Does Development Bias the Evolution of Beetle Horns?

许多种类的甲虫都有“角”，这种角只由雄性产生。这些角可能是极端的（像麋鹿的角一样引人注目），并且雄性对角生产的投资被认为是大量的。在许多种类的甲虫中，只有一些雄性长角。其他的雄性则完全无家可归。在食蚁兽属（鞘翅目：金龟科）的大多数物种中，雄性角的表达是可选的，取决于雄性是否长得大于一个阈值的体型：大于这个体型的雄性会长角，而没有达到这个体型的雄性则不会。雄性角的兼性表达提供了一个不同寻常的机会来研究角是如何发育的，以及雄性产生角是否“昂贵”。先前的研究表明，雄性将发育资源分配给角生产，被迫通过减少分配给其他特征（包括眼睛）来补偿。这表明，在甲虫发育过程中，可能存在分配权衡，限制了甲虫形状的范围，这也可能限制了属内的进化方向。在这里，PI扩展了他之前的工作：(1)使用发育幼虫的操纵实验来证实资源分配权衡的存在和性质；(2)检验这种机制所预测的偏差是否在分类群之间的形状变化中表现出来。这项研究将加深我们对发育机制和进化变化之间关系的理解，并提高我们对甲虫角等夸张第二性特征产生的“成本”的理解。

Emlen 9807932 Many beetle species have "horns" which are only produced by males. These horns can be extreme (as dramatic as the antlers of elk), and investment by males into the production of horns is presumed to be substantial. In many beetle species only some of the males produce horns. other males being completely homless. In most species of the genus Onthophagus (Coleoptera: Scarabaeidae), male horn expression is facultative, and depends on whether or not a male grows larger than a threshold body size: males growing larger than this size produce horns, whereas males not attaining this size do not. Facultative expression of male horns provides an unusual opportunity to study how horns develop, and whether horns are "costly" to males to produce. Prior research revealed that males allocating developmental resources to horn production were forced to compensate by reducing allocation to other traits, including eyes. This suggests that there may be allocation tradeoffs operating during beetle development which restrict the range of beetle shapes that can be produced, and these may also restrict the directions of evolution within the genus. Here the PI extends his prior work by (1) using manipulative experiments on developing larvae to confirm the existence and nature of resource allocation tradeoffs, and (2) testing whether the biases predicted to arise from such a mechanism are manifest in the shape variation among taxa. This study will enhance our understanding of the relationship between developmental mechanisms and evolutionary change, as well as improve our understanding of the "costs" associated with the production of exaggerated secondary sexual characters like beetle horns.