Molecular Biology and Evolution, Vol 14, 485-497, Copyright © 1997 by Society for Molecular Biology and Evolution
S Carranza, J Baguna and M Riutort
In most zoological textbooks, Platyhelminthes are depicted as an early-
emerging clade forming the likely sister group of all the other Bilateria.
Other phylogenetic proposals see them either as the sister group of most of
the Protostomia or as a group derived from protostome coelomate ancestors
by progenesis. The main difficulty in their correct phylogenetic placing is
the lack of convincing synapomorphies for all Platyhelminthes, which may
indicate that they are polyphyletic. Moreover, their internal phylogenetic
relationships are still uncertain. To test these hypotheses, new complete
18S rDNA sequences from 13 species of "Turbellaria" have been obtained and
compared to published sequences of 2 other "Turbellaria," 3 species of
parasitic Platyhelminthes, and several diploblastic and deuterostome and
protostome triploblastics. Maximum-parsimony, maximum-likelihood, and
neighbor-joining methods were used to infer their phylogeny. The results
show the order Catenulida to form an independent early- branching clade and
emerge as a potential sister group of the rest of the Bilateria, while the
rest of Platyhelminthes (Rhabditophora), which includes the parasites, form
a clear monophyletic group closely related to the protostomes. The order
Acoela, morphologically considered as candidates to be ancestral, are shown
to be fast-clock organisms for the 18S rDNA gene. Hence, long-branching of
acoels and insufficient sampling of catenulids and acoels leave their
position still unresolved and call for further studies. Within the
Rhabditophora, our analyses suggest (1) a close relationship between orders
Macrostomida and Polycladida, forming a clear sister group to the rest of
orders; (2) that parasitic platyhelminthes appeared early in the evolution
of the group and form a sister group to a still-unresolved clade made by
Nemertodermatida, Lecithoepitheliata, Prolecithophora, Proseriata,
Tricladida, and Rhabdocoela; and (3) that Seriata is paraphyletic.
ORIGINAL ARTICLE
Are the Platyhelminthes a monophyletic primitive group? An assessment using 18S rDNA sequences
Departament de Genetica, Facultat de Biologia, Universitat de Barcelona, Spain.
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