A környezet hatása kétéltűek kémiai védekezésének változatosságára

A fenotípusos plaszticitás, tehát a genotípus azon képessége, hogy különböző környezetekben különböző fenotípusokat hozzon létre, általánosan elterjedt az élővilágban. Jól ismert, hogy az élőlények módosíthatják viselkedésüket, morfológiájukat és különböző életmenet jellemzőiket ily módon, hogy a környezet kihívásaihoz alkalmazkodjanak. A fajok közötti interakció egy, ökológiailag és evolúciósan is fontos adaptációja a mérgező anyagok felhalmozása és használata, amely ugyancsak széleskörűen elterjedt az élet összes doménjéban. Kémiai védekezésnek nevezzük, ha az élőlények az ilyen anyagokat önvédelemre használják fel ragadozók vagy más biotikus környezeti faktorok ellen. Az azonban ritkán kutatott jelenség, hogy az állatok képesek-e környezeti veszélyekre plasztikusan reagálni a méreganyagaik szintézisének fokozásával vagy azok szelektív kifejezésével. Ebben a disszertációban azt vizsgálom, hogy a barna varangy (Bufo bufo) ebihalai képesek-e különböző környezeti hatásokra főbb méreganyagaik, a bufadienolidok fokozott termelésével válaszolni. Azért, hogy tanulmányozzam ezt a kérdést, laboratóriumi és mezokozmosz kísérleteket végeztem az ebihalakkal, melyekben manipuláltam a ragadozók, a rájuk utaló ingerek, illetve versenytársak jelenlétét. Bizonyítom, hogy a varangyok már korai fejlődési stádiumaiktól kezdve képesek a bufadienolidok aktív termelésére, valamint, hogy csökkentett táplálékmennyiség, illetve versenytársak és ragadozók jelenléte plasztikus kémiai védekezést vált ki az ebihalakban. Tárgyalom e jelenség ökológiai és evolúciós vonatkozásait is.A widespread phenomenon in nature is the ability of a given genotype to produce different phenotypes under different environmental conditions, called phenotypic plasticity. It is well known that organisms can adjust their behaviour, morphology and various life-history traits to adapt to environmental challenges. An ecologically and evolutionary important adaptation in interspecific interactions is the accumulation and use of toxic substances, which is also widespread in all domains of life. The use of such compounds for self-defence against predators or other biotic factors is called chemical defence. It is scarcely studied, however, if animals are capable of displaying plastic responses to threats by upregulating the synthesis of toxins or selectively expressing them. In this dissertation I examine if tadpoles of the common toad (Bufo bufo) are able to plastically respond to different environmental stimuli by increasing the synthesis of their main toxin compounds, the bufadienolides. To investigate this topic I conducted laboratory and mesocosm based experiments on tadpoles of this species and manipulated their exposure to predation threat or competitors. I present evidence that common toad tadpoles are able to actively synthesise bufadienolides from early on in their life, furthermore that they display plastic chemical defences in response to reduced food levels and the presence of conspecific tadpoles and predators. Ecological and evolutionary implications of these findings are discussed

