To be or not to be a flatworm: the acoel controversy.
Since first described, acoels were considered members of the flatworms (Platyhelminthes). However, no clear synapomorphies among the three large flatworm taxa -- the Catenulida, the Acoelomorpha and the Rhabditophora -- have been characterized to date. Molecular phylogenies, on the other hand, commonly positioned acoels separate from other flatworms. Accordingly, our own multi-locus phylogenetic analysis using 43 genes and 23 animal species places the acoel flatworm Isodiametra pulchra at the base of all Bilateria, distant from other flatworms. By contrast, novel data on the distribution and proliferation of stem cells and the specific mode of epidermal replacement constitute a strong synapomorphy for the Acoela plus the major group of flatworms, the Rhabditophora. The expression of a piwi-like gene not only in gonadal, but also in adult somatic stem cells is another unique feature among bilaterians. These two independent stem-cell-related characters put the Acoela into the Platyhelminthes-Lophotrochozoa clade and account for the most parsimonious evolutionary explanation of epidermal cell renewal in the Bilateria. Most available multigene analyses produce conflicting results regarding the position of the acoels in the tree of life. Given these phylogenomic conflicts and the contradiction of developmental and morphological data with phylogenomic results, the monophyly of the phylum Platyhelminthes and the position of the Acoela remain unresolved. By these data, both the inclusion of Acoela within Platyhelminthes, and their separation from flatworms as basal bilaterians are well-supported alternatives.
Institute of Zoology and Center for Molecular Biosciences, University of Innsbruck, Innsbruck, Austria.
Bernhard
B
Egger
Dirk
D
Steinke
Hiroshi
H
Tarui
Katrien
K
De Mulder
Detlev
D
Arendt
Gaëtan
G
Borgonie
Noriko
N
Funayama
Robert
R
Gschwentner
Volker
V
Hartenstein
Bert
B
Hobmayer
Matthew
M
Hooge
Martina
M
Hrouda
Sachiko
S
Ishida
Chiyoko
C
Kobayashi
Georg
G
Kuales
Osamu
O
Nishimura
Daniela
D
Pfister
Reinhard
R
Rieger
Willi
W
Salvenmoser
Julian
J
Smith
Ulrich
U
Technau
Seth
S
Tyler
Kiyokazu
K
Agata
Walter
W
Salzburger
Peter
P
Ladurner
D000818:Animals; D002465:Cell Movement; D049109:Cell Proliferation; D019143:Evolution, Molecular; D015870:Gene Expression; D015801:Helminth Proteins / Q000235:genetics; D017403:In Situ Hybridization; D008957:Models, Genetic; D010802:Phylogeny; D010986:Platyhelminths / Q000145:classification* / Q000166:cytology / Q000235:genetics; D013234:Stem Cells / Q000166:cytology / Q000378:metabolism
D015801:Helminth Proteins