Lehrstuhl für Zoologie und Evolutionsbiologie, Department of Biology, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany.
BMC Evol Biol. 2011 Apr 30;11:116. doi: 10.1186/1471-2148-11-116.
Phenotypic evolution and its role in the diversification of organisms is a central topic in evolutionary biology. A neglected factor during the modern evolutionary synthesis, adaptive phenotypic plasticity, more recently attracted the attention of many evolutionary biologists and is now recognized as an important ingredient in both population persistence and diversification. The traits and directions in which an ancestral source population displays phenotypic plasticity might partly determine the trajectories in morphospace, which are accessible for an adaptive radiation, starting from the colonization of a novel environment. In the case of repeated colonizations of similar environments from the same source population this "flexible stem" hypothesis predicts similar phenotypes to arise in repeated subsequent radiations. The Midas Cichlid (Amphilophus spp.) in Nicaragua has radiated in parallel in several crater-lakes seeded by populations originating from the Nicaraguan Great Lakes. Here, we tested phenotypic plasticity in the pharyngeal jaw of Midas Cichlids. The pharyngeal jaw apparatus of cichlids, a second set of jaws functionally decoupled from the oral ones, is known to mediate ecological specialization and often differs strongly between sister-species.
We performed a common garden experiment raising three groups of Midas cichlids on food differing in hardness and calcium content. Analyzing the lower pharyngeal jaw-bones we find significant differences between diet groups qualitatively resembling the differences found between specialized species. Observed differences in pharyngeal jaw expression between groups were attributable to the diet's mechanical resistance, whereas surplus calcium in the diet was not found to be of importance.
The pharyngeal jaw apparatus of Midas Cichlids can be expressed plastically if stimulated mechanically during feeding. Since this trait is commonly differentiated--among other traits--between Midas Cichlid species, its plasticity might be an important factor in Midas Cichlid speciation. The prevalence of pharyngeal jaw differentiation across the Cichlidae further suggests that adaptive phenotypic plasticity in this trait could play an important role in cichlid speciation in general. We discuss several possibilities how the adaptive radiation of Midas Cichlids might have been influenced in this respect.
表型进化及其在生物多样性中的作用是进化生物学的核心议题。在现代进化综合中被忽视的因素，适应性表型可塑性，最近引起了许多进化生物学家的关注，现在被认为是种群维持和多样化的重要组成部分。祖先源种群表现出表型可塑性的特征和方向可能部分决定了形态空间中的轨迹，这些轨迹对于从新环境的殖民化开始的适应性辐射是可及的。在同一源种群多次殖民类似环境的情况下，这个“灵活茎”假说预测在随后的多次辐射中会出现相似的表型。尼加拉瓜的 Midas 慈鲷（Amphilophus spp.）在几个由源自尼加拉瓜大湖的种群播种的火山口湖中平行辐射。在这里，我们测试了 Midas 慈鲷咽颚的表型可塑性。慈鲷的咽颚器官，是与口腔分离的第二套颚，已知介导生态特化，并且通常在姐妹种之间存在很大差异。
我们进行了一项共同花园实验，用硬度和钙含量不同的食物饲养三组 Midas 慈鲷。分析下咽颚骨，我们发现实验组之间存在显著差异，定性上类似于在特化物种中发现的差异。组间咽颚表达的差异归因于饮食的机械阻力，而饮食中多余的钙并不重要。
如果在进食时受到机械刺激，Midas 慈鲷的咽颚器官可以表现出可塑性。由于这个特征通常在 Midas 慈鲷物种之间存在分化——除了其他特征之外——它的可塑性可能是 Midas 慈鲷物种形成的一个重要因素。Cichlidae 中的咽颚分化普遍存在，这进一步表明，这种特征的适应性表型可塑性可能在慈鲷物种形成中起着重要作用。我们讨论了几个可能性，说明 Midas 慈鲷的适应性辐射在这方面可能受到了怎样的影响。
