title: Divergent role of the Hox gene Antennapedia in spiders is responsible for the convergent evolution of abdominal limb repression.
abstract: Evolution often results in morphologically similar solutions in different organisms, a phenomenon known as convergence. However, there is little knowledge of the processes that lead to convergence at the genetic level. The genes of the Hox cluster control morphology in animals. They may also be central to the convergence of morphological traits, but whether morphological similarities also require similar changes in Hox gene function is disputed. In arthropods, body subdivision into a region with locomotory appendages ("thorax") and a region with reduced appendages ("abdomen") has evolved convergently in several groups, e.g., spiders and insects. In insects, legs develop in the expression domain of the Hox gene Antennapedia (Antp), whereas the Hox genes Ultrabithorax (Ubx) and abdominal-A mediate leg repression in the abdomen. Here, we show that, unlike Antp in insects, the Antp gene in the spider Achaearanea tepidariorum represses legs in the first segment of the abdomen (opisthosoma), and that Antp and Ubx are redundant in the following segment. The down-regulation of Antp in A. tepidariorum leads to a striking 10-legged phenotype. We present evidence from ectopic expression of the spider Antp gene in Drosophila embryos and imaginal tissue that this unique function of Antp is not due to changes in the Antp protein, but likely due to divergent evolution of cofactors, Hox collaborators or target genes in spiders and flies. Our results illustrate an interesting example of convergent evolution of abdominal leg repression in arthropods by altering the role of distinct Hox genes at different levels of their action.
authors.affiliation: Johann-Friedrich-Blumenbach-Institut für Zoologie und Anthropologie, Abteilung für Entwicklungsbiologie, GZMB Ernst-Caspari-Haus,  Georg-August-Universität Göttingen, 37077 Göttingen,  Germany.
authors.forename: Sara
authors.initials: S
authors.lastname: Khadjeh
authors.forename: Natascha
authors.initials: N
authors.lastname: Turetzek
authors.forename: Matthias
authors.initials: M
authors.lastname: Pechmann
authors.forename: Evelyn E
authors.initials: EE
authors.lastname: Schwager
authors.forename: Ernst A
authors.initials: EA
authors.lastname: Wimmer
authors.forename: Wim G M
authors.initials: WG
authors.lastname: Damen
authors.forename: Nikola-Michael
authors.initials: NM
authors.lastname: Prpic
mesh_terms: D000005:Abdomen / Q000033:anatomy & histology*; D000818:Animals; D051943:Antennapedia Homeodomain Protein / Q000235:genetics* / Q000378:metabolism; D057506:Arthropod Antennae / Q000378:metabolism; D005075:Biological Evolution; D004331:Drosophila melanogaster / Q000196:embryology / Q000235:genetics; D004625:Embryo, Nonmammalian / Q000033:anatomy & histology / Q000378:metabolism; D005121:Extremities / Q000033:anatomy & histology*; D018507:Gene Expression Regulation, Developmental; D019476:Insect Proteins / Q000378:metabolism; D008969:Molecular Sequence Data; D034622:RNA Interference; D012097:Repressor Proteins / Q000378:metabolism*; D013112:Spiders / Q000033:anatomy & histology* / Q000196:embryology / Q000235:genetics*
chemical_list: D051943:Antennapedia Homeodomain Protein; D019476:Insect Proteins; D012097:Repressor Proteins