Transgenerational effects and impact of compensatory responses to changes in breeding phenology on antipredator defenses

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.

Parental effects influence life history traits and covary with an environmental cline in common frog populations

Across latitudinal clines, the juvenile developmental rates of ectotherms often covary with the length of the growing season, due to life-history trade-offs imposed by the time-constrained environments. However, as the start of the growing season often varies substantially across years, adaptive parental effects on juvenile developmental rates may mediate the costs of a delayed season. By employing a meta-analysis, we tested whether larval developmental rates across a latitudinal cline of the common frog (Rana temporaria) are affected by fluctuating onsets of breeding, across years. We predicted that larval developmental rate will be inversely related to the onset of breeding, and that northern populations will be more prone to shorten their developmental rate in response to late breeding, as the costs of delayed metamorphosis should be highest in areas with a shorter growing season. We found that the larval period of both northern and southern populations responded to parental environmental conditions to a similar degree in absolute terms, but in different directions. In northern populations, a late season start correlated with decreased development time, suggesting that the evolution of parental effects aids population persistence in time-constrained environments. In southern populations, late season start correlated with increased development time, which could potentially be explained as a predator avoidance strategy. Our findings suggest that local ecological variables can induce adaptive parental effects, but responses are complex, and likely trade-off with other ecological factors.

The effects of different cold-temperature regimes on development, growth, and susceptibility to an abiotic and biotic stressor

Global climate change is expected to both increase average temperatures as well as temperature variability.Increased average temperatures have led to earlier breeding in many spring-breeding organisms. However, individuals breeding earlier will also face increased temperature fluctuations, including exposure to potentially harmful cold-temperature regimes during early developmental stages.Using a model spring-breeding amphibian, we investigated how embryonic exposure to different cold-temperature regimes (control, cold-pulse, and cold-press) affected (a) compensatory larval development and growth, (b) larval susceptibility to a common contaminant, and (c) larval susceptibility to parasites.We found: (a) no evidence of compensatory development or growth, (b) larvae exposed to the cold-press treatment were more susceptible to NaCl at 4-days post-hatching but recovered by 17-days post-hatching, and (c) larvae exposed to both cold treatments were less susceptible to parasites.These results demonstrate that variation in cold-temperature regimes can lead to unique direct and indirect effects on larval growth, development, and response to stressors. This underscores the importance of considering cold-temperature variability and not just increased average temperatures when examining the impacts of climate disruption.

An adaptive transgenerational effect of warming but not of pesticide exposure determines how a pesticide and warming interact for antipredator behaviour

The impact of pesticides on organisms may strongly depend on temperature. While many species will be exposed to pesticides and warming both in the parental and offspring generations, transgenerational effects of pesticides under warming are still poorly studied, particularly for behaviour. We therefore studied the single and combined effects of exposure to the pesticide chlorpyrifos (CPF) and warming both within and across generations on antipredator behaviour of larvae of the vector mosquito Culex pipiens. Within each generation pesticide exposure and warming reduced the escape diving time, making the larvae more susceptible to predation. Pesticide exposure of the parents did not affect offspring antipredator behaviour. Yet, parental exposure to warming determined how warming and the pesticide interacted in the offspring generation. When parents were reared at 24 °C, warming no longer reduced offspring diving times in the solvent control, suggesting an adaptive transgenerational effect to prepare the offspring to better deal with a higher predation risk under warming. Related to this, the CPF-induced reduction in diving time was stronger at 20 °C than at 24 °C, except in the offspring whose parents had been exposed to 24 °C. This dependency of the widespread interaction between warming and pesticide exposure on an adaptive transgenerational effect of warming is an important finding at the interface of global change ecology and ecotoxicology.

Carry-Over Effects Across Metamorphosis of a Pesticide on Female Lifetime Fitness Strongly Depend on Egg Hatching Phenology: A Longitudinal Study under Seminatural Conditions

Current ecological risk assessment of pesticides fails to protect aquatic biodiversity. For the first time, we tested two potential reasons for this failure with regard to carry-over effects across metamorphosis: their dependence on hatching period, and the lack of studies quantifying adult fitness under seminatural conditions. Using the damselfly Coenagrion puella sampled from six populations, we designed an outdoor longitudinal one-year study starting from the egg stage. We exposed the aquatic larvae to the pesticide esfenvalerate (0.11 μg/L) during the initial microcosm part. Next, we monitored the lifetime fitness of the terrestrial adults in an insectary. Exposure to the pesticide negatively impacted not only larval traits, but also drastically reduced lifetime mating success of adult females. The impact of this postmetamorphic effect of the pesticide on the population level was three times more important than the effects in the larval stage. Importantly, this carry-over effect was only present in females that hatched early in the season, and was not mediated by metamorphic traits (age and mass at emergence). We provide proof-of-principle under seminatural conditions for two potential pitfalls that need to be considered when improving risk assessment: carry-over effects on adult fitness can (i) be much more important than effects during the larval stage and may not be captured by metamorphic traits, and (ii) be strongly modulated by egg hatching dates.

Intraspecific priority effects modify compensatory responses to changes in hatching phenology in an amphibian

In seasonal environments, modifications in the phenology of life-history events can alter the strength of time constraints experienced by organisms. Offspring can compensate for a change in timing of hatching by modifying their growth and development trajectories. However, intra- and interspecific interactions may affect these compensatory responses, in particular if differences in phenology between cohorts lead to significant priority effects (i.e. the competitive advantage that early-hatching individuals have over late-hatching ones). Here, we conducted a factorial experiment to determine whether intraspecific priority effects can alter compensatory phenotypic responses to hatching delay in a synchronic breeder by rearing moor frog (Rana arvalis) tadpoles in different combinations of phenological delay and food abundance. Tadpoles compensated for the hatching delay by speeding up their development, but only when reared in groups of individuals with identical hatching phenology. In mixed phenology groups, strong competitive effects by non-delayed tadpoles prevented the compensatory responses and delayed larvae metamorphosed later than in single phenology treatments. Non-delayed individuals gained advantage from developing with delayed larvae by increasing their developmental and growth rates as compared to single phenology groups. Food shortage prolonged larval period and reduced mass at metamorphosis in all treatments, but it did not prevent compensatory developmental responses in larvae reared in single phenology groups. This study demonstrates that strong intraspecific priority effects can constrain the compensatory growth and developmental responses to phenological change, and that priority effects can be an important factor explaining the maintenance of synchronic life histories (i.e. explosive breeding) in seasonal environments.