Beck Christopher W
Institute of Ecology, University of Georgia, Athens, GA 30602, USA Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29801, USA e-mail:
Oecologia. 1997 Oct;112(2):187-192. doi: 10.1007/s004420050299.
Recent experiments suggest that timing of metamorphosis is fixed during development in some anurans, insects, and freshwater invertebrates. Yet, these experiments do not exclude a growth rate optimization model for the timing of metamorphosis. I manipulated food resources available to larvae of squirrel treefrogs (Hyla squirella) to determine if there is a loss of plasticity in duration of larval period during development and to critically test growth rate models for the timing of metamorphosis. Size-specific resource levels for individual tadpoles were switched from low to high or high to low at three developmental stages spaced throughout larval development. The effects of changes in resource availability on larval period and mass at metamorphosis were measured. Switching food levels after late limb bud development did not significantly affect larval period in comparison to constant food level treatments. Therefore, developmental rate in H. squirella is better described by a fixed developmental rate model, rather than a growth rate optimization model. The timing of fixation of developmental rate in H. squirella is similar to that found in other anuran species, suggesting a taxonomically widespread developmental constraint on the plasticity of larval period duration. Mass at metamorphosis was not significantly affected by the timing of changes in food levels; the amount of food available later in development determined the size at metamorphosis. Larval period and mass at metamorphosis were negatively correlated in only one of two experiments, which contrasts with the common assumption of a phenotypic trade-off between decreased larval period and increased mass at metamorphosis.
近期的实验表明，在某些无尾目动物、昆虫和淡水无脊椎动物的发育过程中，变态的时间是固定的。然而，这些实验并未排除变态时间的生长速率优化模型。我对松鼠树蛙（Hyla squirella）幼体可获得的食物资源进行了操控，以确定在发育过程中幼体期的持续时间是否存在可塑性丧失，并严格检验变态时间的生长速率模型。在整个幼体发育过程中分布的三个发育阶段，将单个蝌蚪的特定大小资源水平从低切换到高或从高切换到低。测量了资源可用性变化对幼体期和变态时体重的影响。与恒定食物水平处理相比，在四肢芽后期发育后切换食物水平对幼体期没有显著影响。因此，松鼠树蛙的发育速率用固定发育速率模型来描述更好，而不是生长速率优化模型。松鼠树蛙发育速率固定的时间与在其他无尾目物种中发现的时间相似，这表明在幼体期持续时间的可塑性方面存在一个分类学上广泛存在的发育限制。变态时的体重并未受到食物水平变化时间的显著影响；发育后期可获得的食物量决定了变态时的大小。在两个实验中，只有一个实验显示幼体期和变态时的体重呈负相关，这与幼体期缩短和变态时体重增加之间存在表型权衡的常见假设形成对比。
