Abstracts (first author)

Talk 

Phenotypic divergence among spadefoot toad species reflects accommodation of mechanisms underlying developmental plasticity

Author(s): Gomez-Mestre I, Kulkarni S, Buchholz DR

Summary:

Selection in heterogeneous environments favours plasticity as it allows organisms to adapt to rapidly changing conditions. Developmental plasticity allows populations to withstand rapid environmental changes and confers an overall faster rate of adaptation. Conversely, if plasticity costs are high and the environment stabilises, selection results in genetic assimilation, which could result in trait divergence and species diversification. Current evolutionary theory contemplates that phenotypic divergence between species may initiate as environmentally-induced expression of alternative phenotypes. Descendant lineages of a plastic ancestor evolving in stable divergent environments may lose plasticity over time, their development becoming specialised to produce fixed phenotypes matching each environment. In that case, we would expect ancestral plasticity to mirror differences among taxa and that the same mechanism allowing ancestral plasticity was also the main mechanism explainig species divergences. In that light, we are studying mechanisms of plasticity behind the evolutionary divergence of spadefoot toads. Old World species (Pelobates) breed in long lasting ponds and have long but plastic larval periods, whereas New World species (Scaphiopus) have specialised in ephemeral ponds and have evolved very short larval periods. We hypothesise that Scaphiopus has undergone genetic accommodation of ancestral plasticity, which has resulted in canalised short larval periods. To test this hypothesis we have studied the mechanisms underlying developmental acceleration in response to pond drying and compared it across species. We have found that Pelobates tadpoles, which reflect the ancestral state of the group, increase their metabolic rate, and thyroid hormone and corticosterone concentrations in response to decreased water levels. All these parameters, however, seem to have been canalised in Scaphiopus, lending support to the hypothesis of genetic accommodation.



Abstracts (coauthor)

Summary:

The efficiency of induced antipredator responses critically depends upon accurate cue recognition and hence requires some joint predator-prey evolutionary history because otherwise preys may fail to recognize or respond efficiently against alien predators. Thus, amphibian larvae often fail to trigger antipredator responses against invasive predators. Few studies analyzed the ecological consequences of predator-tadpole interactions on the rest of the aquatic communities of temporary ponds, but amphibians’ impact on the community structure and dynamics of aquatic systems may be conditioned by their interactions with competitors and predators. Thus, predators can potentially have cascading effects on pond communities via alterations in amphibian larval density and/or their phenotype. We used a mesocosm array at Doñana National Park to test for density dependent and density-independent effects of native and invasive predators on survival and growth rate of amphibian larvae, also quantifying its consequences on the food web. Invasive predators caused greater mortalities of amphibian larvae than native predators, hence having a stronger potential for direct density-dependent effects on amphibian guilds and their interactions in the trophic web. Moreover, crayfish increased turbidity and nutrient content in the water and had a direct negative impact on plants that was carried over into the following hydrological cycle. Thus, our results highlight how this invasive species poses a greater predatory threat to amphibians than other native predators, and can also be highly disruptive of the entire pond structure.

Summary:

Chemical detection of predator cues is crucial for aquatic prey, because it allows predator avoidance and activation of plastic antipredatory defenses. This is the case of many larval amphibians that respond to water-borne cues from potential predators by strongly reducing activity levels. However, as a consequence of increasing anthropogenic activity, a variety of harmful contaminants are dumped in freshwater ecosystems, where they can create interferences in the cue recognition system of tadpoles. Here we analyze the potential effects of two contaminants (i. e., humic acid and ammonium nitrate) on the ability of tadpoles of the western spadefoot toad (Pelobates cultripes) to recognize chemical cues from a common predator, nymphs of the dragonfly Anax imperator. We compared swimming activity of tadpoles in presence and absence of water-borne chemical cues from dragonflies, at different concentrations of humic acid and ammonium nitrate. Changes in tadpole activity associated to toxicity were non-significant. In contrast, the interaction between predator cues and presence of contaminants was significant: tadpoles effectively responded to predator cues in the absence of pollutants by reducing their swimming activity, but remained unresponsive to predator cues when either humic acid or ammonium nitrate were added, even at low concentrations. These interferences due to chemical anthropogenic pollution may pose a threat to the cue recognition systems of prey-predator interactions evolved in amphibian populations.

