Smith Frank W, Angelini David R, Jockusch Elizabeth L
Department of Ecology & Evolutionary Biology, University of Connecticut, 75 N. Eagleville Rd., U-3043, Storrs, CT 06269-3043, USA.
Department of Ecology & Evolutionary Biology, University of Connecticut, 75 N. Eagleville Rd., U-3043, Storrs, CT 06269-3043, USA; Department of Biology, Colby College, 5734 Mayflower Hill, Waterville, ME 04901, USA.
Mech Dev. 2014 May;132:13-27. doi: 10.1016/j.mod.2014.02.002. Epub 2014 Feb 15.
The antenna was the first arthropod ventral appendage to evolve non-leg identity. Models of antennal evolution have been based on comparisons of antennal and leg identity specification mechanisms in Drosophila melanogaster, a species in which appendages develop from highly derived imaginal discs during the larval period. We test for conservation of the Drosophila antennal identity specification mechanism at metamorphosis in Tribolium castaneum and three other flour beetle species (Tribolium confusum, Tribolium brevicornis and Latheticus oryzae) in the family Tenebrionidae. In Drosophila, loss of function of four transcription factors-homothorax, extradenticle, Distal-less, and spineless-causes large-scale transformations of the antenna to leg identity. Distal-less and spineless function similarly during metamorphosis in T. castaneum. RNA interference (RNAi) targeting homothorax (hth) or extradenticle (exd) caused transformation of the proximal antenna to distal leg identity in flour beetles, but did not affect the identity of the distal antenna. This differs from the functional domain of these genes in early instar Drosophila, where they are required for identity specification throughout the antenna, but matches their functional domain in late instar Drosophila. The similarities between antennal identity specification at metamorphosis in flour beetles and in late larval Drosophila likely reflect the conservation of an ancestral metamorphic developmental mechanism. There were two notable differences in hth/exd loss of function phenotypes between flies and beetles. Flour beetles retained all of their primary segments in both the antenna and legs, whereas flies undergo reduction and fusion of primary segments. This difference in ground state appendage morphology casts doubt on interpretations of developmental ground states as evolutionary atavisms. Additionally, adult Tribolium eyes were transformed to elytron-like structures; we provide a developmental hypothesis for this evolutionarily surprising transformation.
触角是首个进化出非腿部特征的节肢动物腹侧附肢。触角进化模型基于对黑腹果蝇触角和腿部特征决定机制的比较，在该物种中，附肢在幼虫期由高度特化的成虫盘发育而来。我们测试了拟谷盗及其他三种拟步甲科面粉甲虫（杂拟谷盗、短角拟谷盗和米扁虫）变态发育过程中果蝇触角特征决定机制的保守性。在果蝇中，四种转录因子——同胸、额外齿、无翅和无棘——功能丧失会导致触角大规模转变为腿部特征。无翅和无棘在拟谷盗变态发育过程中的功能相似。靶向同胸（hth）或额外齿（exd）的RNA干扰（RNAi）导致面粉甲虫近端触角转变为远端腿部特征，但不影响远端触角的特征。这与这些基因在果蝇早期龄期的功能域不同，在早期龄期，它们是整个触角特征决定所必需的，但与它们在果蝇晚期龄期的功能域匹配。面粉甲虫变态发育时触角特征决定与果蝇幼虫晚期的相似性可能反映了一种祖先变态发育机制的保守性。果蝇和甲虫在hth/exd功能丧失表型上有两个显著差异。面粉甲虫在触角和腿部都保留了所有的初级节段，而果蝇会经历初级节段的减少和融合。这种基础状态附肢形态的差异让人怀疑将发育基础状态解释为进化返祖现象的观点。此外，成年拟谷盗的眼睛转变为类似鞘翅的结构；我们为这种在进化上令人惊讶的转变提供了一个发育假说。
