V4 region of small subunit rDNA indicates polyphyly of the Fellodistomidae(Digenea) which is supported by morphology and life-cycle data

There is no morphological synapomorphy for the disparate digeneans, the Fel lodistomidae Nicoll, 1909. Although all known life-cycles of the group incl ude bivalves as first intermediate hosts, there is no convincing morphologi cal synapomorphy that can be used to unite the group. Sequences from the V4 region of small subunit (18S) rRNA genes were used to infer phylogenetic r elationships among 13 species of Fellodistomidae from four subfamilies and eight species from seven other digenean families: Bivesiculidae; Brachylaim idae; Bucephalidae; Gorgoderidae; Gymnophallidae; Opecoelidae; and Zoogonid ae. Outgroup comparison was made initially with an aspidogastrean. Various species from the other digenean families were used as outgroups in subseque nt analyses. Three methods of analysis indicated polyphyly of the Fellodist omidae and at least two independent radiations of the subfamilies, such tha t they were more closely associated with other digeneans than to each other . The Tandanicolinae was monophyletic (100% bootstrap support) and was weak ly associated with the Gymnophallidae (< 50-55% bootstrap support). Monophy ly of the Baccigerinae was supported with 78-87% bootstrap support, and mon ophyly of the Zoogonidae + Baccigerinae received 77-86% support. The remain ing fellodistomid species, Fellodistomum fellis, F. agnotum and Coomera bra yi (Fellodistominae) plus Proctoeces maculatus and Complexobursa sp. (Proct oecinae), formed a separate clade with 74-92% bootstrap support. On the bas is of molecular, morphological and life-cycle evidence, the subfamilies Bac cigerinae and Tandanicolinae are removed from the Fellodistomidae and promo ted to familial status. The Baccigerinae is promoted under the senior synon ym Faustulidae Poche, 1926, and the Echinobrevicecinae Dronen, Blend & McEa chran, 1994 is synonymised with the Faustulidae. Consequently, species that were formerly in the Fellodistomidae are now distributed in three families : Fellodistomidae; Faustulidae (syn. Baccigerinae Yamaguti, 1954); and Tand anicolidae Johnston, 1927. We infer that the use of bivalves as intermediat e hosts by this broad range of families indicates multiple host-switching e vents within the radiation of the Digenea.