Summary:

Phenotypic plasticity is the ability of a genotype to modify its phenotype in response to reliable environmental cues. The knowledge of the mechanisms involved in plasticity is the key to understand its ecological, developmental and evolutionary consequences. In the last decade, epigenetic changes have been pointed out as explanations for several phenotypically plastic changes. In particular, DNA methylation has been linked to phenotypic plasticity and evidence is quickly accumulating across different taxa. Here we tested if the predator-induced phenotype observed in many amphibian larvae was the consequence of changes in gene expression derived from changes in the pattern of DNA methylation. Tadpoles produce anti-predator phenotypes that include a conspicuous increase in relative tail depth and a reduction of activity rate. To test if these plastic responses are epigenetically regulated, we exposed Pelobates cultripes tadpoles to the presence or absence of chemical cues from predators, while previously treating or not tadpoles with 5-azacytidine, a DNA methylation inhibitor, in a 2x2 factorial experiment. Our experimental treatments were: control (clean water), predator cues (dragonfly nymphs, Anax imperator), 5-azacytidine, and 5-azacytidine plus predator cues. Tadpoles exposed to predator cues showed the expected antipredator morphology, whereas tadpoles exposed to predator cues but earlier treated with 5-azacytidine showed an intermediate phenotype between the control and predator exposed tadpoles. They also showed a substantial decrease in activity rate as compared to control tadpoles and even to predator-exposed tadpoles. We are currently undertaking a molecular analysis with methylation-sensitive amplified length polymorphism (MSAPs), to quantify the changes in epigenetic marks. In view of these results, it seems like epigenetic regulation of predator-induced phenotypic changes in Pelobates tadpoles may be at least partially driven by DNA-methylation.

Summary:

Alien predators are one of the major causes of rapid decline and extinction of native species, because they often create novel ecological contexts in which the antipredatory responses of native organisms are no longer adaptive. The red swamp crayfish, Procambarus clarkii, is a harmful invasive species in aquatic systems worldwide that is causing great ecological impact on native amphibian populations through intense predation of eggs and tadpoles. Larval amphibians are often capable of innately responding to the presence of chemical cues from local predators through changes in morphology and behaviour. Nonetheless, naïve tadpoles are often incapable of recognising alien predators with whom they have no shared evolutionary history. For this reason, amphibian species are especially vulnerable to the introduction of new predators. However, given enough time, native populations might acquire, by means of rapid adaptation or the evolution of phenotypic plasticity, the ability to escape from alien predators. Here we show that naïve tadpoles of the western spadefoot toad, Pelobates cultripes, cannot innately recognise water-borne cues from P. clarkii. Nevertheless, we also show that P. cultripes tadpoles can learn to recognise the cues of this alien predator as a threat when they are exposed to predator cues combined with conspecific alarm cues. Furthermore, we show that tadpoles that learned to recognise the new predator experienced higher survival during staged predation trials with invasive crayfish. This cognitive ability of tadpoles might be critical for amphibian populations to trigger inducible defences against alien predators, thus tempering the immediate impact of invasions through behavioural plasticity and persist long enough for genetic variants to appear and respond to selection.

Contacts

Chairman: Octávio S. Paulo
Tel: 00 351 217500614 direct
Tel: 00 351 217500000 ext22359
Fax: 00 351 217500028
email: mail@eseb2013.com

Address

XIV Congress of the European Society for Evolutionary Biology

Organization Team
Department of Animal Biology (DBA)
Faculty of Sciences of the University of Lisbon
P-1749-016 Lisbon
Portugal

Website

Computational Biology & Population Genomics Group 
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