A backboned phylogeny for invertebrates: the case and problems of

A backboned phylogeny for invertebrates: the case and problems of eutardigrades
(Tardigrada)
Matteo Vecchi1, Noemi Guil2, Lorena Rebecchi1, Roberto Guidetti1
1
Department of Life Sciences, University of Modena and Reggio Emilia, Modena (Italy).
2
Department of Biodiversity and Evolutionary Biology, Museo nacional de Ciencias Naturales
(CSIC), Madrid (Spain).
Tardigrades are a small phylum of micrometazoans capable to undergo cryptobiosis (i.e. an
ametabolic state of life in response to adverse environmental conditions). Phylogenetic studies on
tardigrades relied almost only on molecular characters, while morphological based phylogenies are
scarce. Beside the problems of working with microscopic animals, the main limitation in the use of
molecular characters is the difficulty in obtaining specimens for the analyses, as many species and
genera have been found only once, in remote places and in small amount, leading to a poor taxon
sampling. Morphological characters instead can be easily obtained from literature and slides
collections, but they are very limited in number, difficult to score and prone to homoplasy. Total
evidence analyses with concatenated morphological and molecular dataset didn’t provide
satisfactory results. To overcome these limitations a backboned approach has been applied. In this
approach a scaffold from molecular phylogenies is used to constrain the Bayesian analyses of
morphological characters. This approach has been used before to place fossil taxa in phylogenies
with extant taxa for wich was avaible a molecular phylogeny. The main limitation in applying this
approach to eutardigrades is the different level of taxon sampling of molecular (exemplar terminals)
and morphological (supraspecific terminals) data. A way to overcome this problem is the leveling
of terminal taxa to the same taxonomic level. In this case study, the supraspecific (genus) terminals
are used. This approach led to an eutardigrades phylogenetic tree inclusive of all genera never
obtained before.