Searching factors causing implausible non-monophyly: ssu rDNA phylogeny of Isopoda Asellota (Crustacea: Peracarida) and faster evolution in marine than in freshwater habitats

This contribution addresses two questions: which alignment patterns are causing non-monophyly of the Asellota and what is the phylogenetic history of this group? The Asellota are small benthic crustaceans occurring in most aquatic habitats. In view of the complex morphological apomorphies known for this group, monophyly of the Asellota has never been questioned. Using ssu rDNA sequences of outgroups and of 16 asellote species from fresh water, littoral marine habitats and from deep-sea localities, the early divergence between the lineages in fresh water and in the ocean, and the monophyly of the deep-sea taxon Munnopsidae are confirmed. Relative substitution rates of freshwater species are much lower than in other isopod species, rates being highest in some littoral marine genera (Carpias and Jaera). Furthermore, more sequence sites are variable in marine than in freshwater species, the latter conserve outgroup character states. Monophyly is recovered with parsimony methods, but not with distance and maximum likelihood analyses, which tear apart the marine from the freshwater species. The information content of alignments was studied with spectra of supporting positions. The scarcity of signal (=apomorphic nucleotides) supporting monophyly of the Asellota is attributed to a short stem-line of this group or to erosion of signal in fast evolving marine species. Parametric boostrapping in combination with spectra indicates that a tree model cannot explain the data and that monophyly of the Asellota should not be rejected even though many topologies do not recover this taxon.