title: Head patterning and Hox gene expression in an onychophoran and its implications for the arthropod head problem.
abstract: The arthropod head problem has puzzled zoologists for more than a century. The head of adult arthropods is a complex structure resulting from the modification, fusion and migration of an uncertain number of segments. In contrast, onychophorans, which are the probable sister group to the arthropods, have a rather simple head comprising three segments that are well defined during development, and give rise to the adult head with three pairs of appendages specialised for sensory and food capture/manipulative purposes. Based on the expression pattern of the anterior Hox genes labial, proboscipedia, Hox3 and Deformed, we show that the third of these onychophoran segments, bearing the slime papillae, can be correlated to the tritocerebrum, the most anterior Hox-expressing arthropod segment. This implies that both the onychophoran antennae and jaws are derived from a more anterior, Hox-free region corresponding to the proto and deutocerebrum of arthropods. Our data provide molecular support for the proposal that the onychophoran head possesses a well-developed appendage that corresponds to the anterior, apparently appendage-less region of the arthropod head.
authors.affiliation: University Museum of Zoology and Department of Zoology, Downing Street, Cambridge, CB2 3EJ, UK. j.eriksson@qmul.ac.uk
authors.forename: Bo Joakim
authors.initials: BJ
authors.lastname: Eriksson
authors.forename: Noel N
authors.initials: NN
authors.lastname: Tait
authors.forename: Graham E
authors.initials: GE
authors.lastname: Budd
authors.forename: Ralf
authors.initials: R
authors.lastname: Janssen
authors.forename: Michael
authors.initials: M
authors.lastname: Akam
mesh_terms: D000818:Animals; D000845:Annelida / Q000196:embryology / Q000235:genetics*; D001181:Arthropods / Q000196:embryology / Q000235:genetics; D019521:Body Patterning / Q000235:genetics*; D018076:DNA, Complementary / Q000737:chemistry / Q000235:genetics; D004625:Embryo, Nonmammalian / Q000033:anatomy & histology / Q000196:embryology / Q000378:metabolism; D019143:Evolution, Molecular; D020869:Gene Expression Profiling; D018507:Gene Expression Regulation, Developmental; D006257:Head / Q000033:anatomy & histology / Q000196:embryology; D018398:Homeodomain Proteins / Q000145:classification / Q000235:genetics*; D017403:In Situ Hybridization; D007448:Invertebrates / Q000145:classification / Q000196:embryology / Q000235:genetics; D008953:Models, Anatomic; D008969:Molecular Sequence Data; D010802:Phylogeny; D017422:Sequence Analysis, DNA
chemical_list: D018076:DNA, Complementary; D018398:Homeodomain Proteins