Traditionally, the family Campulidae has been associated either
with
the family Fasciolidae, parasites of ruminants, or the
Acanthocolpidae, parasites of fishes, based on morphological similarities.
Since morphology does not seem to resolve
clearly the problem of the relationships of campulids, we have used the
sequences of the 18S rRNA gene of the campulids
Zalophotrema hepaticum, Campula oblonga and
Nasitrema globicephalae, the fasciolid Fasciola hepatica,
the acanthocolpid
Stephanostomum baccatum and the outgroup Schistosoma mansoni
to infer a phylogeny. Maximum parsimony and neighbour-joining
methods were applied. Both methods indicated that campulids are closer
to
acanthocolpids than fasciolids.
In order to confirm this relationship, we generated a second phylogeny
using all the partial sequences of the 18S published
for trematodes: Lobatostoma manteri, Echinostoma caproni,
Calicophoron calicophorum, Tetracerasta blepta, Gyliauchen
sp.
and Opistorchis viverrini, plus those mentioned above, and
Dicrocoelium dendriticum. The aspidogastrean L. manteri
was
used as the outgroup. Results were identical to the first analysis.
According to this and the most recent Digenean phylogeny,
which considers campulids and acanthocolpids as sister groups, we
suggest that a common origin for these 2 groups would
imply a host-switching process. The life-cycle of acanthocolpids
includes marine gastropods as first intermediate hosts,
and fishes as second intermediate and definitive hosts. In this
context, the hypothesis would be that trematodes whose
cycle ended in fishes were able to switch to mammalian hosts.
New 18S rRNA sequences from neomenioid aplacophorans and the possible
origin of persistent exogenous contamination