title: Gene expression patterns in an onychophoran reveal that regionalization predates limb segmentation in pan-arthropods.
abstract: In arthropods, such as Drosophila melanogaster, the leg gap genes homothorax (hth), extradenticle (exd), dachshund (dac), and Distal-less (Dll) regionalize the legs in order to facilitate the subsequent segmentation of the legs. We have isolated homologs of all four leg gap genes from the onychophoran Euperipatoides kanangrensis and have studied their expression. We show that leg regionalization takes place in the legs of onychophorans even though they represent simple and nonsegmented appendages. This implies that leg regionalization evolved for a different function and was only later co-opted for a role in leg segmentation. We also show that the leg gap gene patterns in onychophorans (especially of hth and exd) are similar to the patterns in crustaceans and insects, suggesting that this is the plesiomorphic state in arthropods. The reversed hth and exd patterns in chelicerates and myriapods are therefore an apomorphy for this group, the Myriochelata, lending support to the Myriochelata and Tetraconata clades in arthropod phylogeny.
authors.affiliation: Department of Earth Sciences, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden. ralf.janssen@geo.uu.se
authors.forename: Ralf
authors.initials: R
authors.lastname: Janssen
authors.forename: Bo Joakim
authors.initials: BJ
authors.lastname: Eriksson
authors.forename: Graham E
authors.initials: GE
authors.lastname: Budd
authors.forename: Michael
authors.initials: M
authors.lastname: Akam
authors.forename: Nikola-Michael
authors.initials: NM
authors.lastname: Prpic
mesh_terms: D000818:Animals; D001181:Arthropods / Q000235:genetics / Q000254:growth & development*; D019521:Body Patterning / Q000235:genetics*; D004625:Embryo, Nonmammalian / Q000033:anatomy & histology / Q000378:metabolism; D047108:Embryonic Development / Q000235:genetics; D005121:Extremities / Q000254:growth & development*; D005260:Female; D018507:Gene Expression Regulation, Developmental; D010802:Phylogeny