Takahashi Mizuki K, Parris Matthew J
Department of Biology, University of Memphis, Memphis, TN, 38152, USA.
Oecologia. 2008 Nov;158(1):23-34. doi: 10.1007/s00442-008-1125-8. Epub 2008 Aug 16.
Polyphenism, which allows a single genotype to express multiple discrete phenotypes in response to environmental cues, is an adaptive trait in heterogeneous environments. Pond hydroperiod is an important ecological parameter affecting amphibian life history, and variation in local pond hydrology has been hypothesized to play a role in species divergence via changes in polyphenism. The eastern newt (Notophthalmus viridescens) expresses life cycle polyphenism. Larvae develop along three possible pathways: metamorphosis to aquatic lunged adult via a terrestrial juvenile (eft) stage, metamorphosis directly to an aquatic lunged adult, or maturation directly to an aquatic gilled adult without metamorphosis (i.e., paedomorphosis). Subspecies of N. viridescens vary in their polyphenic patterns, suggesting possible adaptation to different environments. However, no studies have experimentally tested how genetic and environmental components contribute to the observed differences among subspecies and whether such differences may facilitate divergence. We tested whether adaptation to local pond hydrology via polyphenic changes existed among subspecies by rearing larvae of three subspecies (N. v. dorsalis, N. v. louisianensis, and N. v. viridescens) along three hydroperiod regimes (short, long, and constant) in outdoor artificial ponds. We found that larval N. v. viridescens obligately metamorphosed to efts under all hydroperiods, whereas N. v. dorsalis and N. v. louisianensis exhibited plasticity: larvae metamorphosed to efts under drying conditions but metamorphosed directly to aquatic adults or became paedomorphic in constant water. Also, N. v. viridescens metamorphosed to efts faster and at a smaller body size than the other two subspecies. These data suggest that subspecies of N. viridescens are adapted to different pond hydroperiods, supporting the potential for polyphenism to facilitate divergence. Canalizing selection for certain alternative phenotypes within a single species in which other populations remain plastic may play an important role in the initiation of ecological divergence.
多型现象允许单一基因型根据环境线索表达多种离散的表型，是异质环境中的一种适应性状。池塘水文周期是影响两栖动物生活史的一个重要生态参数，据推测，当地池塘水文变化通过多型现象的改变在物种分化中发挥作用。东方蝾螈（Notophthalmus viridescens）表现出生命周期多型现象。幼虫沿着三种可能的途径发育：通过陆生幼体（ eft）阶段变态为水生有肺成体，直接变态为水生有肺成体，或直接成熟为无变态的水生有鳃成体（即幼态持续）。绿红东美螈的亚种在其多型模式上存在差异，表明可能适应不同的环境。然而，尚无研究通过实验测试遗传和环境成分如何导致亚种间观察到的差异，以及这些差异是否可能促进分化。我们通过在室外人工池塘中沿着三种水文周期模式（短、长和恒定）饲养三个亚种（绿红东美螈指名亚种N. v. dorsalis、绿红东美螈路易斯安那亚种N. v. louisianensis和绿红东美螈指名亚种N. v. viridescens）的幼虫，来测试亚种间是否存在通过多型变化对当地池塘水文的适应性。我们发现，绿红东美螈指名亚种的幼虫在所有水文周期下都必然变态为eft，而绿红东美螈指名亚种和绿红东美螈路易斯安那亚种表现出可塑性：幼虫在干燥条件下变态为eft，但在恒定水体中直接变态为水生成体或幼态持续。此外，绿红东美螈指名亚种比其他两个亚种更快且以更小的体型变态为eft。这些数据表明，绿红东美螈的亚种适应不同的池塘水文周期，支持了多型现象促进分化的可能性。在一个物种中，对某些替代表型进行定向选择，而其他种群保持可塑性，这可能在生态分化的起始中起重要作用。
