One of these things is not like the other: Mixed predator cues result in lopsided phenotypic responses in a Neotropical tadpole

Many organisms have evolved to produce different phenotypes in response to environmental variation. Dendropsophus ebraccatus tadpoles develop opposing shifts in morphology and coloration when they are exposed to invertebrate vs vertebrate predators. Each of these alternate phenotypes are adaptive, conferring a survival advantage against the predator with which tadpoles were reared but imposing a survival cost with the mismatched predator. Here, we measured the phenotypic response of tadpoles to graded cues and mixed cues of both fish and dragonfly nymphs. Prey species like D. ebraccatus commonly co-occur with both of these types of predators, amongst many others as well. In our first experiment, tadpoles increased investment in defensive phenotypes in response to increasing concentrations of predator cues. Whereas morphology only differed in the strongest predation cue, tail spot coloration differed even at the lowest cue concentration. In our second experiment, tadpoles reared with cues from both predators developed an intermediate yet skewed phenotype that was most similar to the fish-induced phenotype. Previous studies have shown that fish are more lethal than dragonfly larvae; thus tadpoles responded most strongly to the more dangerous predator, even though the number of prey consumed by each predator was the same. This may be due to D. ebraccatus having evolved a stronger response to fish or because fish produce more kairomones than do dragonflies for a given amount of food. We demonstrate that not only do tadpoles assess predation risk via the concentration of predation cues in the water, they produce a stronger response to a more lethal predator even when the strength of cues is presumed to be identical.

Characterization of Two Mitogenomes of Hyla sanchiangensis (Anura: Hylidae), with Phylogenetic Relationships and Selection Pressure Analyses of Hylidae

Hyla sanchiangensis (Anura: Hylidae) is endemic to China and is distributed across Anhui, Zhejiang, Fujian, Guangdong, Guangxi, Hunan, and Guizhou provinces. The mitogenomes of H. sanchiangensis from two different sites (Jinxiu, Guangxi, and Wencheng, Zhejiang) were sequenced. Phylogenetic analyses were conducted, including 38 mitogenomes of Hylidae from the NCBI database, and assessed the phylogenetic relationship of H. sanchiangensis within the analyzed dataset. Two mitogenomes of H. sanchiangensis showed the typical mitochondrial gene arrangement with 13 protein-coding genes (PCGs), two ribosomal RNA genes (12S rRNA and 16S rRNA), 22 transfer RNA (tRNA) genes, and one non-coding control region (D-loop). The lengths of the 12S rRNA and 16S rRNA genes from both samples (Jinxiu and Wencheng) were 933 bp and 1604 bp, respectively. The genetic distance (p-distance transformed into percent) on the basis of the mitogenomes (excluding the control region) of the two samples was calculated as 4.4%. Hyla sanchiangensis showed a close phylogenetic relationship with the clade of (H. annectans + H. tsinlingensis), which was supported by ML and BI analyses. In the branch-site model, five positive selection sites were found in the clade of Hyla and Dryophytes: Cytb protein (at position 316), ND3 protein (at position 85), and ND5 protein (at position 400) have one site, respectively, and two sites in ND4 protein (at positions 47 and 200). Based on the results, we hypothesized that the positive selection of Hyla and Dryophytes was due to their experience of cold stress in historical events, but more evidence is needed to support this conclusion.

Plasticity and flexibility in the anti-predator responses of treefrog tadpoles

Abstract

Tadpoles can respond to perceived predation risk by adjusting their life history, morphology, and behavior in an adaptive way. Adaptive phenotypic plasticity can evolve by natural selection only if there is variation in reaction norms and if this variation is, at least in part, heritable. To provide insights into the evolution of adaptive phenotypic plasticity, we analyzed the environmental and parental components of variation in predator-induced life history (age and size at metamorphosis), morphology (tail depth), and behavior of Italian treefrog tadpoles (Hyla intermedia). Using an incomplete factorial design, we raised tadpoles either with or without caged predators (dragonfly larvae, gen. Aeshna) and, successively, we tested them in experimental arenas either with or without caged predators. Results provided strong evidence for an environmental effect on all three sets of characters. Tadpoles raised with caged predators (dragonfly larvae, gen. Aeshna) metamorphosed earlier (but at a similar body size) and developed deeper tails than their fullsib siblings raised without predators. In the experimental arenas, all tadpoles, independent of their experience, flexibly changed their activity and position, depending on whether the cage was empty or contained the predator. Tadpoles of the two experimental groups, however, showed different responses: those raised with predators were always less active than their predator-naive siblings and differences slightly increased in the presence of predators. Besides this strong environmental component of phenotypic variation, results provided evidence also for parental and parental-by-environment effects, which were strong on life-history, but weak on morphology and behavior. Interestingly, additive parental effects were explained mainly by dams. This supports the hypothesis that phenotypic plasticity might mainly depend on maternal effects and that it might be the expression of condition-dependent mechanisms.

Significance statement

Animals, by plastically adjusting their phenotypes to the local environments, can often sensibly improve their chances of survival, suggesting the hypothesis that phenotypic plasticity evolved by natural selection. We test this hypothesis in the Italian treefrog tadpoles, by investigating the heritable variation in the plastic response to predators (dragonfly larvae). Using an incomplete factorial common-garden experiment, we showed that tadpoles raised with predators metamorphosed earlier (but at similar body size), developed deeper tails, and were less active than their siblings raised without predators. The plastic response varied among families, but variation showed a stronger maternal than paternal component. This suggests that plasticity might largely depend on epigenetic factors and be the expression of condition-dependent mechanisms.

