Popadíc A, Panganiban G, Rusch D, Shear W A, Kaufman T C
Department of Biology and Howard Hughes Medical Institute, Indiana University, Bloomington, IN 47405, USA.
Dev Genes Evol. 1998 May;208(3):142-50. doi: 10.1007/s004270050165.
Mandibles are feeding appendages functioning as "jaws" in the arthropod groups in which they occur. Which part of this appendage is involved in food manipulation (limb tip versus limb base), has been used to suggest phylogenetic relationships among some of the major taxa of arthropods (myriapods, crustaceans, and insects). As a way to independently verify the conclusions drawn from previous morphological analyses, we have studied the expression pattern of the gene Distal-less (Dll), which specifies the distal part of appendages. Our results show, in contrast to the traditional view, that both insect and crustacean adult mandibles are gnathobasic, handling food with the basal portion of the appendage. Furthermore, as is evident by the reduction in the number of Dll-expressing cells in the later developmental stages, adult diplopod jaws are also gnathobasic. Thus, jaws of all mandibulates (myriapods, crustaceans, and insects) seem to have a similar gnathobasic structure. We have also found that Dll is expressed in the labra of all arthropod taxa examined, suggesting that this structure is of appendicular derivation. Additionally, the spinnerets and book lungs of spiders, long considered on other grounds to be modified appendages, express Dll, confirming this interpretation. This study shows that, in addition to their use in phylogenetic and population genetic studies, molecular markers can be very useful for inferring the origins of a particular morphological feature.
在具有上颚的节肢动物类群中，上颚是作为“颌”发挥作用的取食附肢。该附肢的哪一部分参与食物操控（附肢末端还是基部），已被用于推断一些主要节肢动物类群（多足纲动物、甲壳纲动物和昆虫）之间的系统发育关系。作为一种独立验证先前形态学分析得出的结论的方法，我们研究了Distal-less（Dll）基因的表达模式，该基因决定附肢的远端部分。我们的研究结果表明，与传统观点相反，昆虫和甲壳纲动物的成年上颚都是颚基型的，用附肢的基部处理食物。此外，从后期发育阶段中表达Dll的细胞数量减少可以明显看出，成年倍足纲动物的颚也是颚基型的。因此，所有有颚类动物（多足纲动物、甲壳纲动物和昆虫）的颚似乎都具有相似的颚基结构。我们还发现，Dll在所研究的所有节肢动物类群的唇片中都有表达，这表明该结构起源于附肢。此外，长期以来基于其他理由被认为是经过改造的附肢的蜘蛛的纺器和书肺也表达Dll，证实了这一解释。这项研究表明，分子标记除了用于系统发育和群体遗传学研究外，对于推断特定形态特征的起源也非常有用。