A környezet hatása kétéltűek kémiai védekezésének változatosságára

A widespread phenomenon in nature is the ability of a given genotype to produce different phenotypes under different environmental conditions, called phenotypic plasticity. It is well known that organisms can adjust their behaviour, morphology and various life-history traits to adapt to environmental challenges. An ecologically and evolutionary important adaptation in interspecific interactions is the accumulation and use of toxic substances, which is also widespread in all domains of life. The use of such compounds for self-defence against predators or other biotic factors is called chemical defence. It is scarcely studied, however, if animals are capable of displaying plastic responses to threats by upregulating the synthesis of toxins or selectively expressing them. In this dissertation I examine if tadpoles of the common toad (Bufo bufo) are able to plastically respond to different environmental stimuli by increasing the synthesis of their main toxin compounds, the bufadienolides. To investigate this topic I conducted laboratory and mesocosm based experiments on tadpoles of this species and manipulated their exposure to predation threat or competitors. I present evidence that common toad tadpoles are able to actively synthesise bufadienolides from early on in their life, furthermore that they display plastic chemical defences in response to reduced food levels and the presence of conspecific tadpoles and predators. Ecological and evolutionary implications of these findings are discussed.A fenotípusos plaszticitás, tehát a genotípus azon képessége, hogy különböző környezetekben különböző fenotípusokat hozzon létre, általánosan elterjedt az élővilágban. Jól ismert, hogy az élőlények módosíthatják viselkedésüket, morfológiájukat és különböző életmenet jellemzőiket ily módon, hogy a környezet kihívásaihoz alkalmazkodjanak. A fajok közötti interakció egy, ökológiailag és evolúciósan is fontos adaptációja a mérgező anyagok felhalmozása és használata, amely ugyancsak széleskörűen elterjedt az élet összes doménjéban. Kémiai védekezésnek nevezzük, ha az élőlények az ilyen anyagokat önvédelemre használják fel ragadozók vagy más biotikus környezeti faktorok ellen. Az azonban ritkán kutatott jelenség, hogy az állatok képesek-e környezeti veszélyekre plasztikusan reagálni a méreganyagaik szintézisének fokozásával vagy azok szelektív kifejezésével. Ebben a disszertációban azt vizsgálom, hogy a barna varangy (Bufo bufo) ebihalai képesek-e különböző környezeti hatásokra főbb méreganyagaik, a bufadienolidok fokozott termelésével válaszolni. Azért, hogy tanulmányozzam ezt a kérdést, laboratóriumi és mezokozmosz kísérleteket végeztem az ebihalakkal, melyekben manipuláltam a ragadozók, a rájuk utaló ingerek, illetve versenytársak jelenlétét. Bizonyítom, hogy a varangyok már korai fejlődési stádiumaiktól kezdve képesek a bufadienolidok aktív termelésére, valamint, hogy csökkentett táplálékmennyiség, illetve versenytársak és ragadozók jelenléte plasztikus kémiai védekezést vált ki az ebihalakban. Tárgyalom e jelenség ökológiai és evolúciós vonatkozásait is

Experimental evidence for beneficial effects of projected climate change on hibernating amphibians

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Experimental evidence for beneficial effects of projected climate change on hibernating amphibians

Amphibians are the most threatened vertebrates today, experiencing worldwide declines. In recent years considerable effort was invested in exposing the causes of these declines. Climate change has been identified as such a cause; however, the expectable effects of predicted milder, shorter winters on hibernation success of temperate-zone Amphibians have remained controversial, mainly due to a lack of controlled experimental studies. Here we present a laboratory experiment, testing the effects of simulated climate change on hibernating juvenile common toads (Bufo bufo). We simulated hibernation conditions by exposing toadlets to current (1.5 °C) or elevated (4.5 °C) hibernation temperatures in combination with current (91 days) or shortened (61 days) hibernation length. We found that a shorter winter and milder hibernation temperature increased survival of toads during hibernation. Furthermore, the increase in temperature and shortening of the cold period had a synergistic positive effect on body mass change during hibernation. Consequently, while climate change may pose severe challenges for amphibians of the temperate zone during their activity period, the negative effects may be dampened by shorter and milder winters experienced during hibernation

Endocrine disruptors in breeding ponds and reproductive health of toads in agricultural, urban and natural landscapes

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Novel genetic sex markers reveal unexpected lack of, and similar susceptibility to, sex reversal in free-living common toads in both natural and anthropogenic habitats

Anthropogenic environmental changes are affecting biodiversity and microevolution worldwide. Ectothermic vertebrates are especially vulnerable because environmental changes can disrupt their sexual development and cause sex reversal, a mismatch between genetic and phenotypic sex. This can potentially lead to sex‐ratio distortion and population decline. Despite these implications, there is scarce empirical knowledge on the incidence of sex reversal in nature. Populations in anthropogenic environments may be exposed to sex‐reversing stimuli more frequently, which may lead to higher sex‐reversal rate or, alternatively, these populations may adapt to resist sex reversal. We developed PCR‐based genetic sex markers for the common toad (Bufo bufo) to assess the prevalence of sex reversal in wild populations living in natural, agricultural and urban habitats, and the susceptibility of the same populations to two ubiquitous oestrogenic pollutants in a common garden experiment. We found negligible sex‐reversal frequency in free‐living adults despite the presence of various endocrine‐disrupting pollutants in their breeding ponds. Individuals from different habitat types showed similar susceptibility to sex reversal in the laboratory: all genetic males developed female phenotype when exposed to 1 µg L(−1) 17α‐ethinylestradiol (EE2) during larval development, whereas no sex reversal occurred in response to 1 ng L(−1) EE2 and a glyphosate‐based herbicide with 3 µg L(−1) or 3 mg L(−1) glyphosate. The latter results do not support that populations in anthropogenic habitats would have either increased propensity for or higher tolerance to chemically induced sex reversal. Thus, the extremely low sex‐reversal frequency in wild toads compared to other ectothermic vertebrates studied before might indicate idiosyncratic, potentially species‐specific resistance to sex reversal

