Division of Biology, Kansas State University, Manhattan, KS 66506, USA.
Curr Biol. 2009 Dec 29;19(24):2057-65. doi: 10.1016/j.cub.2009.11.014. Epub 2009 Dec 10.
The vast diversity in morphology of insect wings provides an excellent model to study morphological evolution. The best-described wing modification is the specification of halteres in Drosophila by a Hox-dependent mechanism, in which a Hox gene affects the expression of genes important for wing development to modify the resulting structure. We have previously shown that highly modified beetle elytra are Hox-free structures despite their divergent morphology, suggesting another mode of evolutionary modification.
To understand how elytra have evolved without Hox input, we have analyzed wing development in a coleopteran, Tribolium castaneum. Based on Drosophila mutant phenotypes, we first hypothesized that changes in the wing gene network might have contributed to elytral evolution. However, we found that the wing gene network defined in Drosophila is largely conserved in Tribolium and is also used to pattern the elytra. Instead, we found evidence that the exoskeleton formation has been co-opted downstream of the conserved wing gene network multiple times. We also show evidence that one of these co-options happened prior to the others, suggesting that repeated co-options may have strengthened an advantageous trait. In addition, we found that the Tribolium apterous genes are not only essential for exoskeletalization of the elytra but also are required for the proper identity of the hindwing-an unexpected role that we find to be conserved in Drosophila.
Our findings suggest that elytral evolution has been achieved by co-opting a beneficial trait several times while conserving the main framework of wing patterning genes.
昆虫翅膀的形态多样性为研究形态进化提供了极好的模型。描述得最好的翅膀修饰是果蝇中由 Hox 依赖性机制指定平衡棒，其中 Hox 基因影响对翅膀发育很重要的基因的表达，从而修饰产生的结构。我们之前已经表明，尽管高度修饰的甲虫鞘翅具有不同的形态，但它们是无 Hox 的结构，这表明了另一种进化修饰的模式。
为了了解没有 Hox 输入的情况下鞘翅是如何进化的，我们分析了鞘翅目昆虫，Tribolium castaneum 的翅膀发育。基于果蝇突变表型，我们首先假设翅膀基因网络的变化可能促成了鞘翅的进化。然而，我们发现，在 Drosophila 中定义的翅膀基因网络在 Tribolium 中很大程度上是保守的，并且也用于模式化鞘翅。相反，我们发现有证据表明，外骨骼形成已经多次被保守的翅膀基因网络下游共同募集。我们还提供了证据表明，其中一次共同募集发生在其他之前，这表明重复的共同募集可能加强了有利的特征。此外，我们发现 Tribolium 无翅基因不仅对鞘翅的外骨骼化是必需的，而且对后翅的正确身份也是必需的，这是我们在果蝇中发现的一个意想不到的作用。
我们的研究结果表明，鞘翅的进化是通过多次募集有益特征，同时保守翅膀图案基因的主要框架来实现的。