Phylogenetic patterns of trait and trait plasticity evolution: Insights from tadpoles

Environmental heterogeneity has led to the widespread evolution of phenotypic plasticity in all taxonomic groups. Although phenotypic plasticity has been examined from multiple perspectives, few studies have examined evolutionary patterns of plasticity within a phylogeny. We conducted common-garden experiments on 20 species of tadpoles, spanning three families, exposed for 4 weeks to a control, predator cues, or reduced food (i.e., increased intraspecific competition). We quantified tadpole activity, growth, and relative morphology and found widespread differences in species responses to predator cues and reduced food. We detected pervasive phylogenetic signals in traits within each environment, but the phylogenetic signal was much less common in the trait plasticities. Among different models of continuous character evolution, Brownian Motion and Ornstein Uhlenbeck models provided better fits to the data than the Early Burst model. Tadpole activity level in predator environments had much higher evolutionary rates than in the control and reduced-food environments; we did not see this pattern in the other traits. In comparing traits versus trait plasticities, activity evolved much faster than the plasticity of activity whereas morphological traits evolved much slower than morphological plasticities. Collectively, these results suggest that traits and trait plasticities can exhibit dramatically different evolutionary patterns.

Repeatability and heritability of social reaction norms in a wild agamid lizard

In the evolutionary transition from solitary to group living, it should be adaptive for animals to respond to the environment and choose when to socialize to reduce conflict and maximize access to resources. Due to the associated proximate mechanisms (e.g. neural network, endocrine system), it is likely that this behavior varies between individuals according to genetic and non-genetic factors. We used long-term behavioral and genetic data from a population of eastern water dragons (Intellagama lesueurii) to explore variation in plasticity of social behavior, in response to sex ratio and density. To do so, we modeled individual variation in social reaction norms, which describe individuals' mean behavior and behavioral responses to changes in their environment, and partitioned variance into genetic and non-genetic components. We found that reaction norms were repeatable over multiple years, suggesting that individuals consistently differed in their behavioral responses to changes in the social environment. Despite high repeatability of reaction norm components, trait heritability was below our limit of detection based on power analyses (h² < 0.12), leading to very little power to detect heritability of plasticity. This was in contrast to a relatively greater amount of variance associated with environmental effects. This could suggest that mechanisms such as social learning and frequency-dependence may shape variance in reaction norms, which will be testable as the dataset grows.

Nothing as it seems: behavioural plasticity appears correlated with morphology and colour, but is not in a Neotropical tadpole

In response to environmental stressors, organisms often demonstrate flexible responses in morphology, life history or behaviour. However, it is currently unclear if such plastic responses are coordinated or operate independently of one another. In vertebrates, this may partly result from studies examining population- or species-level mean responses, as opposed to finer grained analyses of individuals or families. We measured predator-specific morphological and coloration plasticity in 42 families of tadpoles of the treefrog Dendropsophus ebraccatus and behavioural plasticity from 18 of these families, allowing us to examine the correlation between three predator-induced plastic responses. For all three plastic responses, tadpoles showed strong opposing responses to each of two predators, providing the appearance of covariation in plasticity. However, the examination of individual families revealed a strong correlation between morphological and coloration plasticity, but no correlations between either morphology or colour and behavioural plasticity. Thus, our analysis shows that some aspects of the plastic phenotype develop together while others function independently. This highlights the importance of examining individual- and family-level variation for understanding the adaptive significance of developmental plasticity, which is crucial for a holistic appreciation of phenotypic plasticity and its importance in ecology and evolution.

Tadpoles of hybridising fire-bellied toads (B. bombina and B. variegata) differ in their susceptibility to predation

The role of adaptive divergence in the formation of new species has been the subject of much recent debate. The most direct evidence comes from traits that can be shown to have diverged under natural selection and that now contribute to reproductive isolation. Here, we investigate differential adaptation of two fire-bellied toads (Anura, Bombinatoridae) to two types of aquatic habitat. Bombina bombina and B. variegata are two anciently diverged taxa that now reproduce in predator-rich ponds and ephemeral aquatic sites, respectively. Nevertheless, they hybridise extensively wherever their distribution ranges adjoin. We show in laboratory experiments that, as expected, B. variegata tadpoles are at relatively greater risk of predation from dragonfly larvae, even when they display a predator-induced phenotype. These tadpoles spent relatively more time swimming and so prompted more attacks from the visually hunting predators. We argue in the discussion that genomic regions linked to high activity in B. variegata should be barred from introgression into the B. bombina gene pool and thus contribute to gene flow barriers that keep the two taxa from merging into one.

Right phenotype, wrong place: predator-induced plasticity is costly in a mismatched environment

Like many animals, tadpoles often produce different, predator-specific phenotypes when exposed to risk of predation. It is generally assumed that such plasticity enhances survival in the presence of the predator and is costly elsewhere, but evidence remains surprisingly scarce. We measured (1) the survival trade-off of opposing phenotypes developed by Dendropsophus ebraccatus tadpoles when exposed to different predators and (2) which specific aspects of morphology drive any potential survival benefit or cost. Tadpoles developed predator-specific phenotypes after being reared with caged fish or dragonfly predators for two weeks. In 24 h predation trials with either a fish or a dragonfly, survival was highest in the groups with their matched predator, and lowest among with those the mismatched predator, with predator-naive controls being relatively intermediate. Then, using a large group of phenotypically variable predator-naive tadpoles, we found that increased survival rates are directly related to the morphological changes that are induced by each predator. This demonstrates that induced phenotypes are indeed adaptive and the product of natural selection. Furthermore, our data provide clear evidence of an environmental cost for phenotypic plasticity in a heterogeneous environment. Such costs are fundamental for understanding the evolution and maintenance of inducible phenotypes.