MAXMEN, Amy; MARTINDALE, Mark/Q; GIRIBET, Gonzalo; Harvard University; University of Hawaii; Harvard University: Hox Genes in a Basal Group of Euarthropods: Pycnogonida

In attempt to understand the underlying mechanisms involved in the evolution of innumerable morphologies, researchers have turned to the Hox genes, a family of transcription factors involved in patterning body segments. The Hox genes have been explored in representatives of the four major classes of euarthropods; chelicerates, myriapods, crustaceans, and hexapods (including the insects). However, there is one additional group, Pycnogonida, which is misleadingly referred to as the sea spiders. Several cladistic analyses have placed the pycnogonids as sister taxa to the remaining euarthopods. Pycnogonid fossils from the Cambrian period indicated that this basal group has emerged early in arthopod evolution and also has maintained a conserved body plan over time. Morphologically, the pycnonids are certainly arthropods, yet display many characters which are shared not with arthropods, but rather with other ecdysozoans such as tardigrades, priapulans, and nematodes. Debates concerning Ecdysozoa versus Articulata have been confounded by the use of inferred groundplan character states. The study of pycnogonids, an actual basal arthropod, will be a crucial contribution to a growing body of research on arthropod evolution. Here, 11 Hox-type sequences are reported for the first time from the pycnogonid Anoplodactylus portus , all of which could be assigned to 10 Hox classes proposed to exist in the �ancestral� arthropod Hox cluster. Because shifts in Hox gene expression can be a source in the evolution of diversity, in situ hybridization experiments for a portion of the Hox genes were performed. Consistent with the unique protonymphon larval phenotype, the experiments result in a pattern of Hox gene deployment that differs from what has been found in other arthropods examined to date.