The phylogenetic position of the Isopoda in the Peracarida (Crustacea: Malacostraca)

摘要: > Abstract The sister group to the isopod crustaceans remains a controversial proposition. Previous studies have used idealised composite taxa or few exemplars, resulting in confl icting assertions about placement of Isopoda among Malacostraca. A recent morphological study argued for an Amphipoda-Isopoda clade, whereas molecular using SSU rDNA (18S) data found other relationships. Within isopods, morphologically-specialised Phreatoicidea are generally regarded as earliest-derived taxon, based on their fossil record and several published cladograms. These hypotheses were tested 18S sequences 202 characters from 75 exemplars (52 isopods 23 malacostracans). partitions analysed separately combined, sequence dynamic homology. To fi nd best substitution, insertion-deletion gap insertion costs, scores nding accepted monophyletic select optimal parameters. Separately both rejected clade hypothesis. analysis placed phreatoicideans high tree, rather basal position. basally branching Phreatoicidea-Asellota group. combined Apseudomorpha (Tanaidacea) + with still well separated ancestral root. parasitic subgroups Cymothoida (families Cymothoidae Gnathiidae, superfamily Bopyroidea) comprised branch which is novel hypothesis that argues against monophyly suborder Cymothoida. alone inadequate at this phylogenetic level provided require additional evidence detailed DNA markers confi rmation.