Freshwater snails of the family Lymnaeidae are of a great parasitological importance
because of the very numerous helminth species they transmit, mainly trematodiases
of large medical and veterinary impact. The present knowledge on the genetics of lymnaeids
and on their parasite-host inter-relationships is far from being sufficient. The family
is immersed in a systematic-taxonomic confusion. The necessity for a tool which enables
species distinction and population characterization is evident. This paper aims to
review the European Lymnaeidae basing on the second internal transcribed spacer ITS-2
of the nuclear ribosomal DNA. The ITS-2 sequences of 66 populations of 13 European
and 1 North American lymnaeid species, including the five generic (or subgeneric)
taxa Lymnaea sensu stricto, Stagnicola, Omphiscola, Radix and Galba, have been obtained.
The ITS-2 proves to be a useful marker for resolving supraspecific, specific and population
relationships in Lymnaeidae. Three different groupings according to their ITS-2 length
could be distinguished: Radix and Galba may be considered the oldest taxa (370-406
bp lengths), and Lymnaea s. str., European Stagnicola and Omphiscola (468-491 bp lengths)
the most recent, American Stagnicola and Hinkleyia being intermediate (434-450 bp
lengths). This hypothesis agrees with the phylogeny of lymnaeids based on palaeontological
data, chromosome numbers and radular dentition. ITS-2 sequences present a conserved
central region flanked by two variable lateral regions corresponding to the 5' and
3' ends. The number of repeats of two microsatellites found in this conserved central
region allows to differentiate Radix from all other lymnaeids. Phylogenetic trees
showed four clades: (A) Lymnaea s. str., European Stagnicola and Omphiscola; (B) Radix
species; (C) Galba truncatula; and (D) North American stagnicolines. ITS-2 results
suggest that retaining Stagnicola as a subgenus of Lymnaea may be the most appropriate
and that genus status for Omphiscola is justified. Radix shows a complexity suggesting
different evolutionary lines, whereas G. truncatula appears to be very homogeneous.
North American and European stagnicolines do not belong to the same supraspecific
taxon; the genus Hinkleyia may be used for the American stagnicolines. Genetic distances
and sequence differences allowed us to distinguish the upper limit to be expected
within a single species and to how different sister species may be. S. palustris,
S. fuscus and S. corvus proved to be valid species, but S. turricula may not be considered
a species independent from S. palustris. Marked nucleotide divergences and genetic
distances detected between different S. fuscus populations may be interpreted as a
process of geographic differentiation developping in the present. Among Radix, six
valid species could be distinguished: R. auricularia, R. ampla, R. peregra (=R. ovata;=R.
balthica), R. labiata, R. lagotis and Radix sp. The information which the ITS-2 marker
furnishes is of applied interest concerning the molluscan host specificity of the
different trematode species. The phylogenetic trees inferred from the ITS-2 sequences
are able to differentiate between lymnaeids transmitting and those non-transmitting
fasciolids, as well as between those transmitting F. hepatica and those transmitting
F. gigantica. The Fasciola specificity is linked to the two oldest genera which moreover
cluster together in the phylogenetic trees, suggesting an origin of the Fasciola ancestors
related to the origin of this branch. European Trichobilharzia species causing human
dermatitis are transmitted only by lymnaeids of the Radix and Lymnaea s. str.-Stagnicola
groups. Results suggest the convenience of reinvestigating compatibility differences
after accurate lymnaeid species classification by ITS-2 sequencing. Similarly, ITS-2
sequencing would allow a step forward in the appropriate rearrangement of the actual
systematic confusion among echinostomatids.