Does The Glucocorticoid Stress Response Make Toads More Toxic? An Experimental Study on The Regulation of Bufadienolide Toxin Synthesis

Chemical defense is a crucial component of fitness in many organisms, yet the physiological regulation of defensive toxin synthesis is poorly understood, especially in vertebrates. Bufadienolides, the main defensive compounds of toads, are toxic to many predators and other natural enemies, and their synthesis can be upregulated by stressors, including predation risk, high conspecific density, and pollutants. Thus, higher toxin content may be the consequence of a general endocrine stress response in toads. Therefore, we hypothesized that bufadienolide synthesis may be stimulated by elevated levels of corticosterone (CORT), the main glucocorticoid hormone of amphibians, or by upstream regulators that stimulate CORT production. To test these alternatives, we treated common toad tadpoles with exogenous CORT (exoCORT) or metyrapone (MTP, a CORT-synthesis inhibitor that stimulates upstream regulators of CORT by negative feedback) in the presence or absence of predation cues for 2 or 6 days, and subsequently measured their CORT release rates and bufadienolide content. We found that CORT release rates were elevated by exoCORT, and to a lesser extent also by MTP, regardless of treatment length. Bufadienolide content was significantly decreased by treatment with exoCORT for 6 days but was unaffected by exposure to exoCORT for 2 days or to MTP for either 6 or 2 days. The presence or absence of predation cues affected neither CORT release rate nor bufadienolide content. Our results suggest that changes in bufadienolide synthesis in response to environmental challenges are not driven by CORT but may rather be regulated by upstream hormones of the stress response

Chemical defence effective against multiple enemies: Does the response to conspecifics alleviate the response to predators?

1. Animals living in groups with high conspecific densities typically decrease their level of plastic anti-predatory defence because its benefits diminish with reduced per capita predation risk (a benefit of aggregation), whereas its costs increase due to intensifying competition and increased infection risk. Furthermore, phenotypic responses that provide protection from predators are also often disadvantageous against competitors and infections. 2. Such a trade-off may be absent when the same phenotype provides an effective defence against both predators and competitors, as is the case with some chemical defences. For such multifunctional defensive traits, both predation risk and high conspecific density are expected to increase defence expression while exposure to both predators and conspecifics may result in non-additive effects whereby the defence level induced by two enemies is lower than the sum of responses induced by either of them alone. 3. We tested this theoretical prediction by studying the effects of multiple enemies on chemical defence in a vertebrate animal. We investigated patterns of change in toxin production of common toad Bufo bufo tadpoles following exposure to different conspecific densities and the simultaneous presence or absence of chemical cues on predation risk. 4. We found that tadpoles significantly increased their production of bufadienolide toxins in response to high tadpole density, as well as to predation risk when tadpole density was low. Although the response in bufadienolide production to predation risk was not significant at high tadpole density, the magnitude of anti-predatory response did not differ significantly between low and high tadpole densities. 5. These results show that toad tadpoles adjust their chemical defence to conspecific density and to predation risk simultaneously, and these two effects are more likely additive than non-additive, at least within the range of densities and predation-risk levels studied here. Nevertheless, the trend we found suggests that toxin levels induced by very high conspecific density might weaken the chemical response to predators, which is relevant for the evolutionary ecology of chemical defences, as well as for the conservation of fauna impacted by toxic invaders. A free Plain Language Summary can be found within the Supporting Information of this article

Metamorphic common toads keep chytrid infection under control, but at a cost

Batrachochytrium dendrobatidis (Bd) causes chytridiomycosis, an infectious disease of amphibians, which has contributed to population declines in hundreds of species worldwide. Common toads (Bufo bufo) exhibit low resistance and relatively high tolerance to Bd infection, which may partly be attributable to bufadienolide toxins secreted in their granular skin glands. Bufadienolides are known to provide an effective defense against several pathogens, parasites, and predators. The toxin production of bufonids is a plastic trait, inducible by several environmental factors. Here, we experimentally infected juvenile common toads with Bd and investigated if the toadlets could clear the infection over time, whether the infection induced bufadienolide production, and whether the infection caused decreased body mass. We found that prevalence remained 100% throughout the entire experimental period, but infection intensity did not increase and it was significantly lower on day 30 than on day 20. At the same time, compared to controls, infected toadlets produced lesser amounts of bufadienolides and their body mass was also lower. These results suggest that although young toadlets may not be able to clear Bd infection on their own, they may be able to keep infection intensities under control. Nonetheless, even if toadlets do not succumb to the disease, the costs of chronic infection may still compromise their fitness