Department of Genetics, Environment and Evolution, University College London, Darwin Building, Gower Street, London, WC1E 6BT, UK.
Evodevo. 2013 Jan 2;4(1):1. doi: 10.1186/2041-9139-4-1.
The biting edge of the primitive arthropod mandible consists of a biting incisor process and a crushing molar process. These structures are thought to be derived from a structure known as an endite but the precise details of this are not understood. Various hypotheses concerning the number of endites present in the arthropod mandible have been proposed.In the developing embryo, the mandible has an inner and outer lobe that are likely to develop into the incisor and molar processes of the larval mandible; these two lobes are commonly held to be derived from separate endites and to be serially homologous to the galea and lacinia endites of the maxillary appendage respectively (Machida).
We undertook a study of the development of the embryonic mandible of the beetle Tribolium castaneum using the expression of developmental genes as markers of the developing endites in the mandible and maxilla.The Tribolium ortholog of paired (Tc-prd) has expression domains in the developing maxillary and labial endites as well as the inner and outer lobes of the mandible. Following the expression of Tc-prd in the developing mandible through to late stage embryos shows that the molar and incisor process develop from the inner and outer lobes respectively.In addition to Tc-prd, we compared the expression of genes in the endites of the maxilla to the mandible to draw conclusions about the number of endites in the mandible. Homologs of dachshund are typically expressed in the endites of mandibulate gnathal appendages. Comparison of the expression of Tc-prd, Tribolium dachshund (Tc-dac) and Tribolium wingless (Tc-wg) between the endites of the maxilla and the mandible suggest that, while there are two endites in the maxilla only a single endite is present in the mandible.
Comparative gene expression suggests that the Tribolium mandible has a single endite from which both mandible lobes are derived. Our results do not support Machida's hypothesis homologising the incisor and molar processes of the mandible to the galea and lacinia endites of the maxilla. We propose, instead, that both incisor and molar processes are derived from a single endite serially homologous to the lacinia of the maxilla.
原始节肢动物的下颌的咬合边缘由一个咬合切齿过程和一个压碎的臼齿过程组成。这些结构被认为是从一个称为外颚的结构衍生而来的，但具体细节尚不清楚。关于节肢动物下颌中存在的外颚数量，已经提出了各种假设。在发育中的胚胎中，下颌有一个内叶和一个外叶，它们可能发育成幼虫下颌的切齿和臼齿过程；这两个叶通常被认为是由单独的外颚衍生而来，并且分别与上颌附肢的上颚和下颚外颚同源（Machida）。
我们使用发育基因的表达作为下颌和上颌发育外颚的标记，研究了甲虫 Tribolium castaneum 的胚胎下颌的发育。成对的 Tribolium 同源物（Tc-prd）在发育中的上颌和唇外颚以及下颌的内叶和外叶中具有表达域。在发育中的下颌中表达 Tc-prd 直至晚期胚胎表明，臼齿和切齿过程分别由内叶和外叶发育而来。除了 Tc-prd 之外，我们还比较了上颌和下颌外颚中的基因表达，以得出下颌中外颚数量的结论。 Dachshund 的同源物通常在上颚和下颚的外颚中表达。上颌和下颌外颚中 Tc-prd、Tribolium dachshund（Tc-dac）和 Tribolium wingless（Tc-wg）的表达比较表明，上颌中有两个外颚，而下颌中只有一个外颚。
比较基因表达表明，Tribolium 下颌有一个单一的外颚，两个下颌叶都从中衍生而来。我们的结果不支持 Machida 的假设，即下颌的切齿和臼齿过程与上颌的上颚和下颚外颚同源。相反，我们提出，切齿和臼齿过程都来自于一个与上颌的下颚外颚同源的单一外颚。
