title: A conserved genetic mechanism specifies deutocerebral appendage identity in insects and arachnids.
abstract: The segmental architecture of the arthropod head is one of the most controversial topics in the evolutionary developmental biology of arthropods. The deutocerebral (second) segment of the head is putatively homologous across Arthropoda, as inferred from the segmental distribution of the tripartite brain and the absence of Hox gene expression of this anterior-most, appendage-bearing segment. While this homology statement implies a putative common mechanism for differentiation of deutocerebral appendages across arthropods, experimental data for deutocerebral appendage fate specification are limited to winged insects. Mandibulates (hexapods, crustaceans and myriapods) bear a characteristic pair of antennae on the deutocerebral segment, whereas chelicerates (e.g. spiders, scorpions, harvestmen) bear the eponymous chelicerae. In such hexapods as the fruit fly, Drosophila melanogaster, and the cricket, Gryllus bimaculatus, cephalic appendages are differentiated from the thoracic appendages (legs) by the activity of the appendage patterning gene homothorax (hth). Here we show that embryonic RNA interference against hth in the harvestman Phalangium opilio results in homeonotic chelicera-to-leg transformations, and also in some cases pedipalp-to-leg transformations. In more strongly affected embryos, adjacent appendages undergo fusion and/or truncation, and legs display proximal defects, suggesting conservation of additional functions of hth in patterning the antero-posterior and proximo-distal appendage axes. Expression signal of anterior Hox genes labial, proboscipedia and Deformed is diminished, but not absent, in hth RNAi embryos, consistent with results previously obtained with the insect G. bimaculatus. Our results substantiate a deep homology across arthropods of the mechanism whereby cephalic appendages are differentiated from locomotory appendages.
authors.affiliation: Division of Invertebrate Zoology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA psharma@amnh.org.
authors.forename: Prashant P
authors.initials: PP
authors.lastname: Sharma
authors.affiliation: Department of Biology, University of Florida, Gainesville, FL 32611, USA.
authors.forename: Oscar A
authors.initials: OA
authors.lastname: Tarazona
authors.affiliation: Department of Biology, University of Florida, Gainesville, FL 32611, USA Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
authors.forename: Davys H
authors.initials: DH
authors.lastname: Lopez
authors.affiliation: Department of Biological and Medical Sciences, Oxford Brookes University, Oxford OX3 0BP, UK.
authors.forename: Evelyn E
authors.initials: EE
authors.lastname: Schwager
authors.affiliation: Department of Biology, University of Florida, Gainesville, FL 32611, USA.
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Cohn
authors.affiliation: Division of Invertebrate Zoology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA.
authors.forename: Ward C
authors.initials: WC
authors.lastname: Wheeler
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
authors.forename: Cassandra G
authors.initials: CG
authors.lastname: Extavour
mesh_terms: D000818:Animals; D001097:Arachnida / Q000196:embryology / Q000235:genetics*; D060829:Arthropod Proteins / Q000235:genetics* / Q000378:metabolism; D019521:Body Patterning; D005121:Extremities / Q000196:embryology; D018507:Gene Expression Regulation, Developmental; D006737:Horseshoe Crabs / Q000196:embryology / Q000235:genetics; D007313:Insecta / Q000196:embryology / Q000235:genetics; D008969:Molecular Sequence Data; D034622:RNA Interference; D012605:Scorpions / Q000196:embryology / Q000235:genetics; D017422:Sequence Analysis, DNA
chemical_list: D060829:Arthropod Proteins
keywords: Arthropoda; antenna; chelicera; deutocerebrum; opiliones; serial homology