Üveges Bálint, Fera Gábor, Móricz Ágnes M, Krüzselyi Dániel, Bókony Veronika, Hettyey Attila
Lendület Evolutionary Ecology Research Group, Plant Protection Institute, Centre for Agricultural Research, Hungarian Academy of Sciences, Herman Ottó út 15, Budapest, 1022, Hungary.
Department of Pathophysiology, Plant Protection Institute, Centre for Agricultural Research, Hungarian Academy of Sciences, Herman Ottó út 15, Budapest, 1022, Hungary.
BMC Evol Biol. 2017 Jun 13;17(1):137. doi: 10.1186/s12862-017-0956-5.
Chemical defences are widespread in animals, but how their production is adjusted to ecological conditions is poorly known. Optimal defence theory predicts that inducible defences are favoured over constitutive defences when toxin production is costly and the need for it varies across environments. However, if some environmental changes occur predictably (e.g. coupled to transitions during ontogeny), whereas others are unpredictable (e.g. predation, food availability), changes in defences may have constitutive as well as plastic elements. To investigate this phenomenon, we raised common toad (Bufo bufo) tadpoles with ad libitum or limited food and in the presence or absence of chemical cues on predation risk, and measured their toxin content on 5 occasions during early ontogeny.
The number of compounds showed limited variation with age in tadpoles and was unaffected by food limitation and predator cues. The total amount of bufadienolides first increased and later decreased during development, and it was elevated in young and mid-aged tadpoles with limited food availability compared to their ad libitum fed conspecifics, whereas it did not change in response to cues on predation risk. We provide the first evidence for the active synthesis of defensive toxin compounds this early during ontogeny in amphibians. Furthermore, the observation of increased quantities of bufadienolides in food-restricted tadpoles is the first experimental demonstration of resource-dependent induction of elevated de novo toxin production, suggesting a role for bufadienolides in allelopathy.
Our study shows that the evolution of phenotypic plasticity in chemical defences may depend on the ecological context (i.e. predation vs. competition). Our results furthermore suggest that the age-dependent changes in the diversity of toxin compounds in developing toads may be fixed (i.e., constitutive), timed for the developmental stages in which they are most reliant on their chemical arsenal, whereas inducible plasticity may prevail in the amount of synthesized compounds.
化学防御在动物中广泛存在，但人们对其产生过程如何根据生态条件进行调整却知之甚少。最优防御理论预测，当毒素产生成本高昂且不同环境下对毒素的需求不同时，诱导性防御比组成性防御更具优势。然而，如果某些环境变化是可预测的（例如与个体发育过程中的转变相关），而其他变化是不可预测的（例如捕食、食物可获得性），那么防御的变化可能既有组成性成分，也有可塑性成分。为了研究这一现象，我们将普通蟾蜍（Bufo bufo）蝌蚪饲养在食物充足或有限的环境中，并使其处于有或没有捕食风险化学信号的环境中，在个体发育早期的5个时间点测量它们的毒素含量。
蝌蚪体内化合物的数量随年龄变化有限，且不受食物限制和捕食者信号的影响。蟾毒内酯的总量在发育过程中先增加后减少，与食物充足的同物种蝌蚪相比，食物有限的幼年和中年蝌蚪体内的蟾毒内酯含量升高，而其含量并未因捕食风险信号而改变。我们首次提供了两栖动物在个体发育早期就积极合成防御性毒素化合物的证据。此外，在食物受限的蝌蚪中观察到蟾毒内酯数量增加，这是首次通过实验证明资源依赖诱导从头合成毒素增加，表明蟾毒内酯在化感作用中发挥作用。
我们的研究表明，化学防御中表型可塑性的进化可能取决于生态背景（即捕食与竞争）。我们的结果还表明，发育中的蟾蜍体内毒素化合物多样性随年龄的变化可能是固定的（即组成性的），在它们最依赖化学武器库的发育阶段定时出现，而合成化合物的数量可能主要表现为诱导性可塑性。
