Setton Emily V W, March Logan E, Nolan Erik D, Jones Tamsin E, Cho Holly, Wheeler Ward C, Extavour Cassandra G, Sharma Prashant P
Department of Integrative Biology, University of Wisconsin-Madison, 430 Lincoln Drive, Madison, WI 53706, USA.
Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA.
Dev Biol. 2017 Oct 1;430(1):224-236. doi: 10.1016/j.ydbio.2017.07.016. Epub 2017 Jul 29.
The deutocerebral (second) head segment is putatively homologous across Arthropoda, in spite of remarkable disparity of form and function of deutocerebral appendages. In Mandibulata this segment bears a pair of sensory antennae, whereas in Chelicerata the same segment bears a pair of feeding appendages called chelicerae. Part of the evidence for the homology of deutocerebral appendages is the conserved function of homothorax (hth), which has been shown to specify antennal or cheliceral fate in the absence of Hox signaling, in both mandibulate and chelicerate exemplars. However, the genetic basis for the morphological disparity of antenna and chelicera is not understood. To test whether downstream targets of hth have diverged in a lineage-specific manner, we examined the evolution of the function and expression of spineless (ss), which in two holometabolous insects is known to act as a hth target and distal antennal determinant. Toward expanding phylogenetic representation of gene expression data, here we show that strong expression of ss is observed in developing antennae of a hemimetabolous insect, a centipede, and an amphipod crustacean. By contrast, ss orthologs are not expressed throughout the cheliceral limb buds of spiders or harvestmen during developmental stages when appendage fate is specified. RNA interference-mediated knockdown of ss in Oncopeltus fasciatus, which bears a simple plesiomorphic antenna, resulted in homeotic distal antenna-to-leg transformation, comparable to data from holometabolous insect counterparts. Knockdown of hth in Oncopeltus fasciatus abrogated ss expression, suggesting conservation of upstream regulation. These data suggest that ss may be a flagellar (distal antennal) determinant more broadly, and that this function was acquired at the base of Mandibulata.
尽管中脑（第二）头部节段的附肢在形态和功能上存在显著差异，但在节肢动物门中普遍认为其具有同源性。在有颚类动物中，该节段有一对感觉触角，而在螯肢动物中，同一节段有一对称为螯肢的摄食附肢。中脑附肢同源性的部分证据是同胸（hth）基因的保守功能，在有颚类和螯肢类动物中，已证明在没有Hox信号的情况下，hth基因能决定触角或螯肢的命运。然而，触角和螯肢形态差异的遗传基础尚不清楚。为了测试hth的下游靶点是否以谱系特异性方式发生了分化，我们研究了无脊椎（ss）基因功能和表达的进化，在两种全变态昆虫中，已知该基因作为hth的靶点和触角远端的决定因素。为了扩大基因表达数据的系统发育代表性，我们在此表明，在一种半变态昆虫、一种蜈蚣和一种双足甲壳类动物发育中的触角中观察到了ss的强烈表达。相比之下，在蜘蛛或盲蛛的螯肢肢芽发育阶段，当附肢命运确定时，ss直系同源基因并未在整个肢芽中表达。在具有简单近祖型触角的豆象Oncopeltus fasciatus中，通过RNA干扰介导敲低ss，导致触角远端出现同源异型转化为腿的现象，这与全变态昆虫对应物的数据相当。在豆象Oncopeltus fasciatus中敲低hth可消除ss的表达，表明上游调控具有保守性。这些数据表明，ss可能更广泛地是鞭毛（触角远端）的决定因素，并且这种功能是在有颚类动物基部获得的。
