has induced the evolution of qualitatively different antipredator defenses in populations with longer coexistence time. These responses suggest that strong selection by invasive predators may drive rapid evolutionary change in invaded communities. Examining responses of prey species to biological invasions using multiple populations will help us better forecast the impact of invasive predators in natural communities.

Morphological and life-history responses of anurans to predation by an invasive crayfish: an integrative approach

Predator-induced phenotypic plasticity has been widely documented in response to native predators, but studies examining the extent to which prey can respond to exotic invasive predators are scarce. As native prey often do not share a long evolutionary history with invasive predators, they may lack defenses against them. This can lead to population declines and even extinctions, making exotic predators a serious threat to biodiversity. Here, in a community-wide study, we examined the morphological and life-history responses of anuran larvae reared with the invasive red swamp crayfish, Procambarus clarkii, feeding on conspecific tadpoles. We reared tadpoles of nine species until metamorphosis and examined responses in terms of larval morphology, growth, and development, as well as their degree of phenotypic integration. These responses were compared with the ones developed in the presence of a native predator, the larval dragonfly Aeshna sp., also feeding on tadpoles. Eight of the nine species altered their morphology or life history when reared with the fed dragonfly, but only four when reared with the fed crayfish, suggesting among-species variation in the ability to respond to a novel predator. While morphological defenses were generally similar across species (deeper tails) and almost exclusively elicited in the presence of the fed dragonfly, life-history responses were very variable and commonly elicited in the presence of the invasive crayfish. Phenotypes induced in the presence of dragonfly were more integrated than in crayfish presence. The lack of response to the presence of the fed crayfish in five of the study species suggests higher risk of local extinction and ultimately reduced diversity of the invaded amphibian communities. Understanding how native prey species vary in their responses to invasive predators is important in predicting the impacts caused by newly established predator–prey interactions following biological invasions.

Does exposure to cues of fish predators fed different diets affect morphology and performance of Northern Leopard Frog (Lithobates pipiens) larvae?

Phenotypic plasticity allows animals to change their morphological and life-history traits when exposed to predator cues, which modifies performance and can enhance survival but engender costs. Thus, the extent of plastic changes should vary in relation to the perceived risk of predation. We tested the hypothesis that plastic changes in morphology (and their effect on performance) and life history of developing Northern Leopard Frog (Lithobates pipiens (Schreber, 1782)) larvae vary when exposed to cues of fish predators fed different diets. During development, we exposed tadpoles to control cues, cues from brown bullhead (Ameiurus nebulosus (Lesueur, 1819)) fed trout pellets, or cues from A. nebulosus fed L. pipiens tadpoles. Tadpoles exposed to predatory fish cues had smaller bodies, deeper tail fins, slower growth and development rates, and better turning performance than tadpoles that were not exposed to predatory fish cues, but we found limited evidence that the predator’s diet had an effect on phenotypic plasticity. Predator diet affected tail morphology and activity, but the latter effect was only marginally significant. Lithobates pipiens tadpoles clearly respond to predatory fish cues, but it remains unclear whether their response is modulated by the predator’s diet.

Effects of predation environment and food availability on somatic growth in the Livebearing Fish Brachyrhaphis rhabdophora (Pisces: Poeciliidae)

Variation in somatic growth rates is of great interest to biologists because of the relationship between growth and other fitness-determining traits, and it results from both genetic and environmentally induced variation (i.e. plasticity). Theoretical predictions suggest that mean somatic growth rates and the shape of the reaction norm for growth can be influenced by variation in predator-induced mortality rates. Few studies have focused on variation in reaction norms for growth in response to resource availability between high-predation and low-predation environments. We used juvenile Brachyrhaphis rhabdophora from high-predation and low-predation environments to test for variation in mean growth rates and for variation in reaction norms for growth at two levels of food availability in a common-environment experiment. To test for variation in growth rates in the field, we compared somatic growth rates in juveniles in high-predation and low-predation environments. In the common-environment experiment, mean growth rates did not differ between fish from differing predation environments, but the interaction between predation environment and food level took the form of a crossing reaction norm for both growth in length and mass. Fish from low-predation environments exhibited no significant difference in growth rate between high and low food treatments. In contrast, fish from high-predation environments exhibited variation in growth rates between high and low food treatments, with higher food availability resulting in higher growth rates. In the field, individuals in the high-predation environment grow at a faster rate than those in low-predation environments at the smallest sizes (comparable to sizes in the common-environment experiment). These data provide no evidence for evolved differences in mean growth rates between predation environments. However, fish from high-predation environments exhibited greater plasticity in growth rates in response to resource availability suggesting that predation environments may exhibit increased variation in food availability for prey fish and consequent selection for plasticity.